-
K U Akli,
R B Stephens,
M H Key,
T Bartal,
F N Beg,
S Chawla,
C D Chen,
R Fedosejevs,
R R Freeman,
H Friesen, [......],
P K Patel,
Y Ping,
H Sawada,
D Schumacher,
W Theobald,
Y Y Tsui,
L D Van Woerkom,
M S Wei,
B Westover,
T Yabuuchi
[show abstract]
[hide abstract]
ABSTRACT: We have conducted experiments on both the Vulcan and Titan laser facilities to study hot electron generation and transport in the context of fast ignition. Cu wires attached to Al cones were used to investigate the effect on coupling efficiency of plasma surround and the pre-formed plasma inside the cone. We found that with thin cones 15% of laser energy is coupled to the 40μm diameter wire emulating a 40μm fast ignition spot. Thick cone walls, simulating plasma in fast ignition, reduce coupling by x4. An increase of pre-pulse level inside the cone by a factor of 50 reduces coupling by a factor of 3.
Journal of Physics Conference Series 09/2010; 244(2):022026.
-
T Yabuuchi,
B S Paradkar,
M S Wei, J A King,
F N Beg,
R B Stephens,
N Nakanii,
M Hatakeyama,
H Habara,
K Mima,
K A Tanaka,
J T Larsen
[show abstract]
[hide abstract]
ABSTRACT: The effect of preplasma on fast electron generation and transport has been studied using an intense-laser pulse I =2 10 18 W / cm 2 at the Osaka University. An external long pulse laser beam E 1.5 J was used to create various levels of preplasmas in front of a planar target for a systematic study. K x-ray emission from a fluorescence layer copper was absolutely counted and its spatial distribution was monitored. Experimental data show K x-ray signal reduction up to 60% with an increase in the preplasma level. In addition, a ring structure of K x rays was observed with a large preplasma. The underlying physics of the ring structure production was studied by integrating the modeling using a radiation hydrodynamics code and a hybrid particle-in-cell code. Modeling shows that the ring structure is due to the thermoelectric magnetic field excited by the long pulse laser irradiation and an electrostatic field due to the fast electrons in the preplasma. © 2010 American Institute of Physics. doi:10.1063/1.3447878 Fast ignition FI inertial confinement fusion is a rela-tively new concept and offers high gains compared to the central hot spot concept. 1 It has separate compression and heating phases, which sets relatively less stringent require-ments on the symmetric compression of the fuel. In ad-vanced FI, a gold cone is inserted into the capsule to provide a clear path for the short pulse laser to deposit its energy near the compressed fuel. 2 An important question that needs to be addressed is the preplasma effect on the laser to fast electron conversion and transport. This preplasma is produced by the inherent prepulse or amplified stimulated emission ASE in a laser system. The preplasma effect will be important when high energy lasers of 10–100 kJ are used for FI, where the Joule-level prepulse and ASE can create a large scale pre-plasma inside the cone. In addition, preplasma can also be created by preheating of the cone walls by x rays during the implosion. Furthermore, the high pressure of the implosion can break the cone tip, producing a jet of preplasma. The characteristics of the fast electrons temperature, divergence, and number can be changed due to laser self-focusing or filamantation in the preplasma. 3–5 The electrostatic and mag-netic fields 6 in the preplasma can affect the fast electron transport. The effect of a preplasma inside the cone is of particular importance; however the cone's closed geometry causes difficulty in diagnosing the scale length and density profile of such a preplasma. In this letter, effects of a large preplasma on the fast electron generation and transport have been systematically investigated using planar multilayer targets at the laser inten-sity of the order of 10 18 W / cm 2 this intensity is two orders of magnitude smaller than what is envisioned for the full scale FI, but is similar to that used in recent integrated
Physics of Plasmas 01/2010; 17:060704. · 2.15 Impact Factor
-
A. J. MacKinnon,
K. U. Akli,
T. Bartal,
F. N. Beg,
S. Chawla,
C. D. Chen,
H. Chen,
S. Chen,
E. Chowdhury,
R. Fedosejevs, [......],
D. Offermann,
V. Ovchinnikov,
J. Pasley,
P. K. Patel,
Y. Ping,
D. W. Schumacher,
R. B. Stephens,
Y. Y. Tsui,
M. S. Wei,
L. D. Van Woerkom
[show abstract]
[hide abstract]
