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ABSTRACT: Direct-drive–ignition designs with plastic CH ablators create plasmas of long density scale lengths
(Ln�500 lm) at the quarter-critical density (Nqc) region of the driving laser. The twoplasmon–
decay (TPD) instability can exceed its threshold in such long-scale-length plasmas
(LSPs). To investigate the scaling of TPD-induced hot electrons to laser intensity and plasma
conditions, a series of planar experiments have been conducted at the Omega Laser Facility with
2-ns square pulses at the maximum laser energies available on OMEGA and OMEGA EP.
Radiation–hydrodynamic simulations have been performed for these LSP experiments using the
two-dimensional hydrocode DRACO. The simulated hydrodynamic evolution of such long-scalelength
plasmas has been validated with the time-resolved full-aperture backscattering and
Thomson-scattering measurements. DRACO simulations for CH ablator indicate that (1) ignitionrelevant
long-scale-length plasmas of Ln approaching �400 lm have been created; (2) the density
scale length at Nqc scales as LnðlmÞ ’ ðRDPP � I1=4=2Þ; and (3) the electron temperature Te at Nqc
scales as TeðkeVÞ ’ 0:95 �
ffiffi
I
p
; with the incident intensity (I) measured in 1014 W/cm2 for plasmas
created on both OMEGA and OMEGA EP configurations with different-sized (RDPP) distributed
phase plates. These intensity scalings are in good agreement with the self-similar model
predictions. The measured conversion fraction of laser energy into hot electrons fhot is found to
have a similar behavior for both configurations: a rapid growth [fhot ’ fc � ðGc=4Þ6 for Gc<4]
followed by a saturation of the form, fhot ’ fc � ðGc=4Þ1:2 for Gc�4, with the common wave gain
is defined as Gc ¼ 3 � 10
�2 � IqcLnk0=Te; where the laser intensity contributing to common-wave
gain Iqc, Ln, Te at Nqc, and the laser wavelength k0 are, respectively, measured in [1014 W/cm2],
[lm], [keV], and [lm]. The saturation level fc is observed to be fc^10–2 at around Gc^4. The
hot-electron temperature scales roughly linear with Gc. Furthermore, to mitigate TPD instability in
long-scale-length plasmas, different ablator materials such as saran and aluminum have been
investigated on OMEGA EP. Hot-electron generation has been reduced by a factor of 3–10 for
saran and aluminum plasmas, compared to the CH case at the same incident laser intensity. DRACO
simulations suggest that saran might be a better ablator for direct-drive–ignition designs as it
balances TPD mitigation with an acceptable hydro-efficiency. VC
Physics of Plasmas 03/2013; 20:032704. · 2.15 Impact Factor
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P B Radha,
F J Marshall,
J A Marozas,
A Shvydky,
I Gabalski,
T R Boehly,
T J B Collins,
R S Craxton,
D H Edgell,
R Epstein,
J A Frenje, D H Froula,
V N Goncharov,
M Hohenberger,
R L Mccrory,
P W Mckenty,
D D Meyerhofer,
R D Petrasso,
T C Sangster,
S Skupsky
[show abstract]
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ABSTRACT: Polar-drive (PD) experiments on the OMEGA [Boehly et al., Opt. Commun. 133, 495 (1997)] laser are described. Continuous pulse shapes, where a low-power foot is followed by a rise to the main pulse, and triple-picket pulse shapes, where three pickets precede the main pulse, are used to irradiate warm plastic shell capsules. Both of these pulse shapes set the target on a low, ignition-relevant adiabat of $3.5. The areal density is modeled very well in these implosions indicating that shock timing is well modeled in PD geometry. It is shown that the symmetry can be predictably varied by changing the beam pointings. Symmetry is also well reproduced across the two pulse shapes. Limitations of OMEGA experiments are discussed. Preliminary designs for PD implosion experiments on the NIF, with the goal of addressing ignition-relevant issues for PD, including symmetry are presented. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4803083]
Physics of Plasmas 02/2013; 20. · 2.15 Impact Factor
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ABSTRACT: Time-resolved measurements of electron and ion temperatures using Thomson scattering have been combined with proton radiography data for comprehensive characterization of individual laser-produced plasma bubbles or the interaction of bubble pairs, where reconnection of azimuthal magnetic fields occurs. Measurements of ion and electron temperatures agree with lasnex simulations of single plasma bubbles, which include the physics of magnetic fields. There is negligible difference in temperatures between a single plasma bubble and the interaction region of bubble pairs, although the ion temperature may be slightly higher due to the collision of expanding plasmas. These results are consistent with reconnection in a β∼8 plasma, where the release of magnetic energy (<5% of the electron thermal energy) does not appreciably affect the hydrodynamics.
