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Prolate-Spheroid (“Rugby-Shaped”) Hohlraum for Inertial Confinement Fusion
Abstract
A novel rugby-ball shaped hohlraum is designed in the context of the indirect-drive scheme of inertial-confinement fusion (ICF). Experiments were performed on the OMEGA laser and are the first use of rugby hohlraums for ICF studies. Analysis of experimental data shows that the hohlraum energetics is well understood. We show that the rugby-ball shape exhibits advantages over cylinder, in terms of temperature and of symmetry control of the capsule implosion. Simulations indicate that rugby hohlraum driven targets may be candidates for ignition in a context of early Laser MegaJoule experiments with reduced laser energy.
... [8][9][10][11] The inner beam propagation in cylinders with CH capsules requires a high wavelength difference for a strong cross-beam energy transfer (CBET) to achieve adequate symmetry. 12 To improve the energy-coupling efficiency 13 and reduce the need for a strong CBET, a series of hohlraums are proposed with spherical [15][16][17][18][19][20][21] or rugby shapes, such as an ellipse, 8 a parabola, 13 or an arc with a large radius. 22,23 The resulting drive temperature and symmetry have been significantly improved. ...
... [8][9][10][11] The inner beam propagation in cylinders with CH capsules requires a high wavelength difference for a strong cross-beam energy transfer (CBET) to achieve adequate symmetry. 12 To improve the energy-coupling efficiency 13 and reduce the need for a strong CBET, a series of hohlraums are proposed with spherical [15][16][17][18][19][20][21] or rugby shapes, such as an ellipse, 8 a parabola, 13 or an arc with a large radius. 22,23 The resulting drive temperature and symmetry have been significantly improved. ...
... 22,23 The resulting drive temperature and symmetry have been significantly improved. 13,20,[23][24][25] Further, a new hohlraum shape described by the Lame curve 14 was recently proposed by varying the order of the rugby shapes to improve the drive performance; it has been reported that the energy coupling efficiency, radiation symmetry, and LPI can also be improved. The shapes of the designed hohlraums are represented by explicit functions such as f ðx; yÞ ¼ ðx=aÞ k þ ðy=bÞ k À 1, and only three parameters ða; b; kÞ can be optimized, which limits the potential for improvement in the hohlraum performance. ...
The hohlraum shape attracts considerable attention because there is no successful ignition method for laser-driven inertial confinement fusion at the National Ignition Facility. The available hohlraums are typically designed with simple conic curves, including ellipses, parabolas, arcs, or Lame curves, which allow only a few design parameters for the shape optimization, making it difficult to improve the performance, e. g., the energy coupling efficiency or radiation drive symmetry. A novel free-form hohlraum design and optimization approach based on the non-uniform rational basis spline (NURBS) model is proposed. In the present study, (1) all kinds of hohlraum shapes can be uniformly represented using NURBS, which is greatly beneficial for obtaining the optimal available hohlraum shapes, and (2) such free-form uniform representation enables us to obtain an optimal shape over a large design domain for the hohlraum with a more uniform radiation and higher drive temperature of the fuel capsule. Finally, a hohlraum is optimized and evaluated with respect to the drive temperature and symmetry at the Shenguang III laser facility in China. The drive temperature and symmetry results indicate that such a free-form representation is advantageous over available hohlraum shapes because it can substantially expand the shape design domain so as to obtain an optimal hohlraum with high performance. (C) 2014 AIP Publishing LLC.
... Another strategy is to work specifically on the coupling efficiency parameter, i.e. the ratio of the energy absorbed by the capsule to the laser energy, which is with parametric instabilities a crucial drawback of indirect drive. An alternate configuration is proposed in section 3, with a rugby-shaped hohlraum [4] exhibiting a better coupling efficiency and symmetry control of the implosion than a cylindrical one for the specific energy balance of the 160 beams configuration. In the last section we finally describe a novel ablator configuration [5] still under investigation, in order to release the surface roughness specifications of the capsule. ...
... Bending the walls toward the center of the hohlraum helps increasing the outer ring contribution near the level of the inner ring and leads to a better drive of the capsule. Since 2002, the rugby-shaped hohlraum concept has been tested at small scale on Omega [4,7] and has been also studied for non cryogenic ignition capsules [8]. We have performed integrated FCI2 simulations of the nominal A1040 capsule in a gas-filled rugby hohlraum with the same equatorial radius than the nominal 60 quads hohlraum (3.1mm), slightly smaller length (10mm instead of 10.46mm), and smaller LEHs (1.4mm instead of 1.75mm) [4]. ...
... Since 2002, the rugby-shaped hohlraum concept has been tested at small scale on Omega [4,7] and has been also studied for non cryogenic ignition capsules [8]. We have performed integrated FCI2 simulations of the nominal A1040 capsule in a gas-filled rugby hohlraum with the same equatorial radius than the nominal 60 quads hohlraum (3.1mm), slightly smaller length (10mm instead of 10.46mm), and smaller LEHs (1.4mm instead of 1.75mm) [4]. Variations on the laser pulses shown in the inset ofFigure 4 show that the threshold to ignite the A1040 capsule in a rugby-shaped hohlraum is around 270TW; furthermore the absorbed laser energy and maximal power laser required ...
The LMJ experimental plans include the attempt of ignition and burn of an ICF capsule with 40 laser quads, delivering up to 1.4MJ and 380TW. New targets needing reduced laser energy with only a small decrease in robustness are then designed for this purpose. A first strategy is to use scaled-down cylindrical hohlraums and capsules, taking advantage of our better understanding of the problem, set on theoretical modelling, simulations and experiments. Another strategy is to work specifically on the coupling efficiency parameter, i.e. the ratio of the energy absorbed by the capsule to the laser energy, which is with parametric instabilities a crucial drawback of indirect drive. An alternative design is proposed, made up of the nominal 60 quads capsule, named A1040, in a rugby-shaped hohlraum. Robustness evaluations of these different targets are in progress.
... The laser is able to deliver up to 1.2 MJ and 390 TW at peak power, at 0.351 m. As described in Ref. 8, two strategies can be considered for this reduced energy configuration: either keeping the A1040 capsule and using a rugby-shaped Hohlraum, 9,10 or scaling down the A1040 capsule and Hohlraum. The objective of the former strategy is to improve the coupling efficiency, the ratio of the capsule absorbed energy to the laser energy, by reducing the wall area. ...
... The main issue in this type of configuration is the control of the radiation asymmetry on the capsule: The outer cone contribution to the irradiation turns out to be too low and must be enhanced. 8 Two options, at least, can be considered. First we figure out the best trade-off in the cylindrical configuration by varying the beam's pointing and by using beam phasing. ...
... ͑5͔͒. As described in Ref. 8, bending the wall ͑increasing ␣͒ allows us to reduce the wall losses, and thus the laser energy required. However simultaneously, the radiation escaping out the LEH increases, which balances the wall loss reduction. ...
Laser plasma interaction (LPI) is a critical issue in ignition target design. Based on both scaling laws and two-dimensional calculations, this article describes how we can constrain a laser megajoule (LMJ) [
J. Ebrardt and J. M. Chaput, J. Phys.: Conf. Ser. 112, 032005 (2008)
] target design by mitigating LPI. An ignition indirect drive target has been designed for the 2/3 LMJ step. It requires 0.9 MJ and 260 TW of laser energy and power, to achieve a temperature of 300 eV in a rugby-shaped Hohlraum and give a yield of about 20 MJ. The study focuses on the analysis of linear gain for stimulated Raman and Brillouin scatterings. Enlarging the focal spot is an obvious way to reduce linear gains. We show that this reduction is nonlinear with the focal spot size. For relatively small focal spot area, linear gains are significantly reduced by enlarging the focal spot. However, there is no benefit in too large focal spots because of necessary larger laser entrance holes, which require more laser energy. Furthermore, this leads to the existence, for a given design, of a minimum value for linear gains for which we cannot go below.
... Though all present laser facilities for the indirect-drive approach are configured for the hohlraums with a cylindrical symmetry [1,[26][27][28][29][30][31][32][33][34], various hohlraum configurations have been proposed and investigated [35,36]. In Ref. ...
... Though all present laser facilities for the indirect-drive approach are configured for the hohlraums with a cylindrical symmetry [1,[26][27][28][29][30][31][32][33][34], various hohlraum configurations have been proposed and investigated [35,36]. In Ref. [37], the authors used the 60-beam OMEGA laser to test the concept of the tetrahedral hohlraum with four equivalent laser entrance holes (LEHs), achieving a convergence ratio of ∼10 under radiation drive temperatures of 180 to 229 eV without symmetry tuning. ...
In inertial confinement approaches to fusion, the asymmetry of target implosion is a major obstacle to achieving high gain in the laboratory. A recently proposed octahedral spherical hohlraum makes it possible to naturally create spherical target irradiation without supplementary symmetry control. Before any decision is made to pursue an ignition-scale laser system based on the octahedral hohlraum, one needs to test the concept with the existing facilities. Here, we report a proof-of-concept experiment for the novel octahedral hohlraum geometry on the cylindrically configured SGIII laser facility without a symmetry control. All polar and equatorial self-emission images of the compressed target show a near round shape of convergence ratio 15 under both square and shaped laser pulses. The observed implosion performances agree well with the ideal spherical implosion simulation. It also shows limitations with using the existing facilities and adds further weight to the need to move to a spherical port geometry for future ignition laser facilities.