ABSTRACT: Isochoric heating of inertially confined fusion plasmas by laser driven MeV electrons or protons is an area of great topical interest in the inertial confinement fusion community, particularly with respect to the fast ignition (FI) concept for initiating burn in a fusion capsule. In order to investigate critical aspects needed for a FI point design, experiments were performed to study 1) laser‐to‐electrons or protons conversion issues and 2) laser‐cone interactions including prepulse effects. A large suite of diagnostics was utilized to study these important parameters. Using cone—wire surrogate targets it is found that pre‐pulse levels on medium scale lasers such as Titan at Lawrence Livermore National Laboratory produce long scale length plasmas that strongly effect coupling of the laser to FI relevant electrons inside cones. The cone wall thickness also affects coupling to the wire. Conversion efficiency to protons has also been measured and modeled as a function of target thickness, material. Conclusions from the proton and electron source experiments will be presented. Recent advances in modeling electron transport and innovative target designs for reducing igniter energy and increasing gain curves will also be discussed. In conclusion, a program of study will be presented based on understanding the fundamental physics of the electron or proton source relevant to FI.
AIP Conference Proceedings. 09/2009; 1161(1):25-25.
-
W Theobald,
C Stoeckl,
P A Jaanimagi,
P M Nilson,
M Storm,
D D Meyerhofer,
T C Sangster,
D Hey,
A J MacKinnon,
H-S Park, [......],
R B Stephens,
K U Akli,
K Highbarger,
R L Daskalova,
L Van Woerkom,
R R Freeman,
J S Green,
G Gregori,
K Lancaster,
P A Norreys
[show abstract]
[hide abstract]
ABSTRACT: A dual-channel, curved-crystal spectrograph was designed to measure time-integrated x-ray spectra in the approximately 1.5 to 2 keV range (6.2-8.2 A wavelength) from small-mass, thin-foil targets irradiated by the VULCAN petawatt laser focused up to 4x10(20) W/cm(2). The spectrograph consists of two cylindrically curved potassium-acid-phthalate crystals bent in the meridional plane to increase the spectral range by a factor of approximately 10 compared to a flat crystal. The device acquires single-shot x-ray spectra with good signal-to-background ratios in the hard x-ray background environment of petawatt laser-plasma interactions. The peak spectral energies of the aluminum He(alpha) and Ly(alpha) resonance lines were approximately 1.8 and approximately 1.0 mJ/eV sr (approximately 0.4 and 0.25 J/A sr), respectively, for 220 J, 10 ps laser irradiation.
The Review of scientific instruments 09/2009; 80(8):083501. · 1.52 Impact Factor
-
A J MacKinnon,
K U Akli,
T Bartal,
F N Beg,
S Chawla,
C D Chen,
H Chen,
S Chen,
E Chowdhury,
R Fedosejevs, [......],
D Offermann,
V Ovchinnikov,
J Pasley,
P K Patel,
Y Ping,
D W Schumacher,
R B Stephens,
Y Y Tsui,
M S Wei,
L D Van Woerkom
[show abstract]
[hide abstract]
ABSTRACT: Isochoric heating of inertially confined fusion plasmas by laser driven MeV electrons or protons is an area of great topical interest in the inertial confinement fusion community, particularly with respect to the fast ignition (FI) concept for initiating burn in a fusion capsule. In order to investigate critical aspects needed for a FI point design, experiments were performed to study 1) laser-to-electrons or protons conversion issues and 2) laser-cone interactions including prepulse effects. A large suite of diagnostics was utilized to study these important parameters. Using cone wire surrogate targets it is found that pre-pulse levels on medium scale lasers such as Titan at Lawrence Livermore National Laboratory produce long scale length plasmas that strongly effect coupling of the laser to FI relevant electrons inside cones. The cone wall thickness also affects coupling to the wire. Conversion efficiency to protons has also been measured and modeled as a function of target thickness, material. Conclusions from the proton and electron source experiments will be presented. Recent advances in modeling electron transport and innovative target designs for reducing igniter energy and increasing gain curves will also be discussed. In conclusion, a program of study will be presented based on understanding the fundamental physics of the electron or proton source relevant to FI.
