J. M. Wallace

An analytical and numerical investigation of ion acoustic waves in a two‐ion plasma

The ion acoustic dispersion relation for a plasma containing two distinct ion species is investigated over a wide range of plasma conditions. An approximate general analytic solution to the dispersion relation has been found, and is shown, by comparison to accurate numerical solutions of the individual modes, to be remarkably precise. This solution provides for the first time a systematic account of the totality of ion acoustic modes of the two‐ion system. It has been found that ion acoustic modes consist of two types of modes: (a) at least one, and, at most, two weakly damped modes for which ‖ωI/ωR‖≪1, and (b) an infinity of critically damped modes for which ωI/ωR≂−1. The critically damped modes are organized into two distinct categories: (a) modes for which ‖ω‖/k≳vF (vF is the thermal speed of the fast ion species); and (b) modes for which vS<‖ω‖/k<vF (vS is the thermal speed of the slow ion species). The critically damped modes with ‖ω‖/k≳vF are further organized into three distinct classes: (1) modes ...

Symmetry experiments in gas‐filled hohlraums at NOVA

Understanding drive symmetry in gas‐filled hohlraums is currently of interest because the baseline design of the indirect drive ignition target for the planned National Ignition Facility uses a gas‐filled hohlraum. This paper reports on the results of a series of experiments performed at the Nova laser [C. Bibeau et al. Appl. Opt. 31, 5799 (1992)] facility at Lawrence Livermore National Laboratory with the goal of understanding time‐dependent drive symmetry in gas filled hohlraums. Time‐dependent symmetry data from capsule implosions and reemission targets in gas‐filled hohlraums are discussed. Results of symmetry measurements using thin wall gas‐filled hohlraums are also discussed. The results show that the gas is effective in impeding the motion of the wall blowoff material, and that the resulting implosion performance of the capsule is not significantly degraded from vacuum results. The implosion symmetry in gas differs from vacuum results with similar laser pointing indicating a shift in beam position...

The role of symmetry in indirect‐drive laser fusion

Good radiation drive symmetry will be crucial for achieving ignition in laboratory inertial fusion experiments. The indirect‐drive inertial confinement fusion (ICF) method utilizes the soft x‐ray field in a radiation‐containing cavity, or hohlraum, to help achieve a high degree of symmetry. Achievement of the conditions necessary for ignition and gain will require experimental fine tuning of the drive symmetry. In order to make tuning possible, a significant effort has been devoted to developing symmetry measurement techniques. These techniques have been applied to a series of experiments that give a graphic picture of the symmetry conditions in the complex hohlraum environment. These experiments have been compared with detailed, fully integrated theoretical modeling. The ultimate goal of this work is the detailed understanding of symmetry conditions and the methods for their control. Comparison with experiments provides crucial benchmarking for the modeling—a key element in planning for ignition.

Shock structuring due to fabrication joints in targets

The use of copper-doped beryllium ablators on National Ignition Facility [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] targets, in place of plastic, can require the bonding together of hemispheres with a joint of differing composition. Indirect drive experiments have been conducted on the Nova laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)], and the resulting shock structuring compared with code simulations. It is concluded that one of the available codes, the RAGE code [R. M. Baltrusaitis et al., Phys. Fluids 8, 2471 (1996)] provides useful insight into the effect of joints. This code is then employed to obtain a physical picture of the shock front nonuniformity in terms of a secondary rarefaction and an oblique shock interacting with the main shock that propagates in the absence of the joint. A simple analysis reinforces this picture.

Demonstration of time-dependent symmetry control in hohlraums by drive-beam staggering

Indirect-drive inertial confinement fusion makes use of cavities constructed of high atomic number materials to convert laser power into x-rays for ablatively driving an implosion capsule. Obtaining spatially uniform drive on the capsule requires a careful balancing between the laser absorption region (high drive) and the laser entrance holes (low drive). This balancing is made difficult because of plasma expansion, and the associated movement of the laser absorption region with time. This paper reports the first experimental demonstration of compensation for this motion by using different laser beams at different times, in agreement with modeling.