... In practice, simple mathematical models such as view-factor codes are usually used to compute the radiation flux on the capsule, especially for the preliminary design and optimization of thermonuclear target structure and shape in [7] and [8] , which play an important and complementary role to such codes in [9] and [10] . Therefore, the view-factor based codes such as VISRAD and IRAD3D are often used in ICF experiments in [11][12][13][14] and [15] to plan laser beam pointing, and evaluate the radiation symmetry. ...
... and for the matrix C in Eq. (13 ), ...
Radiation computation is very important for high energy density experiments design in the laser-driven Inertial Confinement Fusion. The view-factor based models are often used to calculate the radiation on the capsule inside a hohlraum. However, it usually takes much time to solve them when the number of equations is very large.
In this paper, an efficient iteration approach GPU is presented. The core idea is: (1) guaranteed symmetry, strictly diagonally dominant, and positive definite properties underlying the models are described, (2) a preconditioned conjugate gradient iteration approach is presented to compute the radiation based on such guaranteed properties, and (3) such approach is then parallelized and implemented for GPU so that the large scale models, especially for the non-linear model, can be efficiently solved in reasonable time.
Finally, two experimental targets for Shenguang laser facilities built in China are demonstrated and compared to validate the efficiency of the presented approach. The results show that, the models’ computation (1) can be speeded up with successive over-relax iteration method by eight times as compared with Cholesky factorization based direct approach, (2) can be accelerated more with the preconditioned conjugate gradient iteration approach by almost eight times, and (3) can be further accelerated about 2 to 4 times as it parallelized and run on the GPU, which enables the large scale models, can be efficiently solved in reasonable time on the usual desktop computers.
... The three kinds of hohlraum geometry are reviewed in this paper. Other hohlraums, such as the spherical hohlraums with 4 LEHs [14,15], the rugby hohlraums [16][17][18][19][20] , et al., which have their own problems, are not discussed here. The flux symmetry and the hohlraum energetic are the most important issues of hohlraum [1][2][3]. ...
... Fig. 10 explains the Ylm asymmetry control scheme. From Eq. (17) in Ref. [6], the 4-order contribution a4 spot,total of laser spots, which is proportion to Pl(cos 2θs), could be zero at the nodes of Pl(cos 2θs) at θs=15.28 0 , 35.06 0 , 54.94 0 , and 74.72 0 . al is chosen to describe the l-order flux asymmetry. ...
In this paper, we give a review of three hohlraum geometries, including cylindrical, octahedral and six-cylinder-port hohlraums, in inertial confinement fusion (ICF) mainly from theoretical side. Every hohlraum has its own strengths and weaknesses. Although there is a problem of drive asymmetry in the cylindrical hohlraums due to some non-ideal factors, the success of ignition is still possible if more laser energy is available beyond the US National Ignition Facility (NIF) in the future. Octahedral hohlraums can provide the high symmetry flux on capsule. However, octahedral hohlraums suffer from several problems due to the complicated three-dimensional plasma conditions inside. And up to now, there is no one target design with the octahedral hohlraums in which each problem can be solved at the same time. Six-cylinder-port hohlraums combine the merits in theory of both cylindrical and octahedral hohlraums to a certain extent. We introduce a target design with good performance by using the six-cylinder-port hohlraums, in which the key issues of concern, such as laser energy, drive symmetry, and laser plasma interaction (LPI), etc, are all balanced.
... Hohlraum design is a critical component of research in indirectly driven inertial confinement fusion. [1][2][3][4][5] A well-designed hohlraum improves the radiation temperature and radiation compression symmetry considerably, thereby helping greatly to ignite deuteriumdeuterium (DD) or deuterium-tritium capsules. A symmetric implosion is stressed, as it can convert more of the available kinetic energy into compression and heating of the fuel capsule. ...
The thermonuclear yield from deuterium-filled capsule implosions driven by Z-pinch dynamic hohlraums on an 8-MA pulsed power facility is diagnosed by Ag activation and neutron time-of-flight (nTOF). However, both detector systems suffer from an excessive bremsstrahlung background. Tests on the Ag activation system suggest that the facility may produce such an intense super-MeV x-ray background that the resulting photonuclear-excitation reactions can overwhelm the potential neutron signal. In the nTOF detectors, the high bremsstrahlung background generates such an excessive afterglow in the scintillator that confirming any thermonuclear yield may require a liquid scintillator with a much lower afterglow together with a gated photomultiplier.
... To capitalize on the knowledge to make the next advance in performance, modifications to the hohlraum design are in progress to improve late-time symmetry control, one of the limiting factors believed to further improvement in symmetry control for longer laser pulse lengths. Figure 6 shows the leading candidates including foam liners [68,69], rugby hohlraums [70][71][72][73], and the I-raum [74]. The foam lined hohlraums use pressure from the heated foam material to slow the wall expansion. ...
Indirect drive converts high power laser light into x-rays using small high- Z cavities called hohlraums. X-rays generated at the hohlraum walls drive a capsule filled with deuterium–tritium (DT) fuel to fusion conditions. Recent experiments have produced fusion yields exceeding 50 kJ where alpha heating provides ~3× increase in yield over PdV work. Closing the gaps toward ignition is challenging, requiring optimization of the target/implosions and the laser to extract maximum energy. The US program has a three-pronged approach to maximize target performance, each closing some portion of the gap. The first item is optimizing the hohlraum to couple more energy to the capsule while maintaining symmetry control. Novel hohlraum designs are being pursued that enable a larger capsule to be driven symmetrically to both reduce 3D effects and increase energy coupled to the capsule. The second issue being addressed is capsule stability. Seeding of instabilities by the hardware used to mount the capsule and fill it with DT fuel remains a concern. Work reducing the impact of the DT fill tubes and novel capsule mounts is being pursed to reduce the effect of mix on the capsule implosions. There is also growing evidence native capsule seeds such as a micro-structure may be playing a role on limiting capsule performance and dedicated experiments are being developed to better understand the phenomenon. The last area of emphasis is the laser. As technology progresses and understanding of laser damage/mitigation advances, increasing the laser energy seems possible. This would increase the amount of energy available to couple to the capsule, and allow larger capsules, potentially increasing the hot spot pressure and confinement time. The combination of each of these focus areas has the potential to produce conditions to initiate thermo-nuclear ignition.
... Indirect drive inertial confinement fusion (ICF) uses a high-Z hohlraum (cylindrical [1,2], rugby or elliptical [3,4] or spherical [5,6] hohlraum) to convert the incident laser energy into thermal X-rays, which ablate a capsule located at the hohlraum center to achieve thermonuclear ignition. A successful indirect drive ignition requires driving the implosion capsule to high velocity (~370 km/s) while keeping the cryogenic deuteriumetritium (DT) fuel layer at low entropy [2], meanwhile the imploded core must remain nearly spherical to avoid quenching the ignition of the central hotspot via minimizing the cooling mechanisms, as well as to avoid decreasing the conversion efficiency of the implosion kinetic energy into hotspot internal energy [7]. ...
The basic energy balance model is applied to analyze the hohlraum energetics data from the Shenguang (SG) series laser facilities and the National Ignition Facility (NIF) experiments published in the past few years. The analysis shows that the overall hohlraum energetics data are in agreement with the energy balance model within 20% deviation. The 20% deviation might be caused by the diversity in hohlraum parameters, such as material, laser pulse, gas filling density, etc. In addition, the NIF’s ignition target designs and our ignition target designs given by simulations are also in accordance with the energy balance model. This work confirms the value of the energy balance model for ignition target design and experimental data assessment, and demonstrates that the NIF energy is enough to achieve ignition if a 1D spherical radiation drive could be created, meanwhile both the laser plasma instabilities and hydrodynamic instabilities could be suppressed.
... The experiments were carried out on the 351-nm Omega laser facility at the University of Rochester. The interaction was studied at the laser entrance hole (LEH) of rugby-ballshaped Holhraums [21,22] filled with a 1-atm methane gas. A spherical capsule was mounted at the center of the Hohlraum. ...
The indirect-drive scheme to inertial confinement fusion uses a large number of laser beams arranged in a symmetric angular distribution. Collective laser plasma instabilities can therefore develop that couple all the incident laser waves located in a cone to the daughter wave growing along the cone symmetry axis [D. F. DuBois et al., Phys. Fluids B 4, 241 (1992)]. With complementary diagnostics of Thomson scattering and of the scattered light, we demonstrate the occurrence of collective stimulated Brillouin sidescattering driving collective acoustic waves in indirect-drive experiments.
... It results in a prolate spheroid shape known as "rugby parabolic shaped hohlraum." 3 Because bending the wall increases the X-ray coupling, other approaches have also been proposed recently such as a spherical hohlraum with octahedral six laser entrance holes. 4 Many experiments have been done over the last years on the Omega Laser Facility to explore rugby hohlraum performances. ...