AIP Conference Proceedings; 01/2009
-
J A King,
K U Akli,
R R Freeman,
J Green,
S P Hatchett,
D Hey,
P Jamangi,
M H Key,
J Koch,
K L Lancaster, [......],
A MacPhee,
P A Norreys,
P K Patel,
T Phillips,
R B Stephens,
W Theobald,
R P J Town,
L Van Woerkom,
B Zhang,
F N Beg
[show abstract]
[hide abstract]
ABSTRACT: Experimental results showing hot electron penetration into Cu wires using Kalpha fluorescence imaging are presented. A 500 J, 1 ps laser was focused at f/3 into hollow aluminum cones joined at their tip to Cu wires of diameters from 10 to 40 mum. Comparison of the axially diminishing absolute intensity of Cu Kalpha with modeling shows that the penetration of the electrons is consistent with one dimensional Ohmic potential limited transport. The laser coupling efficiency to electron energy within the wire is shown to be proportional to the cross sectional area of the wire, reaching 15% for 40 mum wires. Further, we find the hot electron temperature within the wire to be about 750 keV. The relevance of these data to cone coupled fast ignition is discussed.
Physics of Plasmas 01/2009; 16(2):020701 (4 pp.). · 2.15 Impact Factor
-
A G MacPhee,
K U Akli,
F N Beg,
C D Chen,
H Chen,
R Clarke,
D S Hey,
R R Freeman,
A J Kemp,
M H Key, [......],
D T Offermann,
V M Ovchinnikov,
P K Patel,
T W Phillips,
R B Stephens,
R Town,
Y Y Tsui,
M S Wei,
L D Van Woerkom,
A J Mackinnon
[show abstract]
[hide abstract]
ABSTRACT: The ignition concept for electron fast ignition inertial confinement fusion requires sufficient energy be transferred from an approximately 20 ps laser pulse to the compressed fuel via approximately MeV electrons. We have assembled a suite of diagnostics to characterize such transfer, simultaneously fielding absolutely calibrated extreme ultraviolet multilayer imagers at 68 and 256 eV; spherically bent crystal imagers at 4.5 and 8 keV; multi-keV crystal spectrometers; MeV x-ray bremmstrahlung, electron and proton spectrometers (along the same line of sight), and a picosecond optical probe interferometer. These diagnostics allow careful measurement of energy transport and deposition during and following the laser-plasma interactions at extremely high intensities in both planar and conical targets. Together with accurate on-shot laser focal spot and prepulse characterization, these measurements are yielding new insights into energy coupling and are providing critical data for validating numerical particle-in-cell (PIC) and hybrid PIC simulation codes in an area crucial for fast ignition and other applications. Novel aspects of these diagnostics and how they are combined to extract quantitative data on ultrahigh intensity laser-plasma interactions are discussed.
The Review of scientific instruments 11/2008; 79(10):10F302. · 1.52 Impact Factor
-
C. D. Chen, J. A. King,
M. H. Key,
K. U. Akli,
F. N. Beg,
H. Chen,
R. R. Freeman,
A. Link,
A. J. Mackinnon,
A. G. MacPhee,
P. K. Patel,
M. Porkolab,
R. B. Stephens,
L. D. Van Woerkom
[show abstract]
[hide abstract]
ABSTRACT: A Bremsstrahlung spectrometer using k-edge and differential filtering has been used with image plate dosimeters to measure the x-ray fluence from short-pulse laser/target interactions. An electron spectrometer in front of the Bremsstrahlung spectrometer deflects electrons from the x-ray line of sight and simultaneously measures the electron spectrum. The response functions were modeled with the Monte Carlo code INTEGRATED TIGER SERIES 3.0 and the dosimeters calibrated with radioactive sources. An electron distribution with a slope temperature of 1.3 MeV is inferred from the Bremsstrahlung spectra.
