Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session YI1: Fast Ignition II |
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Chair: Richard Town Room: Landmark A |
Friday, November 21, 2008 9:45AM - 10:15AM |
YI1.00001: Advanced Ignition Experiments on OMEGA Invited Speaker: A comprehensive scientific program is being pursued at the Laboratory for Laser Energetics to explore the physics of inertial confinement fusion (ICF) beyond the baseline ``hot-spot'' designs. The OMEGA EP high-energy petawatt laser was completed in April 2008, adjacent to the existing 60-beam OMEGA Laser Facility. OMEGA EP consists of four beamlines with a NIF-like architecture. Two of the beamlines can operate in short-pulse mode, with up to 2.6 kJ each in a 10-ps pulse duration. The two short-pulse beams can be directed into either the OMEGA (co-propagating) or the OMEGA EP target chamber (co-propagating or orthogonal illumination). The combined OMEGA/OMEGA EP facility will be used to study advanced concepts such as the fast-ignitor approach to ICF. Fast ignition separates the fuel assembly and fuel heating by using an ultrafast laser in addition to a driver that compresses the fuel to high density. Fuel-assembly experiments with low-adiabat implosions and cone-in-shell targets on OMEGA have demonstrated high fuel-areal densities of up to 200 mg/cm$^{2}$ and shown that the cone does not significantly perturb the core fuel assembly. Integrated fast-ignitor experiments on OMEGA with cone-in-shell targets are planned for the Summer of 2008. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. Contributors: K.S. Anderson, R. Betti, J. Bromage, J.A. Delettrez, V.Yu Glebov, S.J. Loucks, J.H. Kelly, B.E. Krushwitz, R.L. McCrory, F.J. Marshall, D.D. Meyerhofer, S.F.B. Morse, J. Qiao, T.C. Sangster, W. Seka, S. Skupsky, V.A. Smalyuk, A.A. Solodov, L.J. Waxer, UR/LLE, J.A. Frenje, C.K. Li, R.D. Petrasso, PSFC, MIT, R.B. Stephens, GA. [Preview Abstract] |
Friday, November 21, 2008 10:15AM - 10:45AM |
YI1.00002: Integrated Simulations of Hot-Electron Transport and Ignition for Direct-Drive, Fast-Ignition Targets Invited Speaker: A thorough understanding of future integrated fast-ignition experiments combining compression and heating for high-density thermonuclear fuel require hybrid (fluid+particle) simulations of the implosion and ignition process. Very different spatial and temporal scales need to be resolved to model the entire fast-ignition experiment. The 2-D axisymmetric hydrocode \textit{DRACO}\footnote{P. B. Radha\textit{ et al}., Phys. Plasmas \textbf{12}, 056307 (2005).} and the 2-D/3-D hybrid-PIC code \textit{LSP}\footnote{D. R. Welch \textit{et al}., Phys. Plasmas \textbf{13}, 063105 (2006).} have been integrated to simulate the implosion and heating of direct-drive fast-ignition fusion targets. \textit{DRACO} includes the physics required to simulate compression, ignition, and burn of fast-ignition targets. \textit{LSP} simulates the transport of hot electrons from the place where they are generated by a petawatt laser pulse to the dense fuel core where their energy is absorbed. The results of integrated simulations of optimized spherically symmetric and cone-in-shell DT, high-gain, fast-ignition targets\footnote{R. Betti and C. Zhou, Phys. Plasmas \textbf{12}, 110702 (2005).} will be presented. The minimum energy required for ignition is found for hot electrons with a realistic angular spread and Maxwellian energy-distribution function.\footnote{B. Chrisman, Y. Sentoku, and A. J. Kemp, Phys. Plasmas \textbf{15}, 056309 (2008).} The results from simulations of cone-in-shell plastic targets designed for fast-ignition experiments on OMEGA EP will be presented. Target heating and neutron yield are computed. Resistive Weibel instability is found to break the hot-electron beam into filaments. The global self-generated resistive magnetic field of the beam is found to collimate the hot electrons. The self-generated field increases the coupling efficiency of hot electrons with the target core and reduces the minimum energy required for ignition. This work was supported by the U.S. Department of Energy under Cooperative Agreement Nos. DE-FC02-04ER54789 and DE-FC52-08NA28302. Contributors: K.S. Anderson, R. Betti, V. Gotcheva, J.F. Myatt, J.A. Delettrez, S. Skupsky. [Preview Abstract] |
Friday, November 21, 2008 10:45AM - 11:15AM |
YI1.00003: Magnetic Collimation of Fast Electrons in Laser-Solid Interactions and Fast Ignition Invited Speaker: In laser-solid experiments and in fast ignition electrons are accelerated from the critical surface into an overdense plasma. The basic physics of magnetic field generation by these fast electrons in the overdense plasma and the effect of this magnetic field on the fast electron transport are reviewed. A simple analytical model is developed that neatly describes both the magnetic field generation and its affect on the fast electron transport. The best known and most desirable of these effects is collimation (focusing or pinching). A simple and remarkably general criterion for collimation is derived, taking into account angular scattering, which is simpler, more accurate and more widely applicable than previous results [Phys. Rev. Lett. {\bf 93} 035003 (2003)]. It also shows that careful temporal shaping of the laser pulse could significantly increase collimation without increasing the laser energy; a refinement of the two-pulse collimation scheme [Phys. Rev. Lett. {\bf 100} 025002 (2008)]. It is shown that the magnetic field can also lead to beam hollowing, to an increase in beam divergence and to transport inhibition. The predictions of the analytical model are shown to compare favourably with hybrid code modelling. The implications for laser-solid experiments and for fast ignition are considered. The apparent lack of evidence for magnetic collimation in laser-solid experiments is discussed. [Preview Abstract] |
