Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session YI2: HEDP and Plasma Simulations |
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Chair: Bedros Afeyan, Polymath Research Inc. Room: Rosen Centre Hotel Salon 3/4 |
Friday, November 16, 2007 9:30AM - 10:00AM |
YI2.00001: High-Intensity-Laser--Solid Interactions in the Refluxing Limit Invited Speaker: Small mass targets are of interest in high-intensity-laser--solid interactions due to their unique fast-electron transport properties.\footnote{ S. P. Hatchett \textit{et al}., Phys. Plasmas \textbf{7}, 2076 (2000).}$^{,}$\footnote{ R. A. Snavely\textit{ et al}., Phys. Rev. Lett. \textbf{85}, 2945 (2000).} Electron refluxing in solid-density matter by the Debye sheath fields that are set up at the target surfaces provide a unique environment for the determination of the conversion efficiency of laser energy into fast electrons.\footnote{ W. Theobald\textit{ et al}., Phys. Plasmas \textbf{13}, 043102 (2006).} Previous measurements of the absolute K$_{\alpha }$ yield from copper foils as a function of laser intensity demonstrate excellent agreement with electron refluxing models.$^{3,}$\footnote{ J. Myatt\textit{ et al}., Phys. Plasmas \textbf{14}, 056301 (2007).}$^{ }$In particular, fast-electron conversion efficiencies of around 10{\%} to 20{\%} have been inferred by fitting the absolute K$_{\alpha }$ yields to semi-analytical modeling. It is well known that ionization of the M shell during volumetric heating within such small mass copper targets can cause a deviation in the ratio of the number of emitted K$_{\beta }$ and K$_{\alpha }$ photons below the cold material limit.$^{4}$ This is a direct consequence of bulk target heating due to fast-electron energy loss. Such a deviation could provide a useful code benchmarking parameter on the energy content of the fast electrons and a consistency check on the laser-electron conversion efficiency. This consistency check, however, has proven elusive experimentally. We demonstrate here for the first time the consistency between the fast-electron conversion efficiencies predicted by these two methods using small mass targets. It is demonstrated that a 3.5$\times $ reduction in the ratio of the number of emitted K$_{\beta }$ and K$_{\alpha }$photons is achievable below the cold material limit using 20 $\times $ 20 $\times $ 2 \textit{$\mu $}m copper targets at laser intensities of 2 $\times $ 10$^{19}$ W cm$^{-2}$. These results provide a comparison in preparation for the higher energy-density environments that will be accessible using the future OMEGA EP Laser Facility.\footnote{ C. Stoeckl\textit{ et al}., Fusion Sci. Technol. \textbf{49}, 367 (2006)} This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreements DE-FC52-92SF19460 and DE-FC02-04ER54789. Contributors: W. Theobald, J. Myatt, M. Storm, O.V. Gotchev, C. Mileham, C. Stoeckl, R. Betti,$^{\ast }$ D.D. Meyerhofer,$^{\ast }$ and T.C. Sangster. $^{\ast }$Also at the Fusion Science Center for Extreme States of Matter and Fast Ignition. [Preview Abstract] |
Friday, November 16, 2007 10:00AM - 10:30AM |
YI2.00002: Radiation Transport through Inhomogeneous Materials Invited Speaker: Calculations of radiation transport in heated materials are greatly complicated by the presence of regions in which two or more materials are inhomogeneously mixed. This phenomenon is important in many systems, including inertial confinement fusion (ICF), where mixing can occur from instability growth and in astrophysical systems where density clumps can be found in star-forming regions and molecular clouds. We describe laboratory experiments designed to test the modeling of radiation transport through inhomogeneous plasmas. A laser-heated hohlraum is used as a thermal source to drive radiation through polymer foam containing randomly-distributed gold particles. We present experimental measurements of radiation transport in foams with gold particle sizes ranging from 5-9 micron to sub-micron diameters as well as the homogeneous foam case. We also compare simulation results of the radiation transport to the experiment. This was performed by the Los Alamos National Laboratory under the auspices of the United States Department of Energy under contract no. W-7405-ENG-36. [Preview Abstract] |
Friday, November 16, 2007 10:30AM - 11:00AM |
YI2.00003: Charged Particle Stopping Power Effects on Ignition: An Exact Calculation Invited Speaker: Using a new technique designed to capture both long and short distance physics in a completely finite manner, I will calculate the charged particle stopping power exactly to leading and next-to-leading order in the plasma number density, including an exact treatment of two-body quantum scattering. I should emphasize that this is not a model but a systematic calculation that provides an estimate of its own error and its domain of applicability. This calculation is extremely accurate in the plasma regime realized during ICF ignition, and the 3.5 MeV alpha particle range tends to be 30 to 40\% longer than most models in the literature have predicted. This increases the rho-R required for ignition. [Preview Abstract] |
Friday, November 16, 2007 11:00AM - 11:30AM |
YI2.00004: Toward Petaflop First Principles Kinetic Plasma Simulation Invited Speaker: Due to physical limitations (such as the speed of light), moving data between and even within modern microprocessors is more time consuming than performing computations. As a result, individual processor core performance is stagnant, multicore processors are ubiquitous and traditional programming styles are unable to exploit the potential of modern computers fully. This talk will discuss the architecture and implementation of the 3d electromagnetic relativistic particle-in-cell code VPIC for LANL's Roadrunner supercomputer. Roadrunner is expected to have 13,000 IBM Cell microprocessors (each Cell contains a dual threaded Power core and 8 specialized vector cores) and be capable of over a petaflop ($10^{15}$ floating point operations per second). VPIC minimizes data movement and allows vector extensions of modern processors to be utilized portably. This made it possible to port VPIC quickly while achieving unprecedented performance. The initial port performed 0.13 billion particles pushed and accumulated per second per Cell---equivalent to 1.0 billion per second per 8 Cell node or sustaining Roadrunner at 0.4 petaflop. Higher performance is likely as the port is refined. Regardless, already demonstrated performance will enable previously intractable simulations in numerous areas of plasma physics, including magnetic reconnection and laser plasma interactions. [Preview Abstract] |
Friday, November 16, 2007 11:30AM - 12:00PM |
YI2.00005: Noninvariance of Space- and Time-Scale Ranges under a Lorentz Transformation and the Implications for the Study of Relativistic Interactions Invited Speaker: We present an analysis which shows that the ranges of space and time scales spanned by a system are not invariant under Lorentz transformation [1]. This implies the existence of a frame of reference which minimizes an aggregate measure of the range of space and time scales. Such a frame is derived, for example, for the following cases: free electron laser, laser-plasma accelerator, and particle beams interacting with electron clouds. The implications for experimental, theoretical, and numerical studies are discussed. The most immediate relevance is the reduction by orders of magnitude in computer simulation run times for such systems. A speed-up of 1000 was obtained on a proof-of-principle Particle-In-Cell simulation of a relativistic proton beam experiencing a hose-like instability as propagating through a high density of electrons in a uniform focusing channel. We are in the process of upgrading our simulation tools to be in a position to perform simulations in the optimal frame, and apply them to actual situation for the identified areas of applications. We will present results, as available. [1] Phys. Rev. Lett. 98, 130405 (2007) [Preview Abstract] |
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