Session TI3: Fast Ignition and ICF

Integrated Fast-Ignition Core-Heating Experiments on OMEGA

Integrated fast-ignition core-heating experiments are carried out at the Omega Laser Facility. Plastic (CD) shell targets with a re-entrant gold cone are compressed with a $\sim $20{\-}kJ, UV low-adiabat laser pulse. A 1-kJ, 10-ps pulse from OMEGA EP generates fast electrons in the hollow cone that are transported into the compressed core. The experiments demonstrate a significant enhancement of the neutron yield. The neutron-yield enhancement caused by the high-intensity pulse is 1.5 $\times $ 10$^{7}$, which is more than 150{\%} of the implosion yield. For the first time, measurements of the breakout time of the compression-induced shock wave through the cone were performed for the same targets as used in the integrated experiments. The shock breakout was measured to be $\sim $100 ps after peak neutron production. The experiments demonstrate that the cone tip is intact at the time when the short-pulse laser interacts with the cone. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302, DE-FC02-04ER54789, and DE-FG02-05ER54839. \\[4pt] In collaboration with A. A. Solodov, K. S. Anderson, R. Betti (LLE/FSC); C. Stoeckl, T.R. Boehly, R.S. Craxton, J.A. Delettrez, V.Yu. Glebov, J.P. Knauer, F.J. Marshall, K.L. Marshall, D.D. Meyerhofer,$^{ }$P.M. Nilson, T.C. Sangster, W. Seka (LLE); F.N. Beg (UCSD), H. Habara (ILE), P.K. Patel (LLNL), R.B. Stephens (GA); J.A. Frenje, N. Sinenian (PSFC/MIT).   

Fast Ignition Realization Experiments (FIREX) and Beyond

After 50 years journey from the innovation of lasers, controlled ignition and subsequent burn will be demonstrated within a couple of years at the US National Ignition Facility (NIF). Fast ignition has the high potential to ignite a fuel using only about one tenth of laser energy of NIF [1]. One of the most advanced fast ignition programs is the Fast Ignition Realization Experiment (FIREX) [2]. The goal of its first phase is to demonstrate ignition temperature of 5 keV, followed by the second phase to demonstrate the ignition-and-burn. The first experiment of FIREX-I, reported here, gives a confidence that one can achieve ignition temperature at the heating laser energy of 10 kJ. One beam of LFEX laser was equipped with a pair of tiled gratings and successfully compressed to 1.2 ps with the energy of 1 kJ, providing about 1 PW laser power. The first experiment with the LFEX laser was performed using deuterated polystyrene shells with a gold cone. Ion temperatures of the core plasmas were deduced from the observed neutron yield, fuel density and the fuel mass. The result gives a confidence that ion temperature will increase up to the 5-keV level with using sharp rising rectangular laser pulse. Given the demonstrations of the ignition temperature at FIREX-I and the ignition-and-burn at NIF, the inertial fusion research would then shift from the plasma physics era to electric power era. Success of the high power short pulse laser system LFEX also makes us envisage future ultra high intensity lasers, such as, GEKKO-EXA and ELI [3] with sub exa Watt power. Extremely high intensity achieved in such facilities will open up ultra high-field physics. \\[4pt] [1] E.I. Moses, Nucl. Fusion 49(2009)104022. \\[0pt] [2] H. Azechi et al., Nucl. Fusion 49 (2009) 104024. \\[0pt] [3] http://www.extreme-light-infrastructure.eu/what-is-eli.php   

High-Intensity Laser-to-Hot-Electron Conversion Efficiency from 1 to 2100 J Using the OMEGA EP Laser System

Intense laser--matter interactions generate high-current electron beams. The laser-electron conversion efficiency is an important parameter for fast ignition and for developing intense x-ray sources for flash-radiography and x-ray-scattering experiments. These applications may require kilojoules of laser energy focused to greater than 10$^{18}$~W/cm$^{2}$ with pulse durations of tens of picoseconds. Previous experiments have measured the conversion efficiency with picosecond and subpicosecond laser pulses with energies up to $\sim $500~J. The research extends conversion-efficiency measurements to 1- to 10-ps laser pulses with energies up to 2100~J using the OMEGA EP Laser System and shows that the conversion efficiency is constant (20$\pm $10{\%}) over the entire range The conversion efficiency is measured for interactions with finite-mass, thin-foil targets. A collimated electron jet exits the target rear surface and initiates rapid target charging, causing the majority of laser-accelerated electrons to recirculate (reflux) within the target. The total fast-electron energy is inferred from K-photon spectroscopy. Time-resolved x-ray emission data suggest that electrons are accelerated into the target over the entire laser-pulse duration with approximately constant conversion. This work provides significant insight into high-intensity laser--target interactions. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302 and DE-FC02-04ER54789. \\[4pt] In collaboration with R. Betti, A. A. Solodov (LLE/FSC), R. S. Craxton, J. A. Delettrez, C. Dorrer, L. Gao, P. A. Jaanimagi, J. H. Kelly, B. E. Kruschwitz, D. D. Meyerhofer, J. F. Myatt, T. C. Sangster, C. Stoeckl, W. Theobald, B. Yaakobi, J. D. Zuegel (LLE), A. J. MacKinnon, P. K. Patel (LLNL), K. U. Akli (General Atomics), L. Willingale, K. M. Krushelnick (U. of Michigan).   