Gas-filled rugby-shaped hohlraums have demonstrated high performances compared to a classical similar diameter cylinder hohlraum with a nearly 40% increase of x-ray drive, 10% higher measured peak drive temperature, and an increase in neutron production. Experimental comparisons have been done between rugby, cylinder, and elliptical hohlraums. The impact of these geometry differences on the laser plasma instabilities is examined. Using comparisons with hydrodynamic simulations carried out with the code FCI2 and postprocessed by Piranah, we have been able to reproduce the stimulated Raman and Brillouin scattering spectrum of the different beams. Using a methodology based on a statistical analysis for the gain calculations, we show that the behavior of the laser plasma instabilities in rugby hohlraums can be reproduced. The efficiency of laser smoothing techniques to mitigate these instabilities are discussed, and we show that while rugby hohlraums exhibit more laser plasma instabilities than cylinder hohlraum, the latter can be mitigated in the case of an elliptical hohlraum.
... reduce their cost as any one shot may require a lot of money. Therefore, several codes based on view-factor such as VISRAD [12], were developed to pre-plan laser beams pointing and evaluate the radiation symmetry on the capsule of targets, and are now still used to plan laser beam planning, or estimate radiation symmetry on the fuel capsule for laser facilities such as Omega [13][14][15][16]. In addition, recently, such tools have been reported to be utilized to design view-factor target experiments on the NIF [17,18]. ...
Physical experiment design and optimization is very essential for laser driven inertial confinement fusion due to the high cost of each shot. However, only limited experiments with simple structure or shape on several laser facilities can be designed and evaluated in available codes, and targets are usually defined by programming, which may lead to it difficult for complex shape target design and optimization on arbitrary laser facilities. A unified modeling approach for physical experiment design and optimization on any laser facilities is presented in this paper. Its core idea includes: (1) any laser facility can be flexibly defined and included with two scripts, (2) complex shape targets and laser beams can be parametrically modeled based on features, (3) an automatically mapping scheme of laser beam energy onto discrete mesh elements of targets enable targets or laser beams be optimized without any additional interactive modeling or programming, and (4) significant computation algorithms are additionally presented to efficiently evaluate radiation symmetry on the target. Finally, examples are demonstrated to validate the significance of such unified modeling approach for physical experiments design and optimization in laser driven inertial confinement fusion.
... Fig. 91 shows that the thicker symcaps driven to about the same velocity performed as well or better than the nominal thickness cases. Potentially more efficient rounded cylinder hohlraum designs ("rugby") 151 allowing increased clearance between capsule and hohlraum wall for the inner beams to reduce SRS and/or reduced wall area losses 152 will also be tested at NIF for improved coupling to allow for thicker capsule implosions. Finally, potentially more efficient ablators (Be 153 and highdensity carbon 154 ) will be tested. ...
... While those targets were made of gold and had right circular cylindrical geometries, we find similar performance in terms of X-ray drive, backscatter and X-ray burn through from the spherical silver vacuum halfraums [17] used in the present work. Recent research has demonstrated an increase in the radiation temperature from about 230 to 240 eV for rugby ball shaped halfraums when compared to the equivalent sized right circular cylindrical hohlraums [18,19]. ...
The propagation and energy coupling of intense laser beams in plasmas are critical issues in inertial confinement fusion. Applying magnetic fields to such a setup has been shown to enhance fuel confinement and heating. Here we report on experimental measurements demonstrating improved transmission and increased smoothing of a high-power laser beam propagating in a magnetized underdense plasma. We also measure enhanced backscattering, which our kinetic simulations show is due to magnetic confinement of hot electrons, thus leading to reduced target preheating.
A recently proposed octahedral spherical hohlraum with six laser entrance holes (LEHs) is an attractive concept for an upgraded laser facility aiming at a predictable and reproducible fusion gain with a simple target design. However, with the laser energies available at present, LEH size can be a critical issue. Owing to the uncertainties in simulation results, the LEH size should be determined on the basis of experimental evidence. However, determination of LEH size of an ignition target at a small-scale laser facility poses difficulties. In this paper, we propose to use the prepulse of an ignition pulse to determine the LEH size for ignition-scale hohlraums via LEH closure behavior, and we present convincing evidence from multiple diagnostics at the SGIII facility with ignition-scale hohlraum, laser prepulse, and laser beam size. The LEH closure observed in our experiment is in agreement with data from the National Ignition Facility. The total LEH area of the octahedral hohlraum is found to be very close to that of a cylindrical hohlraum, thus successfully demonstrating the feasibility of the octahedral hohlraum in terms of laser energy, which is crucially important for sizing an ignition-scale octahedrally configured laser system. This work provides a novel way to determine the LEH size of an ignition target at a small-scale laser facility, and it can be applied to other hohlraum configurations for the indirect drive approach.
We report on the experimental measurement of specular reflection (“glint”) of laser beams off the hohlraum wall in inertial confinement fusion experiments at the National Ignition Facility. In a hohlraum, glinted light can escape the opposite laser entrance hole of the hohlraum and is a potential laser energy loss mechanism. The total measured glint on the inner cones of beams is measured to be less than 8 TW (when using the full National Ignition Facility laser), which is <2% of incident peak power. The simulated x-ray flux exceeds the measurement by 10%–20%, and glinted laser light is unable to account for this discrepancy. Similar inner beam glint was measured for ρ = 0.3 and 0.6 mg/cc gas fill hohlraums, but no glint was detected for 1.2 mg/cc densities. Inner beam glint is dominated by the lowest angle 21.5 beams within a 23.5 quad, and it is at most 30% sensitive to different quad polarization arrangements.
The new hohlraum experimental platform and the quasi-3D simulation model are developed to enable the study of the indirect drive experiment using the six-cylinder-port hohlraum for the first time. It is also the first implosion experiment for the six laser-entrance-hole hohlraum to effectively use all the laser beams of the laser facility that is primarily designed for the cylindrical hohlraum. The experiments performed at the 100 kJ Laser Facility produce a peak hohlraum radiation temperature of ∼222 eV for ∼80 kJ and 2 ns square laser pulse. The inferred x-ray conversion efficiency η∼87% is similar to the cylindrical hohlraum and higher than the octahedral spherical hohlraum at the same laser facility, while the low laser backscatter is similar to the outer cone of the cylindrical hohlraum. The hohlraum radiation temperature and M-band (>1.6 keV) flux can be well reproduced by the quasi-3D simulation. The variations of the yield-over-clean and the hot spot shape can also be semiquantitatively explained by the calculated major radiation asymmetry of the quasi-3D simulation. Our work demonstrates the capability for the study of the indirect drive with the six-cylinder-port hohlraum at the cylindrically configured laser facility, which is essential for numerically assessing the laser energy required by the ignition-scale six-cylinder-port hohlraum.
In the laser-driven indirect drive scheme for inertial confinement fusion, the energy coupling efficiency from the hohlraum to the capsule is typically ∼10% due to limited capsule sizes in order to attain quasi-round implosions with currently available laser energy in cylindrical hohlraums. Recent experiments at the National ignition Facility (NIF) showed ∼30% energy coupling efficiency to aluminum capsules by using a rugby-shaped hohlraum to accommodate larger capsules. This paper reports the first experiment at the NIF demonstrating ∼30% energy coupling to a 3 mm-diameter high-energy-density carbon capsule in a rugby hohlraum with a two-shock laser pulse shape. By comparing the measured bang time with a simulated hydrodynamic scaling, ∼430 kJ coupling is inferred with 1.36 MJ laser drive. The symmetry of the hot spot was observed to be more oblate than simulations predicted. X-ray images taken at late time show strong emission at the laser entrance hole of the rugby hohlraum, indicating a closure earlier than expected, which could contribute to the oblate hot spot shape. Implementing wavelength detuning or modifying the hohlraum shape to tune the symmetry in future experiments would allow symmetric implosions while maintaining the high energy coupling 2021 IAEA, Vienna.
Lower-than-expected deuterium–tritium fuel areal densities have been experimentally inferred across a variety of high-convergence, nominally low-adiabat implosion campaigns at the National Ignition Facility (NIF) using cylinder-shaped Hohlraums [Hurricane et al., Phys. Plasmas 26, 052704 (2019)]. A leading candidate explanation is the presence of atomic mix between the fuel and ablator from hydrodynamic instability growth [Clark et al., Phys. Plasmas 26, 050601 (2019)], leading to reduced fuel compressibility and an effectively higher (in-flight) fuel adiabat α. Tolerating a high-α implosion can be obtained with significantly higher capsule absorbed energy E cap according to the one-dimensional (1-D) ignition-threshold-factor analytic scaling [S. Atzeni and J. Meyer-ter-Vehn, Nucl. Fusion 41, 465 (2001)], ITF ∼ E cap · α − 1.8. Recent experiments with large Al shells in rugby-shaped Hohlraums have established high laser-capsule coupling efficiencies of ≽ 30% [Ping et al., Nat. Phys. 15, 138 (2019)], enabling a path to E cap ≽ 0.5 MJ at the NIF and increased performance margin M ≡ ITF − 1. The ability to operate at high adiabat with large capsules using nonstandard Hohlraums leads to the predicted onset of a volume-ignition mode, defined as when both the entire fuel is the “hot spot” and inertial confinement is principally provided by the ablator compared with the compressed fuel. Such an ignition mode, normally reserved for high-Z targets, e.g., double shells [Amendt et al., Phys. Plasmas 14, 056312 (2007)], is predicted to lead to lower fuel convergence and less exposure to mix due to the intended high adiabat—but at the expense of ∼3–4 × reduced (1-D) yield compared with conventional central hot-spot ignition designs.