Friday, November 21, 2008 11:15AM - 11:45AM |
YI1.00004: Fast Ignition with Ultra-High Intensity Lasers Invited Speaker: One of the critical design constraints for fast ignition targets is the need to have a small cross section for the hot spot at the target core while delivering enough power to the hot spot with hot electrons of the proper energy range, $\sim $ 1-3 MeV, to couple to the core. We use PIC simulations of isolated targets to investigate the feasibility of using 1$\mu $m ignition lasers with ultra high intensities, I$>$5x10$^{19}$W/cm$^{2}$, for fast ignition. The absorption of an intense laser in an overdense plasma and the subsequent energy transport through the overdense plasma is explored by examining the interaction of high intensity ignition lasers, up to 8x10$^{20}$W/cm$^{2}$, with a 50$\mu $m radius target using two-dimensional Particle-In-Cell simulations. At these ultra-high intensities, we find that most of the energy transport is in a hot bulk and not in the super-hot tail of the electron distribution. Electrons in a relatively low energy range, below 3MeV, transport 90{\%} of the heat flux through 50$\mu $m of 100n$_{c}$ plasma to the target core. Hot electrons generated at the laser-plasma interface drive plasma turbulence in the background collisionless plasma heating it to MeV temperatures over pico-second time scales. Over the same time scale Weibel instabilities generate large magnetic fields that confine the transverse spread of the heated plasma. The effect of this interaction with the background plasma is to lower the energy of the electrons entering the target core at the cost of energy used to heat the plasma. The fraction of laser power that transits the dense plasma and is deposited into the core increases with laser intensity. At the highest intensity, I=8x10$^{20}$W/cm$^{2}$, 12{\%} of the laser power is deposited in the high density core after 2.5ps and the heat flux into the core is 16{\%} of the incident laser flux. [Preview Abstract] |
Friday, November 21, 2008 11:45AM - 12:15PM |
YI1.00005: Hot electron temperature scaling at $I\lambda ^2>10^{20}\mbox{Wcm}^{-2}$ and implications for fast ignition Invited Speaker: The fast ignition (FI) concept of inertial confinement fusion ignites pre-compressed fuel with laser-generated hot electrons. A laser intensity I$\sim $10$^{20}$ W/cm$^{2}$ is required to deliver 15-20 kJ of electrons in 20 ps into a 40 $\mu $m diameter spot. These electrons must have the energy, $\sim $2 MeV, to penetrate to and stop in the compressed core at a density of $\sim $300 g/cc. Ponderomotive scaling theory and experiments have indicated that the required electron energy is produced at I$\lambda ^{2}\sim $10$^{19}$ W$\mu $m$^{2}$/cm$^{2}$, necessitating a wavelength shorter than the 1 $\mu $m preferred for the ignitor laser. Experimental measurements of the mean electron energy are, however, complicated by the fact that direct measurements of the source electron spectrum are not possible, and indirect measurements, based on K-alpha x-rays, bremsstrahlung radiation, or escaping electrons, are highly dependent on transport modeling for interpretation. Recent experiments on the TITAN laser facility have used multiple-layered planar targets and a combination of these diagnostic techniques to improve the constraints on modeling. The experiments---performed over a range of laser intensities, pulselengths, and pre-pulse levels---together with particle-in-cell modeling suggest that the mean electron energy may be lower than previously considered, which would reduce the necessity in the fast ignition scheme for a short wavelength ignitor laser. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and supported by Contract DE-FG02-05ER54834. [Preview Abstract] |
Friday, November 21, 2008 12:15PM - 12:45PM |
YI1.00006: Study of ultra-intense laser propagation in overdense plasmas for fast ignition Invited Speaker: The scheme of fast ignition of fusion energy relies on the ultra-intense laser energy transport into the compressed core plasma. The popular idea is to use a physical reentrant cone to guide the laser beam from the coronal plasma close enough to the core plasma. An alternative idea is to inject the laser pulse directly into the corona plasma. This requires efficient propagation of laser light in the overdense plasma. We study the so called super-penetration mode of ultra-intense laser pulse penetrating into overdense plasmas with the relativistic effect [K. A. Tanaka et al., Phys. Plasmas 7, 2014(2000), R. Kodama et al., Phys. Plasmas 8, 2268(2001)]. We have experimentally observed the laser light penetration through a large preformed plasma with peak density of 10nc and the plasma channel formation, with the channel direction coinciding with the laser axis [A. L. Lei etal., Phys. Rev. E 76, 066403(2007)]. We find that the laser propagation is dependent on the laser focus position via measuring the laser transmittance though the overdense plasmas. The fast electrons generated are pointed along the laser axis and much collimated compared to the laser-solid interactions. To improve the laser propagation quality in overdense plasmas, we propose to use multiple short laser pulses. The physics of multiple pulse penetration will be discussed. This work is supported by the Japan-China JSPS-CAS Core University Program. [Preview Abstract] |
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