Review of Scientific Instruments 10/2008; 79(10):10E305-10E305-3. · 1.37 Impact Factor
-
T. Ma,
A. G. MacPhee,
M. H. Key,
S. P. Hatchett,
K. U. Akli,
T. W. Barbee,
C. D. Chen,
R. R. Freeman, J. A. King,
A. Link,
A. J. Mackinnon,
D. T. Offermann,
V. Ovchinnikov,
P. K. Patel,
R. B. Stephens,
L. D. Van Woerkom,
B. Zhang,
F. N. Beg
[show abstract]
[hide abstract]
ABSTRACT: Measurements of plasma temperature at the rear surface of foil targets due to heating by hot electrons, which were produced in short pulse high intensity laser matter interactions using the 150 J, 0.5 ps Titan laser, are reported. Extreme ultraviolet (XUV) imaging at 256 and 68 eV energies is used to determine spatially resolved target rear surface temperature patterns by comparing absolute intensities to radiation hydrodynamic modeling. XUV mirrors at these two energies were absolutely calibrated at the Advanced Light Source at the Lawrence Berkeley Laboratory. Temperatures deduced from both imagers are validated against each other within the range of 75–225 eV.
Review of Scientific Instruments 09/2008; 79(9):093507-093507-7. · 1.37 Impact Factor
-
M H Key,
K Akli,
F Beg,
R Betti,
D S Clark,
S N Chen,
R R Freeman,
S Hansen,
S P Hatchett,
D Hey, [......],
M Foord,
M Tabak,
W Theobald,
M Storm,
R P J Town,
S C Wilks,
L VanWoerkom,
M S Wei,
R Weber,
B Zhang
[show abstract]
[hide abstract]
ABSTRACT: Fast ignition research has reached the stage where point designs are becoming crucial to the identification of key issues and the development of projects to demonstrate high gain fast ignition. The status of point designs for cone coupled electron fast ignition and some of the issues they highlight are discussed.
Journal of Physics Conference Series 06/2008; 112(2):022056.
-
L Van Woerkom,
K U Akli,
T Bartal,
F N Beg,
S Chawla,
C D Chen,
E Chowdhury,
R R Freeman,
D Hey,
M H Key, [......],
A Link,
T Ma,
A J MacKinnon,
A G MacPhee,
D Offermann,
V Ovchinnikov,
P K Patel,
D W Schumacher,
R B Stephens,
Y Y Tsui
[show abstract]
[hide abstract]
ABSTRACT: Experimental results from copper cones irradiated with ultraintense laser light are presented. Spatial images and total yields of Cu K alpha fluorescence were measured as a function of the laser focusing properties. The fluorescence emission extends into the cone approximately 300 mum from the cone tip and cannot be explained by ray tracing including cone wall absorption. In addition, the total fluorescence yield from cones is an order of magnitude higher than for equivalent mass foil targets. Indications are that the physics of the laser-cone interaction is dominated by preplasma created from the long duration, low-energy prepulse from the laser.
Physics of Plasmas 05/2008; 15(5):056304-1-6. · 2.15 Impact Factor
-
J S Green,
V M Ovchinnikov,
R G Evans,
K U Akli,
H Azechi,
F N Beg,
C Bellei,
R R Freeman,
H Habara,
R Heathcote, [......],
K Takeda,
K A Tanaka,
W Theobald,
T Tanimoto,
J Waugh,
L Van Woerkom,
N C Woolsey,
M Zepf,
J R Davies,
P A Norreys
[show abstract]
[hide abstract]
ABSTRACT: Metal foil targets were irradiated with 1 mum wavelength (lambda) laser pulses of 5 ps duration and focused intensities (I) of up to 4x10;{19} W cm;{-2}, giving values of both Ilambda;{2} and pulse duration comparable to those required for fast ignition inertial fusion. The divergence of the electrons accelerated into the target was determined from spatially resolved measurements of x-ray K_{alpha} emission and from transverse probing of the plasma formed on the back of the foils. Comparison of the divergence with other published data shows that it increases with Ilambda;{2} and is independent of pulse duration. Two-dimensional particle-in-cell simulations reproduce these results, indicating that it is a fundamental property of the laser-plasma interaction.