Experiments on the National Ignition Facility (NIF) to study hohlraums lined with a 20-mg/cc 400−μm-thick Ta2O5 aerogel at full scale (hohlraum diameter = 6.72 mm) are reported. Driven with a 1.6-MJ, 450-TW laser pulse, the performance of the foam liner is diagnosed using implosion hot-spot symmetry measurements of the high-density carbon (HDC) capsule and measurement of inner beam propagation through a thin-wall 8−μm Au window in the hohlraum. Results show an improved capsule performance due to laser energy deposition further inside the hohlraum, leading to a modest increase in x-ray drive and reduced preheat due to changes in the x-ray spectrum when the foam liner is included. In addition, the outer cone bubble uniformity is improved, but the predicted improvement in inner beam propagation to improve symmetry control is not realized for this foam thickness and density.
Experiments on imploding an Al capsule in a Au rugby hohlraum with up to a 1.5 MJ laser drive were performed on the National Ignition Facility (NIF). The capsule diameter was 3.0 mm with ∼1 MJ drive and 3.4 mm with ∼1.5 MJ drive. Effective symmetry tuning by modifying the rugby hohlraum shape was demonstrated, and good shell symmetry was achieved for 3.4 mm capsules at a convergence of ∼10. The nuclear bang time and the shell velocity from simulations agree with experimental data, indicating ∼500 kJ coupling with 1.5 MJ drive or ∼30% efficiency. The peak velocity reached above 300 km/s for a 120 μm-thick Al capsule. The laser backscatter inside the low-gas-filled rugby hohlraum was very low (<4%) at both scales. The high energy coupling allows implosion designs with increased adiabat, which, in turn, increases the tolerance to detrimental effects of instabilities and asymmetries. These encouraging experimental results open new opportunities for both the mainline single-shell scheme and the double-shell design toward ignition.
The ablative expansion of laser-heated materials is important for determining how hohlraum cavities can be utilized for inertial confinement fusion. The utility of a low-density foam layer to reduce the density of the expanding heated hohlraum wall is demonstrated in a series of experiments on the National Ignition Facility. X-ray radiography measurements of the expanding foam-lined Au wall in low aspect-ratio cylindrical geometry are used to compare the impact of Au-doped CH and Ta2O5 foams between 10 and 40 mg/cc on the wall expansion. HYDRA Simulations are used to estimate the x-ray transmission at the 1/4 nc surface, which is important in understanding the absorption of laser light by the plasma. These demonstrate for the first time that a foam layer reduces the expansion of a hohlraum-like target and illustrate that the interplay between the expanding foam plasma and the shock reflected by the hohlraum wall is critical in optimizing foam-liner parameters to achieve the maximum time for a symmetric drive on a capsule.
The newly available ns-gated laser-entrance-hole (LEH) imager on the National Ignition Facility provides routine, non-perturbative measurements of the x-ray emission from laser-heated plasmas inside the hohlraum as viewed at 19° to the hohlraum axis through one of its LEHs. Multiple images are acquired for a series of times and filter-selected x-ray energy bands within a single shot. The images provide time dependent data on phenomena including the effective radius of the LEH, the length of the gold-plasma “bubble” evolving off the interior wall surface heated by the outer beams, the evolving radius of the x-ray heated hohlraum wall, and the radius of the ablation front of the fusion capsule. These measurements are explained and illustrated with sample data. These techniques are then applied to understand hohlraum behavior as a function of gas fill. For hohlraums with helium gas fill densities of 0.15 to 0.30 mg/cm³, synthetic images computed from simulations agree well with experimental gated LEH images when an inhibited heat transport model [Jones et al., Phys. Plasmas 24, 056312 (2017)] is used. This model can be adjusted to reproduce the expansion rate of the laser-heated plasma bubble in such a way as to improve agreement with the images. At the higher 0.6 mg/cc gas fill, the experimental images show more pronounced 3D features, resulting in slightly less good agreement with the 2D simulations.
A new hohlraum geometry or “Frustraum” is proposed that may enable 2–3× higher capsule absorbed x-ray energy than for nominally sized capsules in standard cylinders. The Frustraum geometry comprises two truncated conical halves (or “frusta”) joined at the waist. An associated larger waist volume above the capsule allows fielding ∼50% larger capsules than the nominal 1 mm (radius) scale. A key feature of the Frustraum is that the outer laser cones strike the Frustraum ends at a higher glancing angle (by ∼23°) compared with a cylinder and generate more specular reflection. A scenario for boosted symmetry control from the outer cones reflecting off a glancing angle hohlraum wall depends on the choice of electron flux limit in the simulations. Recent data from the National Ignition Facility using oversized aluminum shells in rugby-shaped hohlraums [Ping et al., Nat. Phys. 15, 138 (2019)] come closest to approximating a Frustraum and are consistent with a flux limit of 0.03–0.04 in matching the simulated Dante drive history, the backlit trajectory of the Al shell, neutron yield, and implosion time. Applying this simulation methodology to hot-spot ignition designs in a Frustraum shows effective symmetry control and sufficient drive (∼290 eV) to enable high yield, moderate convergence implosions. Simulations suggest that adjusting the obliquity of the Frustraum wall is a robust lever for symmetry tuning. A high adiabat (α = 4.6) ignition design with a shortened laser pulse (<7 ns) is proposed to provide further margin to potential late-time loss of symmetry control from hohlraum filling and anomalous sources of fuel preheat.
The irradiation uniformity of a cylindrical target directly driven by laser beams has been considered, which is relevant for fast ignition electron-transport experiments. The laser intensity distribution on the cylindrical target surface is analyzed and optimized by applying the polar direct drive technique and adjusting the laser beam parameters. Moreover, the rotation of laser spot around its propagation axis is taken into consideration. A case study based on the SG-III prototype laser configuration is presented to demonstrate the optimization approach. The irradiation uniformity is reduced from 10% to 1.6% for perfectly balanced beams, and the effects of uncertainties in beam errors (power imbalance and pointing error) are also studied. Furthermore, differences in laser absorption with different incident angles are taken into account and the results show that highly uniform energy deposition can be achieved.
As usual cylindrical hohlraum with double laser ring cones may lead to serious laser-plasma interaction, such as the simulated Raman scatter and cross-beam energy transfer effect, spherical hohlraum with octahedral 6 Laser Entrance Holes (LEHs) and single cone laser beams, was investigated and reported to have a consistent high radiation symmetry during the whole implosion process. However, it has several potential challenges such as the smaller space left for diagnosis and the assembly of centrally located capsule. In this paper, based on the view-factor model, we investigate the radiation symmetry and the drive temperature on the capsule located in the spherical hohlraum with tetrahedral 4 LEHs and single cone laser beams, since there is more available space for laser disposition and diagnosis. Then, such target is optimized on the laser beam pointing direction to achieve a high radiation performance, i.e., the radiation symmetry and drive temperature on the capsule. Finally, an optimal spherical hohlraum with optimal laser beam pointing has been demonstrated and compared with the spherical hohlraum with octahedral 6 LEHs. The resulting radiation symmetry and the drive temperature shows that it has almost a similar radiation symmetry (the radiation asymmetry variation is no more than 0.2%), and higher drive temperature (the temperature has been increased by 1.73%, and an additional 133 kJ energy of 2 MJ energy for fusion can be saved).
An octahedral spherical hohlraum is a promising candidate in target design for inertial confinement fusion study, because of its potential superiority in uniform radiation and efficient coupling [Lan et al., Phys. Plasmas 21, 010704 (2014)]. Before the experimental investigation for octahedral spherical hohlraum, an energetics experiment is accomplished on the Shenguang-III prototype laser facility by using spherical hohlraums with two cylindrical laser entrance holes. Time evolution of the radiation temperature is obtained with flat response X-ray diode detectors at four different viewing angles with demonstrated repeatability of the measurements. The experimental observations are successfully explained by using a phenomenological model which considers not only the radiation flux contributed from the laser ablated and radiation ablated plasma from hohlraum wall, but also that contributed from the filling plasma inside the hohlraum. This method proves to be a simple but effective way to interpret the time-dependent behaviour of the radiation temperatures at different viewing angles.
In this paper, we give a review of our theoretical and experimental progress in octahedral spherical hohlraum study. From our theoretical study, the octahedral spherical hohlraums with 6 Laser Entrance Holes (LEHs) of octahedral symmetry have robust high symmetry during the capsule implosion at hohlraum-to-capsule radius ratio larger than 3.7. In addition, the octahedral spherical hohlraums also have potential superiority on low backscattering without supplementary technology. We studied the laser arrangement and constraints of the octahedral spherical hohlraums, and gave a design on the laser arrangement for ignition octahedral hohlraums. As a result, the injection angle of laser beams of 50 to 60 degree was proposed as the optimum candidate range for the octahedral spherical hohlraums. We proposed a novel octahedral spherical hohlraum with cylindrical LEHs and LEH shields, in order to increase the laser coupling efficiency and improve the capsule symmetry and to mitigate the influence of the wall blowoff on laser transport. We studied on the sensitivity of the octahedral spherical hohlraums to random errors and compared the sensitivity among the octahedral spherical hohlraums, the rugby hohlraums and the cylindrical hohlraums, and the results show that the octahedral spherical hohlraums are robust to these random errors while the cylindrical hohlraums are the most sensitive. Up till to now, we have carried out three experiments on the spherical hohlraum with 2 LEHs on ShenGuang(SG) laser facilities, including improvement of laser transport by using the cylindrical LEHs in the spherical hohlraums on SGIII prototype laser facility, spherical hohlraum energetics on SGIII prototype laser facility, and comparisons of laser plasma instabilities between the spherical hohlraums and the cylindrical hohlraums on SGIII laser facility.