Physical Review E 11/2012; 86(5-2):056407. · 2.26 Impact Factor
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[show abstract]
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ABSTRACT: The energy in hot electrons produced by the two plasmon decay instability, in planar targets, is measured to be the same when driven by one or two laser beams and significantly reduced with four for a constant overlapped intensity on the OMEGA EP. This is caused by multiple beams sharing the same common electron-plasma wave. A model, consistent with the experimental results, predicts that multiple laser beams can only drive a resonant common two plasmon decay electron-plasma wave in the region of wave numbers bisecting the beams. In this region, the gain is proportional to the overlapped laser beam intensity.
Physical Review Letters 10/2012; 109(15):155007. · 7.37 Impact Factor
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ABSTRACT: A reflective optical transport system has been designed for the OMEGA Thomson-scattering diagnostic. A Schwarzschild objective that uses two concentric spherical mirrors coupled to a Pfund objective provides diffraction-limited imaging across all reflected wavelengths. This enables the operator to perform Thomson-scattering measurements of ultraviolet (0.263 μm) light scattered from electron plasma waves.
The Review of scientific instruments 10/2012; 83(10):10E349. · 1.52 Impact Factor
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ABSTRACT: A technique to measure the shell trajectory in direct-drive inertial confinement fusion implosions is presented. The x-ray self emission of the target is measured with an x-ray framing camera. Optimized filtering limits the x-ray emission from the corona plasma, isolating a sharp intensity gradient very near the ablation surface. This enables one to measure the radius of the imploding shell with an accuracy better than 1 μm and to determine a 200-ps average velocity to better than 2%.
The Review of scientific instruments 10/2012; 83(10):10E530. · 1.52 Impact Factor
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D H Froula,
B Yaakobi,
S X Hu,
P-Y Chang,
R S Craxton,
D H Edgell,
R Follett,
D T Michel,
J F Myatt,
W Seka,
R W Short,
A Solodov,
C Stoeckl
[show abstract]
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ABSTRACT: Measurements of the hot-electron generation by the two-plasmon-decay instability are made in plasmas relevant to direct-drive inertial confinement fusion. Density-scale lengths of 400 μm at n(cr)/4 in planar CH targets allowed the two-plasmon-decay instability to be driven to saturation for vacuum intensities above ~3.5×10(14) W cm(-2). In the saturated regime, ~1% of the laser energy is converted to hot electrons. The hot-electron temperature is measured to increase rapidly from 25 to 90 keV as the laser beam intensity is increased from 2 to 7×10(14) W cm(-2). This increase in the hot-electron temperature is compared with predictions from nonlinear Zakharov models.
Physical Review Letters 04/2012; 108(16):165003. · 7.37 Impact Factor
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ABSTRACT: X-ray Thomson scattering has enabled us to measure the temperature of a shocked layer, produced in the laboratory, that is relevant to shocks emerging from supernovas. High energy lasers are used to create a shock in argon gas which is probed by x-ray scattering. The scattered, inelastic Compton feature allows inference of the electron temperature. It is measured to be 34 eV in the radiative precursor and ∼60 eV near the shock. Comparison of energy fluxes implied by the data demonstrates that the shock wave is strongly radiative.
Physical Review Letters 04/2012; 108(14):145001. · 7.37 Impact Factor
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D H Froula,
I V Igumenshchev,
D T Michel,
D H Edgell,
R Follett,
V Yu Glebov,
V N Goncharov,
J Kwiatkowski,
F J Marshall,
P B Radha,
W Seka,
C Sorce,
S Stagnitto,
C Stoeckl,
T C Sangster
[show abstract]
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ABSTRACT: A series of experiments to determine the optimum laser-beam radius by balancing the reduction of cross-beam energy transfer (CBET) with increased illumination nonuniformities shows that the hydrodynamic efficiency is increased by ∼35%, which leads to a factor of 2.6 increase in the neutron yield when the laser-spot size is reduced by 20%. Over this range, the absorption is measured to increase by 15%, resulting in a 17% increase in the implosion velocity and a 10% earlier bang time. When reducing the ratio of laser-spot size to a target radius below 0.8, the rms amplitudes of the nonuniformities imposed by the smaller laser spots are measured at a convergence ratio of 2.5 to exceed 8 μm and the neutron yield saturates despite increasing absorbed energy, implosion velocity, and decreasing bang time. The results agree well with hydrodynamic simulations that include both nonlocal and CBET models.