Physical Review Letters 02/2008; 100(1):015003. · 7.37 Impact Factor
-
M. H. Key,
J. C. Adam,
K. U. Akli,
M. Borghesi,
M. H. Chen,
R. G. Evans,
R. R. Freeman,
H. Habara,
S. P. Hatchett,
J. M. Hill, [......],
T. Phillips,
L. Romagnani,
R. A. Snavely,
R. Stephens,
C. Stoeckl,
R. Town,
Y. Toyama,
B. Zhang,
M. Zepf,
P. A. Norreys
[show abstract]
[hide abstract]
ABSTRACT: An integrated experiment relevant to fast ignition. A Cu-doped deuterated polymer spherical shell target with an inserted hollow An cone is imploded by a six-beam 900-J, 1-ns laser. A 10-ps, 70-J laser pulse is focused into the cone at, the time of peak compression. The flux of high-energy electrons through the imploded material is determined from the yield of Cu K alpha fluorescence by comparison with a Monte Carlo model. The electrons are estimated to carry about 15% of the laser energy. Collisional and Ohmic heating are modeled, and Ohmic effects are shown to be relatively unimportant. An electron spectrometer shows significantly greater reduction of the transmitted electron flux than is calculated in the model. Enhanced scattering by instability-induced magnetic fields is suggested. An extension of this fluor-based technique to measurement of coupling efficiency. to the ignition hot spot in future larger-scale fast ignition experiments is outlined. (C) 2008 American Institute of Physics.
Physics of Plasmas 01/2008; 15(2). · 2.15 Impact Factor
-
J A King,
J Pasley,
F. Beg,
E Brambrink,
A. Edens,
M Geissel,
D. Headley,
P.K. Rambo,
J. Schwarz,
D.B. Sinars,
R B Stephens
[show abstract]
[hide abstract]
ABSTRACT: Summary form only given. We present results from experiments performed on 100 TW facility at Sandia National Laboratory. The main goals of the experiment were to quantify the pointing stability, Kalpha spot size and pre-pulse effects of the Z-PW laser. The targets were 25 mum thick Cu foils. The specific diagnostics used include a Cu Kalpha spherical crystal imager which provided information on the shot to shot variations in Kalpha spot intensity and position, an optical probe used for transverse shadowgraphy of pre-pulse plasma formation and radiochromic film RCF stacks to characterize the rear surface proton energy. The Copper Kalpha images show an average spot diameter of ~71plusmn11 mum and a pointing error of les50 mum in both the vertical and horizontal directions. The stacks of RCF showed an average peak proton energy of 7.7plusmn2.4 MeV.
Plasma Science, 2007. ICOPS 2007. IEEE 34th International Conference on; 07/2007
-
Peimin Gu,
B. Zhang,
M. H. Key,
S. P. Hatchett,
T. Barbee,
R. R. Freeman,
K. Akli,
D. Hey, J. A. King,
A. J. Mackinnon,
R. A. Snavely,
R. B. Stephens
[show abstract]
[hide abstract]
ABSTRACT: A 68 eV extreme-ultraviolet light imaging diagnostic measures short pulse isochoric heating by electrons and protons in petawatt laser experiments. Temperatures are deduced from the absolute intensities and comparison with modeling using a radiation hydrodynamics code.
Review of Scientific Instruments 11/2006; 77(11):113101-113101-6. · 1.37 Impact Factor
-
A J Mackinnon,
P K Patel,
M Borghesi,
R C Clarke,
R R Freeman,
H Habara,
S P Hatchett,
D Hey,
D G Hicks,
S Kar, [......],
K Lancaster,
D Neely,
A Nikkro,
P A Norreys,
M M Notley,
T W Phillips,
L Romagnani,
R A Snavely,
R B Stephens,
R P J Town
[show abstract]
[hide abstract]
ABSTRACT: Protons accelerated by a picosecond laser pulse have been used to radiograph a 500 microm diameter capsule, imploded with 300 J of laser light in 6 symmetrically incident beams of wavelength 1.054 microm and pulse length 1 ns. Point projection proton backlighting was used to characterize the density gradients at discrete times through the implosion. Asymmetries were diagnosed both during the early and stagnation stages of the implosion. Comparison with analytic scattering theory and simple Monte Carlo simulations were consistent with a 3+/-1 g/cm3 core with diameter 85+/-10 microm. Scaling simulations show that protons>50 MeV are required to diagnose asymmetry in ignition scale conditions.