The hohlraum is very crucial for indirect laser driven Inertial Confinement Fusion. Usually, its shape is designed as sphere, cylinder, or rugby with some kind of fixed functions, such as ellipse or parabola. Recently, a spherical hohlraum with octahedral 6 laser entrance holes (LEHs) has been presented with high flux symmetry [Lan et al., Phys. Plasmas 21, 010704 (2014); 21, 052704 (2014)]. However, there is only one shape parameter, i.e., the hohlraum to capsule radius ratio, being optimized. In this paper, we build the hohlraum with octahedral 6LEHs with a unified free-form representation, in which, by varying additional shape parameters: (1) available hohlraum shapes can be uniformly and accurately represented, (2) it can be used to understand why the spherical hohlraum has higher flux symmetry, (3) it allows us to obtain a feasible shape design field satisfying flux symmetry constraints, and (4) a synthetically optimized hohlraum can be obtained with a tradeoff of flux symmetry and other hohlraum performance. Finally, the hohlraum with octahedral 6LEHs is modeled, analyzed, and then optimized based on the unified free-form representation. The results show that a feasible shape design field with flux asymmetry no more than 1% can be obtained, and over the feasible design field, the spherical hohlraum is validated to have the highest flux symmetry, and a synthetically optimal hohlraum can be found with closing flux symmetry but larger volume between laser spots and centrally located capsule.
Indirect-drive implosions experiments were conducted on the Omega Laser Facility to test the performance of uniformly doped plastic ablators for Inertial Confinement Fusion. The first convergent ablation measurements in gas-filled rugby hohlraums are reported. Ignition relevant limb velocities in the range from 150 to 300 μm.ns-1 have been reached by varying the laser drive energy and the initial capsule aspect ratio. The measured capsule trajectory and implosion velocity are in good agreement with 2D integrated simulations and a zero-dimensional modeling of the implosions. We demonstrate experimentally the scaling law for the maximum implosion velocity predicted by the improved rocket model [Y. Saillard, Nucl. Fusion 46, 1017 (2006)] in the high-ablation regime case.
The irradiation uniformity of a cone-in-shell target directly driven by laser beams has been considered. First, a model is established to include the influence of the cone on laser beam propagation. Then, the irradiation uniformity on the target surface outside the cone during the initial imprinting phase is analyzed, and highly uniform irradiation on the target surface outside the cone is achieved by optimizing the intensity distribution within laser beams, as well as the polar direct drive displacement. As an illustrative example, direct drive irradiation uniformity of a typical cone-in-shell target is improved for Shenguang III laser facility, the illumination non-uniformity is reduced from 5.8% to 1.1%. Irradiation on the cone surface outside the target is also analyzed, and it is found that for the laser-target configuration considered in this work, a gold cone thicker than
50
μ
m
is needed to avoid shock breakout. Moreover, sensitivity to beam uncertainties (power imbalance and pointing error) is analyzed, indicating that this scheme can tolerate a certain amount of beam errors.
Progress toward ignition at the National Ignition Facility (NIF) has been focused on furthering the understanding of implosion performance. Implosion performance depends on the capsule fuel shape, on higher mode asymmetries that may cause hydrodynamic instabilities to quench ignition, on time-dependent asymmetries introduced by the hohlraum target, and on ablator performance. Significant findings in each of these four areas is reported. These investigations have led to improved in-flight capsule shape, a demonstration that a capsule robust to mix can generate high levels of neutrons (7.7 × 1014), hohlraum modifications that should ultimately provide improved beam propagation and better laser coupling, and fielding of capsules with high-density carbon (HDC) ablators. A capsule just fielded with a HDC ablator and filled with DT gas generated a preliminary record level of neutrons at 1.6 × 1015, or 5 kJ of energy. Future plans include further improvements to fuel shape and hohlraum performance, fielding robust capsules at higher laser power and energy, and tuning the HDC capsule. A capsule with a nanocrystalline diamond (HDC) ablator on a DT ice layer will be fielded at NIF later this year.
The uniformity of the compression driver is of fundamental importance for inertial confinement fusion (ICF). In this paper, the illumination uniformity on a spherical capsule during the initial imprinting phase directly driven by laser beams has been considered. We aim to explore methods to achieve high direct drive illumination uniformity on laser facilities designed for indirect drive ICF. There are many parameters that would affect the irradiation uniformity, such as Polar Direct Drive displacement quantity, capsule radius, laser spot size and intensity distribution within a laser beam. A novel approach to reduce the root mean square illumination non-uniformity based on multi-parameter optimizing approach (particle swarm optimization) is proposed, which enables us to obtain a set of optimal parameters over a large parameter space. Finally, this method is applied to improve the direct drive illumination uniformity provided by Shenguang III laser facility and the illumination non-uniformity is reduced from 5.62% to 0.23% for perfectly balanced beams. Moreover, beam errors (power imbalance and pointing error) are taken into account to provide a more practical solution and results show that this multi-parameter optimization approach is effective.
A recent low gas-fill density (0.6 mg/cc 4He) cylindrical hohlraum experiment on the National Ignition Facility has shown high laser-coupling efficiency (>96%), reduced phenomenological laser drive corrections, and improved high-density carbon capsule implosion symmetry [Jones et al., Bull. Am. Phys. Soc. 59(15), 66 (2014)]. In this Letter, an ignition design using a large rugby-shaped hohlraum [Amendt et al., Phys. Plasmas 21, 112703 (2014)] for high energetics efficiency and symmetry control with the same low gas-fill density (0.6 mg/cc 4He) is developed as a potentially robust platform for demonstrating thermonuclear burn. The companion high-density carbon capsule for this hohlraum design is driven by an adiabat-shaped [Betti et al., Phys. Plasmas 9, 2277 (2002)] 4-shock drive profile for robust high gain (>10) 1-D ignition performance and large margin to 2-D perturbation growth.
A recent publication [K. Lan et al., Phys. Plasmas 21, 010704 (2014)] proposed a spherical hohlraum with six laser entrance holes of octahedral symmetry at a specific hohlraum-to-capsule radius ratio of 5.14 for inertial fusion study, which has robust high symmetry during the capsule implosion and superiority on low backscatter without supplementary technology. This paper extends the previous one by studying the laser arrangement and constraints of octahedral hohlraum in detail. As a result, it has serious beam crossing at θ L ≤ 45 ° , and θ L = 50 ° to 60° is proposed as the optimum candidate range for the golden octahedral hohlraum, here θ L is the opening angle that the laser quad beam makes with the Laser Entrance Hole (LEH) normal direction. In addition, the design of the LEH azimuthal angle should avoid laser spot overlapping on hohlraum wall and laser beam transferring outside hohlraum from a neighbor LEH. The octahedral hohlraums are flexible and can be applicable to diverse inertial fusion drive approaches. This paper also applies the octahedral hohlraum to the recent proposed hybrid indirect-direct drive approach.
An ultra-thin layer of uranium nitrides (UN) has been coated on the inner
surface of the depleted uranium hohlraum (DUH), which has been proved by our
experiment can prevent the oxidization of Uranium (U) effectively. Comparative
experiments between the novel depleted uranium hohlraum and pure golden (Au)
hohlraum are implemented on Shenguang III prototype laser facility. Under the
laser intensity of 6*10^14 W/cm2, we observe that, the hard x-ray (> 1.8 keV)
fraction of this uranium hohlraum decreases by 61% and the peak intensity of
total x-ray flux (0.1 keV ~ 5 keV) increases by 5%. Two dimensional radiation
hydrodynamic code LARED are exploited to interpret the above observations. Our
result for the first time indicates the advantage of the UN-coated DUH in
generating the uniform x-ray field with a quasi Planckian spectrum and thus has
important implications in optimizing the ignition hohlraum design.
A direct experimental comparison of rugby-shaped and cylindrical shaped gas-filled hohlraums on the Omega laser facility demonstrates that higher coupling and minimal backscatter can be achieved in the rugby geometry, leading to significantly enhanced implosion performance. A nearly 50% increase of x-ray drive is associated with earlier bangtime and increase of neutron production. The observed drive enhancement from rugby geometry in this study is almost twice stronger than in previously published results.