Moderate-convergence inertial confinement fusion implosions in tetrahedral hohlraums at Omega

A highly uniform thermal x-radiation field for indirect-drive inertial confinement fusion implosions may be obtained by irradiating a four-hole, tetrahedral geometry, spherical hohlraum with all 60 Omega laser beams. Implosion studies and calculations [J. M. Wallace et al., Phys. Rev. Lett. 82, 3807 (1999)] indicate a drive uniformity comparable to that expected for the National Ignition Facility [J. A. Painser et al., Laser Focus World 30, 75 (1994)]. With 60 beams distributed over the cavity wall, tetrahedral hohlraums have a natural insensitivity to power balance and pointing errors. Standard, smooth Nova capsules imploded with this drive indicate that moderate convergence-ratio implosions, Cr∼18, have measured-neutron yield to calculated-clean-one-dimensional-neutronyield ratios similar to those previously investigated using the comparatively poor drive uniformity of Nova cylindrical hohlraums. This may indicate that a nonsymmetry-related neutron yield degradation mechanism, e.g., hydrodynamic mixing ...

Indirect drive experiments utilizing multiple beam cones in cylindrical hohlraums on OMEGA

Current plans for time-dependent control of flux asymmetry in the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, and M. Sorem, “The National Ignition Facility Project,” ICF Quart. 5, 110 (1995)] hohlraums rely on multiple beam cones with different laser power temporal profiles in each cone. Experiments with multiple beam cones have begun on the Omega laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the University of Rochester. In addition to allowing symmetry experiments similar to those performed on Nova [A. Hauer et al., Rev. Sci. Instrum. 66, 672 (1995)], the Omega facility allows multiple beam cones to be moved independently to confirm our ability to model the resulting implosion image shapes. Results indicate that hohlraum symmetry behaves similarly with multiple rings of beams as with a single ring, but with the weighted beam spot position used to parametrize the beam pointing.

Convective threshold of stimulated Brillouin scattering in a two‐ion plasma

The convective threshold of stimulated Brillouin backscatter (SBS) in a laser‐irradiated plasma containing two distinct ion species is investigated over a wide range of plasma conditions and laser intensities. This investigation, carried out in the context of kinetic theory, is a generalization of a previous investigation of ion acoustic waves in a two‐ion plasma, i.e., the unpumped system. The totality of SBS modes in the two‐ion system is obtained using a numerical sweep through the complex‐ω plane. The coupling of the electromagnetic pump wave to the plasma produces only one mode, in addition to the ion acoustic modes. Because this additional mode is not a normal mode of the plasma in the absence of the pump wave, it is called a quasimode. The SBS modes fall into three classes: (a) at least one and, at most, two modes that are weakly damped for sufficiently low laser intensity, reducing to the weakly damped ion acoustic modes as the pump strength vanishes; (b) an infinity of critically damped modes, mo...

Convective gain of stimulated Brillouin scattering in long‐scale length, two‐ion‐component plasmas

The linear kinetic theory is developed for the convective amplification of stimulated Brillouin scattering in a plasma containing two distinct ion species. A computationally tractable expression for the gain coefficient Q is obtained by, first, restricting consideration to growth from the two possible weakly damped ion modes in the two‐ion‐species plasma and, second, invoking the two‐mode approximation for e−1, the ion response function, in the plasma. Furthermore, a practical procedure is presented for efficiently obtaining Q over the mesh of a large‐scale hydrodynamic simulation of a laser‐irradiated target. The theory and its application are demonstrated in the simulation of a simple gas target.

An adiabatic electron fluid particle-in-cell method for simulating ion-driven parametric instabilities

Summary form only given, as follows. A hybrid particle-in-cell method is presented in which the electrons are modeled as an adiabatic fluid with an arbitrary ratio of specific heats /spl gamma/. The electromagnetic field model is based on a temporal WKB approximation which results in a Schrodinger equation for the field envelope. The method is a new tool for simulating ion-driven parametric instabilities which often exist in laser-produced plasmas. The method is general, and does not depend on the number of spatial dimensions. The method will model the plasma behavior correctly even in situations where the electron Debye shielding is not negligible . (For the current inertial confinement fusion regime of interest, k/spl lambda//sub De/=O(1), i.e., the Debye shielding effect is significant.) Test simulations of ion Landau damping and of stimulated Brillouin scattering in both one and two dimensions are performed, and the results are in excellent agreement with linear Vlasov theory.