Physical Review Letters 03/2012; 108(12):125003. · 7.37 Impact Factor
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B Yaakobi,
P -Y Chang,
A Solodov,
C Stoeckl,
D H Edgell,
R S Craxton,
S X Hu,
J F Myatt,
F J Marshall,
W Seka, D H Froula
Physics of Plasmas 01/2012; 19(1). · 2.15 Impact Factor
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[show abstract]
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ABSTRACT: Magnetic fields generated by the Rayleigh-Taylor instability were measured in laser-accelerated planar foils using ultrafast proton radiography. Thin plastic foils were irradiated with similar to 4-kJ, 2.5-ns laser pulses focused to an intensity of similar to 10(14) W/cm(2) on the OMEGA EP Laser System. Target modulations were seeded by laser nonuniformities and amplified during target acceleration by the Rayleigh-Taylor instability. The experimental data show the hydrodynamic evolution of the target and MG-level magnetic fields generated in the broken foil. The experimental data are in good agreement with predictions from 2-D magnetohydrodynamic simulations.
Physical Review Letters 01/2012; 109(11):5. · 7.37 Impact Factor
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D H Froula,
D T Michel,
I V Igumenshchev,
S X Hu,
B Yaakobi,
J F Myatt,
D H Edgell,
R Follett,
V Y Glebov,
V N Goncharov,
T J Kessler,
A V Maximov,
P B Radha,
T C Sangster,
W Seka,
R W Short,
A A Solodov,
C Sorce,
C Stoeckl
[show abstract]
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ABSTRACT: Direct-drive ignition is most susceptible to multiple-beam laser-plasma instabilities, as the single-beam intensities are low (I-s similar to 10(14)W cm(-2)) and the electron temperature in the underdense plasma is high (T-e similar or equal to 3.5 keV). Cross-beam energy transfer is driven by multiple laser beams and can significantly reduce the hydrodynamic efficiency in direct-drive experiments on OMEGA (Boehly et al 1997 Opt. Commun. 133 495). Reducing the radii of the laser beams significantly increases the hydrodynamic efficiency at the cost of an increase in the low-mode modulations. Initial 2D hydrodynamic simulations indicate that zooming, transitioning the laser-beam radius prior to the main drive, does not increase low-mode nonuniformities. The combination of zooming and dynamic bandwidth reduction will provide a 30% effective increase in the drive energy on OMEGA direct-drive implosions. It was shown that two-plasmon decay (TPD) can be driven by multiple laser beams and both planar and spherical experiments were performed to study the hot electrons generated by TPD. The fraction of laser energy converted to hot electrons scales with the hot-electron temperature for all geometries and over a wide range of intensities. At ignition-relevant intensities, the fraction of laser energy converted to hot electrons is measured to decrease by an order of magnitude when the ablator material is changed from carbon-hydrogen to aluminum. The TPD results are compared with a multiple-beam linear theory and a nonlinear Zakharov model.
Plasma Physics and Controlled Fusion 01/2012; 54(12). · 2.42 Impact Factor
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ABSTRACT: A new Thomson scattering diagnostic has been implemented on the Omega Laser facility at the Laboratory for Laser Energetics, University of Rochester [J.M. Soures, et al., Laser and Particle Beams 11, 317 (1993)] to measure the electron feature in the ultraviolet wavelength range 200 nm - 263 nm. A pair of imaging spectrometers and streak cameras collect light scattered from electron plasma fluctuations and ion-acoustic fluctuations simultaneously. These spectra allow an accurate measure of the electron temperature, density, average charge state and plasma flow velocity in a high-density laser plasma regime perviously inaccessible.
Journal of Instrumentation 08/2011; 6(08):P08004. · 1.87 Impact Factor
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B B Pollock,
C E Clayton,
J E Ralph,
F Albert,
A Davidson,
L Divol,
C Filip,
S H Glenzer,
K Herpoldt,
W Lu,
K A Marsh,
J Meinecke,
W B Mori,
A Pak,
T C Rensink,
J S Ross,
J Shaw,
G R Tynan,
C Joshi, D H Froula
[show abstract]
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ABSTRACT: Laser wakefield acceleration of electrons holds great promise for producing ultracompact stages of GeV scale, high-quality electron beams for applications such as x-ray free electron lasers and high-energy colliders. Ultrahigh intensity laser pulses can be self-guided by relativistic plasma waves (the wake) over tens of vacuum diffraction lengths, to give >1 GeV energy in centimeter-scale low density plasmas using ionization-induced injection to inject charge into the wake even at low densities. By restricting electron injection to a distinct short region, the injector stage, energetic electron beams (of the order of 100 MeV) with a relatively large energy spread are generated. Some of these electrons are then further accelerated by a second, longer accelerator stage, which increases their energy to ∼0.5 GeV while reducing the relative energy spread to <5% FWHM.