The National Ignition Campaign (NIC) was a multi-institution effort established under the National Nuclear Security Administration of DOE in 2005, prior to the completion of the National Ignition Facility (NIF) in 2009. The scope of the NIC was the planning and preparation for and the execution of the first 3 yr of ignition experiments (through the end of September 2012) as well as the development, fielding, qualification, and integration of the wide range of capabilities required for ignition. Besides the operation and optimization of the use of NIF, these capabilities included over 50 optical, x-ray, and nuclear diagnostic systems, target fabrication facilities, experimental platforms, and a wide range of NIF facility infrastructure. The goal of ignition experiments on the NIF is to achieve, for the first time, ignition and thermonuclear burn in the laboratory via inertial confinement fusion and to develop a platform for ignition and high energy density applications on the NIF. The goal of the NIC was to develop and integrate all of the capabilities required for a precision ignition campaign and, if possible, to demonstrate ignition and gain by the end of FY12. The goal of achieving ignition can be divided into three main challenges. The first challenge is defining specifications for the target, laser, and diagnostics with the understanding that not all ignition physics is fully understood and not all material properties are known. The second challenge is designing experiments to systematically remove these uncertainties. The third challenge is translating these experimental results into metrics designed to determine how well the experimental implosions have performed relative to expectations and requirements and to advance those metrics toward the conditions required for ignition. This paper summarizes the approach taken to address these challenges, along with the progress achieved to date and the challenges that remain. At project completion in 2009, NIF lacked almost all the diagnostics and infrastructure required for ignition experiments. About half of the 3 yr period covered in this review was taken up by the effort required to install and performance qualify the equipment and experimental platforms needed for ignition experiments. Ignition on the NIF is a grand challenge undertaking and the results presented here represent a snapshot in time on the path toward that goal. The path forward presented at the end of this review summarizes plans for the Ignition Campaign on the NIF, which were adopted at the end of 2012, as well as some of the key results obtained since the end of the NIC.
In preparation of the first ignition attempts on the Laser
Mégajoule (LMJ), an experimental program is being pursued on
OMEGA to investigate LMJ-relevant hohlraums. First, radiation
temperature levels close to 300 eV were recently achieved in
reduced-scale hohlraums with modest backscatter losses. Regarding the
baseline target design for fusion experiments on LMJ, an extensive
experimental database has also been collected for scaled implosions
experiments in both empty and gas-filled rugby-shaped hohlraums. We
acquired a full picture of hohlraum energetics and implosion dynamics.
Not only did the rugby hohlraums show significantly higher x-ray drive
energy over the cylindrical hohlraums, but symmetry control by power
balance was demonstrated, as well as high-performance D2
implosions enabling the use of a complete suite of neutrons diagnostics.
Charged particle diagnostics provide complementary insights into the
physics of these x-ray driven implosions. An overview of these results
demonstrates our ability to control the key parameters driving the
implosion, lending more confidence in extrapolations to ignition-scale
targets.
Some of our recent studies on hohlraum physics are presented, mainly
including simulation study on hohlraum physics experiments on SGIII
prototype, the design of Au + U + Au sandwich hohlraum for ignition
target, and an initial design of elliptical hohlraum and pertinent drive
laser power in order to generate an ignition radiation profile.
CEA implemented an absolutely calibrated broadband soft X-ray spectrometer called DMX on the Omega laser facility at the Laboratory for Laser Energetics (LLE) in 1999 to measure radiant power and spectral distribution of the radiation of the Au plasma. The DMX spectrometer is composed of 20 channels covering the spectral range from 50 eV to 20 keV. The channels for energies below 1.5 keV combine a mirror and a filter with a coaxial photo-emissive detector. For the channels above 5 keV the photoemissive detector is replaced by a conductive detector. The intermediate energy channels (1.5 keV < photon energy < 5 keV) use only a filter and a coaxial detector. A further improvement of DMX consists in flat-response X-ray channels for a precise absolute measurement of the photon flux in the photon energy range from 0.1 keV to 6 keV. Such channels are equipped with a filter, a Multilayer Mirror (MLM), and a coaxial detector. We present as an example the development of channel for the gold M emission lines in the photon energy range from 2 keV to 4 keV which has been successfully used on the OMEGA laser facility. The results of the radiant power measurements with the new MLM channel and with the usual channel composed of a thin titanium filter and a coaxial detector (without mirror) are compared. All elements of the channel have been calibrated in the laboratory of the Physikalisch-Technische Bundesanstalt, Germany's National Metrology Institute, at the synchrotron radiation facility BESSY II in Berlin using dedicated well established and validated methods.
An initial study and design on ignition elliptical hohlraum (ellipraum) is given by using the expended plasma-filling model with criterions. As a result, in an ellipraum with a smaller ratio of major-to-minor axis (a/b), the radius ratio of ellipraum-to-capsule (b/RC) should be larger (hence more sphere-like) to meet the criterions of plasma-filling and laser deposition, meanwhile the required laser energy and peak power are lower and the coupling between different modes is weaker. To produce a 300 eV radiation pulse to ignite a capsule of 1 mm radius, an ellipraum of a/b = 1.6 and b/Rc = 2.8 is superior to a cylinraum with a length-to-diameter ratio of 1.81 and a cylinraum-to-capsule radius ratio of 2.54 in saving more than 10% laser energy and reducing 50% coupling between different modes.
Point design targets have been specified for the initial ignition campaign on the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets contain D-T fusion fuel in an ablator of either CH with Ge doping, or Be with Cu. These shells are imploded in a U or Au hohlraum with a peak radiation temperature set between 270 and 300 eV. Considerations determining the point design include laser-plasma interactions, hydrodynamic instabilities, laser operations, and target fabrication. Simulations were used to evaluate choices, and to define requirements and specifications. Simulation techniques and their experimental validation are summarized. Simulations were used to estimate the sensitivity of target performance to uncertainties and variations in experimental conditions. A formalism is described that evaluates margin for ignition, summarized in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shot-to-shot variability in ITF are evaluated, and sensitivity of the margin to characteristics of the experiment. The formalism is used to estimate probability of ignition. The ignition experiment will be preceded with an experimental campaign that determines features of the design that cannot be defined with simulations alone. The requirements for this campaign are summarized. Requirements are summarized for the laser and target fabrication.
A suite of experimental designs for the Omega laser facility [
Boehly et al., Opt. Commun. 133, 495 (1997)
] using rugby and cylindrical hohlraums is proposed to confirm the energetics benefits of rugby-shaped hohlraums over cylinders under optimal implosion symmetry conditions. Postprocessed Dante x-ray drive measurements predict a 12–17 eV (23%–36%) peak hohlraum temperature (x-ray flux) enhancement for a 1 ns flattop laser drive history. Simulated core self-emission x-ray histories also show earlier implosion times by 200–400 ps, depending on the hohlraum case-to-capsule ratio and laser-entrance-hole size. Capsules filled with 10 or 50 atm of deuterium (DD) are predicted to give in excess of 1010 neutrons in two-dimensional hohlraum simulations in the absence of mix, enabling DD burn history measurements for the first time in indirect-drive on Omega. Capsule designs with 50 atm of D3He are also proposed to make use of proton slowing for independently verifying the drive benefits of rugby hohlraums. Scale-5/4 hohlraum designs are also introduced to provide further margin to potential laser-plasma-induced backscatter and hot-electron production.
Inertial confinement fusion targets must be carefully designed to ignite their central hot spots and burn. Changes in the optimal implosion could reduce the fusion energy or even prevent ignition. Since there are unavoidable uncertainties due to technological defects and not perfect reproducibility from shot to shot, the fusion energy will remain uncertain. The degree with which a target can tolerate larger specifications than specified, and the probability with which a particular yield is exceeded, are possible measures of the robustness of that design. This robustness must be assessed in a very high-dimensional parameter space whose variables include every characteristics of the given target and of the associated laser pulse shape, using high-fidelity simulations. Therefore, these studies would remain computationally very intensive. In this paper we propose an approach which consist first of constructing an accurate metamodel of the yield on the whole parameter space with a reasonable data set of simulations. Then the robustness is very quickly assessed for any set of specifications with this surrogate. The yield is approximated by a neural network, and an iterative method adds new points in the data set by means of D-optimal experimental designs. The robustness study of the baseline Laser Mégajoule target against one-dimensional defects illustrates this approach. A set of 2000 simulations is sufficient to metamodel the fusion energy on a large 22-dimensional parameter space around the nominal point. Furthermore, a metamodel of the robustness margin against all specifications has been obtained, providing guidance for target fabrication research and development.
Results of ablative Rayleigh–Taylor instability growth experiments performed in indirect drive on the OMEGA laser facility [
T. R. Boehly, D. L. Brown, S. Craxton et al., Opt. Commun. 133, 495 (1997)
] are reported. These experiments aim at benchmarking hydrocodes simulations and ablator instabilities growth in conditions relevant to ignition in the framework of the Laser MégaJoule [
C. Cavailler, Plasma Phys. Controlled Fusion 47, 389 (2005)
]. The modulated samples under study were made of germanium-doped plastic (CHGe), which is the nominal ablator for future ignition experiments. The incident x-ray drive was provided using rugby-shaped hohlraums [
M. Vandenboomgaerde, J. Bastian, A. Casner et al., Phys. Rev. Lett. 99, 065004 (2007)
] and was characterized by means of absolute time-resolved soft x-ray power measurements through a dedicated diagnostic hole, shock breakout data and one-dimensional and two-dimensional (2D) side-on radiographies. All these independent x-ray drive diagnostics lead to an actual on-foil flux that is about 50% smaller than laser-entrance-hole measurements. The experimentally inferred flux is used to simulate experimental optical depths obtained from face-on radiographies for an extensive set of initial conditions: front-side single-mode (wavelength λ = 35, 50, and 70 μm) and two-mode perturbations (wavelength λ = 35 and 70 μm, in phase or in opposite phase). Three-dimensional pattern growth is also compared with the 2D case. Finally the case of the feedthrough mechanism is addressed with rear-side modulated foils.