High-power, kilojoule class laser channeling in millimeter scale underdense plasma

The interaction of high-energy, relativistic laser pulses with underdense plasma is of fundamental interest relevant to laser propagation phenomena, particle acceleration, x-ray generation and blast wave studies. In particular, the hole-boring fast-ignition scheme for inertial confinement fusion requires an ignition pulse to channel through millimeter scale underdense plasma to reach the dense fuel core. Experiments were performed using the OMEGA EP facility 9 ps short-pulse laser, with pulse energy $\approx 750 \; \rm{J}$ (corresponding to 90 TW power and focused peak intensity of $5 \times 10^{19} \; \rm{Wcm}^{-2}$) to investigate the interaction with a millimeter-scale, low-density plasma plume, generated by a long-pulse interaction with CH target. The second short-pulse beam generates a proton probe beam from a thin-foil target, which gives high-quality images of the electromagnetic fields generated in the interactions. The evolution and early time expansion of the channel is measured on a single shot using this method. At later times, the channel has evolved to show a strong instability. Plasmas up to near-critical density were also investigated by using low-density foam targets. Large-scale 2D particle-in-cell simulations using the OSIRIS code are performed to model the interactions and investigate the observed physical phenomenon. In collaboration with P.~M.~Nilson, A.~G.~R.~Thomas, J.~Cobble, R.~S.~Craxton, K.~Flippo, A.~Maksimchuk, W.~Nazarov, P.A.~Norreys, T.~C.~Sangster, R.~H.~H.~Scott, C.~Stoeckl, C.~Zulick, and K.~Krushelnick.   

Fast electron transport in laser-driven shock heated warm dense matter and its implication for fast ignition

Understanding of relativistic electron transport from the region of laser interaction in a high-Z cone tip, and onward through a low-Z low density plasma is of great importance to cone guided fast ignition. We have performed experiments using the Titan laser at LLNL to investigate electron transport in warm dense matter (WDM) with foam package targets consisting of Au/CRF foam/Cu/CH. WDM is created by a long laser pulse (300 J, 3 ns, 600 $\mu $m spot) driven shock compression and heating of the low density foam with initial density of 150 mg/cm$^{3}$. Shock propagation in the foam is investigated using the side-on x-ray radiography complemented with radiation hydro simulations. At its maximum compression, low-Z WDM with solid density and a temperature of 5-10 eV is assembled right behind the Au foil. Transport of the high intensity laser (I$_{peak}\sim $10$^{20}$ W/cm$^{2})$ produced relativistic electrons from the Au foil in the WDM is characterized by measuring K$\alpha $ emissions from the Cu fluorescence layer. A large angular spread of fast electrons is observed in the 2D spatial profiles of the K$\alpha $ emission when fast electrons propagate through WDM. In addition, a 5X increase in the number of escaped electrons at a large off-normal angles is seen, consistent with the observed large angular spread. PIC simulations suggest that the large angular spread could be due to the randomization of fast electrons by the intense Weibel-like magnetic fields generated at the interface between the high density Au and the low-Z lower density plasma.\footnote{B. Chrisman et al., Phys. Plasmas 15, 056309 (2008).} Detailed experimental and PIC simulation results will be presented and their implications for fast ignition will be discussed.   

The Potential Role of Electric Fields and Plasma Barotropic Diffusion on the Inertial Confinement Fusion Database

The generation of strong, self-generated electric fields ($\approx$ 1-10 GV/m) in direct-drive, inertial-confinement-fusion capsules has been reported [1], prompting the question whether such fields can have observable consequences on target performance. Two anomalies in the inertial confinement fusion database are well known: (1) an observed $\approx$ 2x greater-than-expected deficit of neutrons in an equimolar D$^{3}$He fuel mixture compared with hydrodynamically equivalent DD [2] mixtures, and (2) a similar shortfall of neutrons when trace amounts of argon are mixed with DD fuel in indirect-drive implosions [3]. A new mechanism based on barodiffusion (or pressure gradient-driven diffusion) in a plasma is proposed that incorporates the presence of shock-generated electric fields to explain the reported anomalies. For Omega-scale implosions the (low Mach number) return shock has an appreciable scale length over which the lighter DD ions can diffuse away from fuel center. The depletion of DD fuel is estimated and found to yield a corresponding reduction in neutrons, consistent with the anomalies observed in experiments for both argon-doped DD fuels and D$^{3}$He equimolar mixtures. The reverse diffusion of the heavier ions towards fuel center also increases the pressure, potentially resulting in lower stagnation pressures and larger imploded cores in agreement with gated self-emission x-ray imaging data. The theory is applied to studying the degree of potential fractionation of THD fuel mixtures for an upcoming ignition tuning campaign on the National Ignition Facility.\\[4pt] [1] Rygg et al., Science 319, 1223 (2008), Li et al., PRL 100, 225001 (2008)\\[0pt] [2] Rygg et al., PoP 13, 052702 (2006)\\[0pt] [3] Lindl et al., PoP 11, 339 (2004).