Physical Review Letters 07/2011; 107(4):045001. · 7.37 Impact Factor
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ABSTRACT: We present simultaneous Thomson-scattering measurements of light scattered from ion-acoustic and electron-plasma fluctuations in a N(2) gas jet plasma. By varying the plasma density from 1.5×10(18) to 4.0×10(19) cm(-3) and the temperature from 100 to 600 eV, we observe the transition from the collective regime to the noncollective regime in the high-frequency Thomson-scattering spectrum. These measurements allow an accurate local measurement of fundamental plasma parameters: electron temperature, density, and ion temperature. Furthermore, experiments performed in the high densities typically found in laser produced plasmas result in scattering from electrons moving near the phase velocity of the relativistic plasma waves. Therefore, it is shown that even at low temperatures relativistic corrections to the scattered power must be included.
The Review of scientific instruments 10/2010; 81(10):10D523. · 1.52 Impact Factor
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C E Clayton,
J E Ralph,
F Albert,
R A Fonseca,
S H Glenzer,
C Joshi,
W Lu,
K A Marsh,
S F Martins,
W B Mori,
A Pak,
F S Tsung,
B B Pollock,
J S Ross,
L O Silva, D H Froula
[show abstract]
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ABSTRACT: The concepts of matched-beam, self-guided laser propagation and ionization-induced injection have been combined to accelerate electrons up to 1.45 GeV energy in a laser wakefield accelerator. From the spatial and spectral content of the laser light exiting the plasma, we infer that the 60 fs, 110 TW laser pulse is guided and excites a wake over the entire 1.3 cm length of the gas cell at densities below 1.5 × 10(18) cm(-3). High-energy electrons are observed only when small (3%) amounts of CO2 gas are added to the He gas. Computer simulations confirm that it is the K-shell electrons of oxygen that are ionized and injected into the wake and accelerated to beyond 1 GeV energy.
Physical Review Letters 09/2010; 105(10):105003. · 7.37 Impact Factor
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J. E. Ralph,
C. E. Clayton,
F. Albert,
B. B. Pollock,
S. F. Martins,
A. E. Pak,
K. A. Marsh,
J. L. Shaw,
A. Till,
J. P. Palastro,
W. Lu,
S. H. Glenzer,
L. O. Silva,
W. B. Mori,
C. Joshi, D. H. Froula
[show abstract]
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ABSTRACT: Experiments conducted using a 200 TW 60 fs laser have demonstrated up to 720 MeV electrons in the self-guided laser wakefield regime using pure helium gas jet targets. The self-trapped charge in a helium plasma was shown to fall off with decreasing electron density with a threshold at 2.5×1018 cm−3, below which no charge is measured above 100 MeV. Self-guiding, however, is shown to continue below this density limitation over distances of 14 mm with an exit spot size of 25 μm. Simulations show that injection of electrons at these densities can be assisted through ionization induced trapping in a mix of helium with 3% oxygen.
Physics of Plasmas 04/2010; 17(5):056709-056709-6. · 2.15 Impact Factor
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ABSTRACT: A series of laser-plasma interaction experiments is presented using gas-filled hohlraums that shed light on the behavior of stimulated Raman scattering and stimulated Brillouin scattering at various plasma conditions encountered in indirect drive ignition designs. Detailed experimental results are presented that quantify the density, temperature, and intensity thresholds for both of these instabilities. In addition to controlling plasma parameters, the National Ignition Campaign relies on optical beam smoothing techniques to mitigate backscatter and the results presented show that polarization smoothing is effective at controlling backscatter. These results provide an experimental basis for the forthcoming experiments on National Ignition Facility.
Physics of Plasmas 03/2010; 17(5):056302-056302-7. · 2.15 Impact Factor
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ABSTRACT: We observe relativistic modifications to the Thomson scattering spectrum in a traditionally classical regime: v(osc)/c = eE(0)/cmomega(0) < 1 and T(e) < 1 keV. The modifications result from scattering off electron-plasma fluctuations with relativistic phase velocities. Normalized phase velocities v/c between 0.03 and 0.12 have been achieved in a N(2) gas-jet plasma by varying the plasma density from 3 x 10(18) cm(-3) to 7 x 10(19) cm(-3) and electron temperature between 85 and 700 eV. For these conditions, the complete temporally resolved Thomson scattering spectrum including the electron and ion features has been measured. A relativistic treatment of the Thomson scattering form factor shows excellent agreement with the experimental data.
Physical Review Letters 03/2010; 104(10):105001. · 7.37 Impact Factor