Rugby-shaped hohlraums have been proposed as a method for x-ray drive enhancement for indirectly driven capsule implosions. This concept has recently been tested in a series of shots on the OMEGA laser facility [
T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)
]. In this paper, experimental results are presented comparing the performance of D2-filled capsules between standard cylindrical Au hohlraums and rugby-shaped hohlraums. The rugby hohlraums demonstrated 18% more x-ray drive energy as compared with the cylinders, and the high-performance design of these implosions (both cylinder and rugby) also provided ≈20× more deuterium (DD) neutrons than any previous indirectly driven campaign on OMEGA and ≈3× more than ever achieved on NOVA [
E. M. Campbell, Laser Part. Beams 9, 209 (1991)
] implosions driven with nearly twice the laser energy. This increase in performance enables, for the first time, a measurement of the neutron burn history and imaging of the neutron core shapes in an indirectly driven implosion. Previous DD neutron yields had been too low to register this key measurement of capsule performance and the effects of dynamic mix. A wealth of additional data on the fuel areal density from the suite of charged particle diagnostics was obtained on a subset of the shots that used D 3He rather than D2 fuel. Comparisons of the experimental results with numerical simulations are shown to be in very good agreement. The design techniques employed in this campaign, e.g., smaller laser entrance holes and hohlraum case-to-capsule ratios, provide added confidence in the pursuit of ignition on the National Ignition Facility [
J. D. Lindl, P. Amendt, R. L. Berger et al., Phys. Plasmas 11, 339 (2004)
].
Numerical simulation of laser driven Inertial Confinement Fusion (ICF) related experiments require the use of large multidimensional hydro codes. Though these codes include detailed physics for numerous phenomena, they deal poorly with electron conduction, which is the leading energy transport mechanism of these systems. Electron heat flow is known, since the work of Luciani, Mora, and Virmont (LMV) [Phys. Rev. Lett. 51, 1664 (1983)], to be a nonlocal process, which the local Spitzer–Harm theory, even flux limited, is unable to account for. The present work aims at extending the original formula of LMV to two or three dimensions of space. This multidimensional extension leads to an equivalent transport equation suitable for easy implementation in a two-dimensional radiation-hydrodynamic code. Simulations are presented and compared to Fokker–Planck simulations in one and two dimensions of space. © 2000 American Institute of Physics.
Nearly 10 years of Nova [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] experiments and analysis have lead to a relatively detailed quantitative and qualitative understanding of radiation drive in laser‐heated hohlraums. Our most successful quantitative modeling tool is two‐dimensional (2‐D) LASNEX numerical simulations [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Analysis of the simulations provides us with insight into the physics of hohlraum drive. In particular we find hohlraum radiation conversion efficiency becomes quite high with longer pulses as the accumulated, high‐Z blow‐off plasma begins to radiate. Extensive Nova experiments corroborate our quantitative and qualitative understanding. © 1996 American Institute of Physics.
The goal of demonstrating ignition on the National Ignition Facility [
J. D. Lindl et al., Phys. Plasmas 11, 339 (2003)
] has motivated a revisit of double-shell (DS) targets as a complementary path to the cryogenic baseline approach. Expected benefits of DS ignition targets include noncryogenic deuterium-tritium (DT) fuel preparation, minimal hohlraum-plasma-mediated laser backscatter, low threshold-ignition temperatures ( ≈ 4 keV) for relaxed hohlraum x-ray flux asymmetry tolerances, and minimal (two-) shock timing requirements. On the other hand, DS ignition presents several formidable challenges, encompassing room-temperature containment of high-pressure DT ( ≈ 790 atm) in the inner shell, strict concentricity requirements on the two shells (<3 μm), development of nanoporous (<100 nm cell size) low-density (<100 mg/cc) metallic foams for structural support of the inner shell and hydrodynamic instability mitigation, and effective control of hydrodynamic instabilities on the high-Atwood-number interface between the DT fuel and the high-Z inner shell. Recent progress in DS ignition designs and required materials science advances at the nanoscale are described herein. Two new ignition designs that use rugby-shaped vacuum hohlraums are presented that utilize either 1 or 2 MJ of laser energy at 3ω. The capability of the National Ignition Facility to generate the requested 2 MJ reverse-ramp pulse shape for DS ignition is expected to be comparable to the planned high-contrast ( ≈ 100) pulse shape at 1.8 MJ for the baseline cryogenic target. Nanocrystalline, high-strength, Au-Cu alloy inner shells are under development using electrochemical deposition over a glass mandrel, exhibiting tensile strengths well in excess of 790 atm. Novel, low-density (85 mg/cc) copper foams have recently been demonstrated using 10 mg/cc SiO2 nanoporous aerogels with suspended Cu particles. A prototype demonstration of an ignition DS is planned for 2008, incorporating the needed novel nanomaterials science developments and the required fabrication tolerances for a realistic ignition attempt after 2010.
Studies on simulation and design of ignition targets for the National Ignition Facility (NIF) are described. Recent effort has emphasized the systematic exploration of the parameter space of possible ignition targets, providing comparisons as specific as possible between the various targets. This study aims at providing guidance for target fabrication R&D, and for other elements of the ignition program. Targets are being considered that span 250–350 eV drive temperatures, capsule energies from 150 to 600 kJ, cocktail and gold hohlraum spectra, and three ablator materials (Be[Cu], CH[Ge] and polyimide). Capsules with graded doped beryllium ablators are found to be very stable with respect to short-wavelength Rayleigh–Taylor growth. Sensitivity to ablator roughness, ice roughness and asymmetry is being explored, as it depends on ablator material, drive temperature and absorbed energy. Three-dimensional simulations are being used to ensure adequate radiation symmetry in three dimensions (3D), and to ensure that coupling of 3D asymmetry and 3D Rayleigh–Taylor does not adversely affect planned performance. Integrated 3D hohlraum simulations indicate that 3D features in the laser illumination pattern affect the hohlraums' performance, and the hohlraum has been redesigned to accommodate these effects.
Targets intended to produce ignition on NIF are being simulated and the simulations are used to set specifications for target fabrication. Recent design work has focused on designs that assume only 1.0 MJ of laser energy instead of the previous 1.6 MJ. To perform with less laser energy, the hohlraum has been redesigned to be more efficient than previously, and the capsules are slightly smaller. The main-line hohlraum design now has a SiO2 foam fill, a wall of U-Dy-Au, and shields mounted between the capsule and the laser entrance holes. Two capsule designs are being considered. One has a graded doped Be(Cu) ablator, and the other graded doped CH(Ge). Both can perform acceptably with recently demonstrated ice layer quality, and with recently demonstrated outer surface roughness. Smoothness of the internal interfaces may be an issue for the Be(Cu) design, and it may be necessary either to polish partially coated shells or to improve process control so that the internal layers are smoother. Complete tables of specifications are being prepared for both targets, to be completed this fiscal year. All the specifications are being rolled together into an error budget indicating adequate margin for ignition with the new designs.
We recall the main features of the LMJ. By using a simple but global model we determined different shells able to give a thermonuclear yield larger than 15 MJ; this model delimited an operating domain for the laser with a 25% margin to take into account the poorly understood phenomena.The different issues are related to the physics of the shell and to the physics of the hohlraum: optimisation of the shell implosion; laser–plasma interaction in the hohlraum; irradiation uniformity given by the hohlraum (2D simulations); implosion with non-uniformities; robustness against the experimental uncertainties; hydrodynamic instabilities during implosion.
Time-resolved drive measurements with thin-walled hohlraum targets on Omega [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] are presented and compared with two-dimensional hydrodynamical simulations. For the first time, radiation fluxes are measured through the laser entrance hole instead of through a diagnostic side hole. We find improved agreement between time dependent experiments and simulations using this new technique. In addition, the drive history obtained in this manner correlates well with the drive onto the capsule at target center.
Germanium-doped CH (CHGe) is one nominal ablator for LMJ target design. To investigate its properties we performed indirect drive planar Rayleigh-Taylor experiments on the OMEGA laser facility [1]. On each shot foil motion and modulations growth were simultaneously measured by side-on and face-on radiography, while drive was assessed by measuring radiation escaping through the hohlraum laser-entrance-hole (LEH). This complete set of data allows a more stringent comparison between the hydrocode simulations and the experimental results. We compare CHGe perturbations growth with those acquired on CHBr in the same experimental configuration. These preliminary results are the first step toward a test-bed validation of CH(Ge) as an ablator on OMEGA and further on LiL [2].