Physical Review Letters 08/2006; 97(4):045001. · 7.37 Impact Factor
-
E Martinolli,
M Koenig,
S D Baton,
J J Santos,
F Amiranoff,
D Batani,
E Perelli-Cippo,
F Scianitti,
L Gremillet,
R Mélizzi, [......],
C Rousseaux,
T A Hall,
M H Key,
R Snavely,
A J MacKinnon,
R R Freeman, J A King,
R Stephens,
D Neely,
R J Clarke
[show abstract]
[hide abstract]
ABSTRACT: We present experimental results on fast-electron energy deposition into solid targets in ultrahigh intensity laser-matter interaction. X-ray K alpha emission spectroscopy with absolute photon counting served to diagnose fast-electron propagation in multilayered targets. Target heating was measured from ionization-shifted K alpha emission. Data show a 200 microm fast-electron range in solid Al. The relative intensities of spectrally shifted Al K alpha lines imply a mean temperature of a few tens of eV up to a 100 microm depth. Experimental results suggest refluxing of the electron beam at target rear side. They were compared with the predictions of both a collisional Monte Carlo and a collisional-electromagnetic, particle-fluid transport code. The validity of the code modeling of heating in such highly transient conditions is discussed.
Physical Review E 05/2006; 73(4 Pt 2):046402. · 2.26 Impact Factor
-
W. Theobald,
K. Akli,
R. Clarke,
J. A. Delettrez,
R. R. Freeman,
S. Glenzer,
J. Green,
G. Gregori,
R. Heathcote,
N. Izumi, [......],
P. Patel,
S. P. Regan,
H. Sawada,
R. Shepherd,
R. Snavely,
R. B. Stephens,
C. Stoeckl,
M. Storm,
B. Zhang,
T. C. Sangster
[show abstract]
[hide abstract]
ABSTRACT: A hot, 2 to 3 keV electron temperature surface plasma was observed in the interaction of a 0.7 ps petawatt laser beam with solid copper-foil targets at intensities >1020 W/cm2. Copper K-shell spectra were measured in the range of 8 to 9 keV using a single-photon-counting x-ray charged-coupled-device camera. In addition to Kα and Kβ inner-shell lines, the emission contained the Cu Heα and Lyα lines, allowing the temperature to be inferred. These lines have not been observed previously with ultrafast laser pulses. For intensities less than 3×1018 W/cm2, only the Kα and Kβ inner-shell emissions are detected. Measurements of the absolute Kα yield as a function of the laser intensity are in general agreement with a model that includes refluxing and confinement of the suprathermal electrons in the target volume.
Physics of Plasmas 04/2006; 13(4):043102-043102-11. · 2.15 Impact Factor
-
H. S. Park,
N. Izumi,
M. H. Key, J. A. King,
J. A. Koch,
O. L. Landen,
P. K. Patel,
D. F. Price,
B. A. Remington,
H. F. Robey, [......],
D. M. Chambers,
R. Eagleton,
T. Goldsack,
R. J. Clarke,
R. Heathcote,
E. Giraldez,
A. Nikroo,
D. A. Steinman,
R. B. Stephens,
B. B. Zhang
Physics of Plasmas 01/2006; 13:056309. · 2.15 Impact Factor
-
M. H. Key,
K. Akli,
F. Beg,
M. H. Chen,
H. K. Chung,
R. R. Freeman,
M. E. Foord,
J. S. Green,
P. M. Gu,
G. Gregori, [......],
R. A. Snavely,
R. B. Stephens,
C. Stoeckl,
M. Tabak,
W. Theobald,
K. Tanaka,
R. Town,
S. C. Wilks,
Y. Yabuuchi,
B. Zhang
Journal de Physique IV (Proceedings) 01/2006; 133:371. · 0.29 Impact Factor