In high flux (1013–15 W/cm2) laser-plasma interaction, a large part of the incoming laser energy is radiated into soft x-rays. To determine both the shape of this spectrum and the conversion efficiency, we have designed and tested an absolutely calibrated broadband soft x-ray spectrometer with a high temporal resolution (100 ps). The detector in this spectrometer is a new coaxial x-ray diode coupled with a fast single shot oscilloscope (French IN7100 with 7 GHz frequency response cutoff). Both absolute calibrations (x-ray response of diodes) and relative calibrations (filters and mirrors) have used the French synchrotron beam lines at Laboratoire pour l’utilisation du Rayonnement Electromagnétique (LURE) in Orsay. The initial version of this instrument was first successfully implemented on laser plasmas experiments at the Phébus facility in France and an improved version is now operating at the Omega laser facility in Rochester, New York. The emitted x-ray spectrum is absolutely measured in 18 broad bands from 50 eV up to 20 keV. The softer bands (<1.5 keV) combine mirror and filter responses coupled with the coaxial diode response to improve hard x-ray rejection. Intermediate energy channels (1.5 keV<hν<5 keV) used only a filter and coaxial diode. For the hardest channels (>5 keV) we replace the x-ray diode (not sufficiently sensitive) with a photoconductive detector (neutron-damaged GaAs). An equivalent instrument will be designed for the future National Ignition Facility (NIF) in the United States and the Laser Mégajoule (LMJ) in France. © 2001 American Institute of Physics.
The objectives of the US indirect-drive target physics program are discussed. The main objective of the program is to demonstrate and model hohlraum characteristics, as well as capsule performance in targets that are scaled in key physics variables from National Ignition Facility (NIF) targets. The program is divided into two parts called Hohlraum and Laser-Plasma Physics (HLP) program and the Hydrodynamically Equivalent Physics (HEP) program. The HLP program addresses x-ray generation and transport, laser-plasma coupling and the development of energy-efficient hohlraums. The HEP experiments address the issues of hydrodynamic instability and mix, as well as the effects of flux asymmetry on capsules, scaled as closely as possible to ignition capsules.
Coupling efficiency, the ratio of the capsule absorbed energy to the driver energy, is a key parameter in ignition target designs. The hohlraum originally proposed for the National Ignition Facility (NIF) [
G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, S228 (2004)
] coupled ∼ 11% of the absorbed laser energy to the capsule as x rays. Described here is a second generation of the hohlraum target which has a higher coupling efficiency, ∼ 16%. Because the ignition capsule’s ability to withstand three-dimensional effects increases rapidly with absorbed energy, the additional energy can significantly increase the likelihood of ignition. The new target includes laser entrance hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive inertial confinement fusion. The LEH shields are high Z disks placed inside the hohlraum on the symmetry axis to block the capsule’s view of the relatively cold LEHs. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: (1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the pole; (2) because the capsule pole does not have a view of the cold LEHs, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple 140 kJ of x rays to a capsule out of 865 kJ of absorbed laser energy and produce ∼ 10 MJ of yield. In the current designs, which continue to be optimized, the addition of the LEH shields saves ∼ 95 kJ of energy (about 10%) over hohlraums without LEH shields.
The original ignition “point designs” (circa 1992) for the National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] were made energetically conservative to provide margin for uncertainties in laser absorption, x-ray conversion efficiency and hohlraum-capsule coupling. Since that time, extensive experiments on Nova [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] and Omega [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] and their related analysis indicate that NIF coupling efficiency may be almost “as good as we could hope for.” Given close agreement between experiment and theory/modeling, one can credibly explore target enhancements which couple more of NIFs energy to an ignition capsule. These include using optimized mixtures of materials to reduce x-ray wall losses, slightly reduced laser entrance holes, and laser operation strategies which increase the amount of energy one can extract from NIF. It is found that 3–4× increases in absorbed capsule energy appear possible, providing a potentially more robust target and ∼10× increase in capsule yield.
Tetrahedral hohlraums, by which are understood spherical hohlraums with four laser entrance holes (LEH’s) placed at or near the vertices of a tetrahedron, are proposed for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)] and the upgraded OMEGA laser [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)]. All but four of the 48 NIF beams can irradiate a tetrahedral hohlraum, assuming that 72 beam ports are provided to accommodate direct drive. On OMEGA, the target chamber provides an exact tetrahedral symmetry, permitting the irradiation of tetrahedral hohlraums with all 60 beams. Hohlraum designs are optimized using a new three‐dimensional view‐factor program called Buttercup, which traces all beam paths through the hohlraum and calculates the radiation flux on the capsule for different values of the albedo. Good irradiation uniformity (∼2% rms) can be obtained on the capsule at all times during the implosion, even with identical beam temporal histories, in contrast to the case of cylindrical hohlraums where ‘‘beam phasing’’ is needed. © 1996 American Institute of Physics.
The system of differential equations for the non-ablated mass, the
average implosion velocity, and the ablation radius of an indirectly
driven capsule in acceleration phase, has been obtained from
conservation principles of hydrodynamics. Two phases are distinguished
during acceleration, according to the uniformity of the velocity in the
non-ablated shell. The results of the integration of this system are
well compared with numerical simulation of optimized capsules. Assuming
that the ablation pressure depends only on the Hohlraum temperature,
the relations between the non-ablated mass, the implosion velocity, and
the ablation radius are obtained for optimized temperature shape. These
relations provide the maximum implosion velocity and the remaining
non-ablated mass in terms of the initial capsule and the maximum
temperature (or the initial capsule mass in terms of the remaining
non-ablated mass) useful to determine the required ablator thickness
for optimized capsules. These results are also compared with numerical
simulations of different capsules.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, when completed in 2008, will contain a 192-beam, 1.8 MJ, 500 TW, ultraviolet laser system together with a 10 m diameter target chamber and room for 100 diagnostics. NIF is housed in a 26 000 m2 environmentally controlled building and is the world's largest and most energetic laser experimental system. NIF provides a scientific centre for the study of inertial confinement fusion and the physics of matter at extreme energy densities and pressures. NIF's energetic laser beams will compress fusion targets to conditions required for thermonuclear burn, liberating more energy than required to initiate the fusion reactions. Other NIF experiments will study physical processes at temperatures and pressures approaching 108 K and 1011 bar, respectively, conditions that exist naturally only in the interior of stars and planets. NIF is currently configured with four laser beams activated in late 2002. These beams are being regularly used for laser performance and physics experiments, and to date nearly 250 system shots have been conducted. NIF's laser beams have generated 106 kJ in 23 ns pulses of infrared light and over 16 kJ in 3.5 ns pulses at the third harmonic (351 nm). A number of target experimental systems are being commissioned in support of experimental campaigns. This paper provides a detailed look at the NIF laser systems, laser and optical performance, and results from laser commissioning shots. We also discuss NIF's high-energy density and inertial fusion experimental capabilities, the first experiments on NIF, and plans for future capabilities of this unique facility.
For several years we have been exploring the possibility of using green (2ω) light for indirect drive ignition on National Ignition Facility (NIF). This paper is a comprehensive review of our progress in this investigation and was the subject of a Teller lecture when one of the authors (LJS) was honoured with the Edward Teller Medal at the IFSA03 conference on September 12th, 2003, at Monterey, CA. While much of the work presented here has been previously published (Suter L.J. et al 2004 Phys. Plasmas 11 2738) and is included for completeness of the review, this paper also includes new research examining the possibility of higher temperature (300 eV) ignition hohlraums driven by green light (section 7).
We have made absolute measurements of x‐ray spectra from 0.1–1.5 keV produced by plasmas from targets irradiated by the Lawrence Livermore National Laboratory Nova laser. These measurements were made using a 15‐channel K‐ and L‐edge filtered x‐ray diode system. Valid interpretation of the results from this type of diagnostic requires some care in eliminating the effect of channel response at photon energies higher than the absorption edge. Significant errors can occur if this is disregarded. We will discuss the techniques used and the magnitude of the effects observed. Integrated x‐ray energy in the 1.5–3‐keV region is inferred from the results.
OMEGA is a 60-terawatt, 60-beam, frequency-tripled Nd:glass laser system designed to perform precision direct-drive inertial-confinement-fusion (ICF) experiments. The upgrade to the system, completed in April 1995, met or surpassed all technical requirements. The acceptance tests demonstrated exceptional performance throughout the system: high driver stability (< 2% variations), precise control of the beam profiles and amplifier gains, 75% frequency-conversion efficiency, beam energy balance less than 8% and stable on-target irradiation of up to 37 kJ UV. We present these results and show that the system performance is well modeled by our propagation and frequency-conversion codes.
Stimulated Brillouin scattering (SBS) has been measured from hohlraums with plasma conditions similar to those predicted for high gain targets. The plasmas differ from the more familiar exploding foil or solid targets in being hot (3 keV), high electron density (1${0}^{21}$ c${\mathrm{m}}^{\char21{}3}$), stationary, confined within a gold cylinder, and uniform over greater than 2 mm. Peak SBS backscatter is 3% in these hohlraums for an interaction beam with intensities of $\left(1$-${}4\right)\ifmmode\times\else\texttimes\fi{}{10}^{15}$ W/c${\mathrm{m}}^{2}$, laser wavelength equal to 0.351 $\mu${}m, $f/4$ or $f/8$ focusing optics, and a variety of beam smoothing implementations.
Solutions to the radiation diffusion equation predict the absorbed energy ("wall loss") within an inertial confinement fusion (ICF) hohlraum. Comparing supersonic versus subsonic solutions suggests that a high metallic foam as hohlraum wall material will reduce hydrodynamic losses, and hence, net absorbed energy by . We derive an analytic expression for the optimal density (for any given drive temperature and pulse-length) that will achieve this reduction factor and which agrees well with numerical simulations. This approach can increase the coupling efficiency of indirectly driven ICF capsules.