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Wakes in Inertial Fusion Plasmas

  • Author(s): Ellis, Ian Norman
  • Advisor(s): Mori, Warren B
  • et al.
Abstract

Plasma wave wakes, which are the collective oscillatory response near the plasma frequency to the propagation of particles or electromagnetic waves through a plasma, play a critical role in many plasma processes. New results from backwards stimulated Raman scattering (BSRS), in which wakes with phase velocities much less than the speed of light are induced by the beating of counter-propagating light waves, and from electron beam stopping, in which the wakes are produced by the motion of relativistically propagating electrons through the dense plasma, are discussed. Both processes play important roles in Inertial Confinement Fusion (ICF). In BSRS, laser light is scattered backwards out of the plasma, decreasing the energy available to compress the ICF capsule and affecting the symmetry of where the laser energy hits the hohlraum wall in indirect drive ICF. The plasma wave wake can also generate superthermal electrons that can preheat the core and/or the ablator. Electron beam stopping plays a critical role in the Fast Ignition (FI) ICF concept, in which a beam of relativistic electrons is used to heat the target core to ignition temperatures after the compression stage. The beam stopping power determines the effectiveness of the heating process. This dissertation covers new discoveries on the importance of plasma wave wakes in both BSRS and electron beam stopping.

In the SRS studies, 1D particle-in-cell (PIC) simulations using OSIRIS are performed, which model a short-duration (~500/&omega0 FWHM) counter-propagating scattered light seed pulse in the presence of a constant pump laser with an intensity far below the absolute instability threshold for plasma waves undergoing Landau damping. The seed undergoes linear convective Raman amplification and dominates over the amplification of fluctuations due to particle discreteness. The simulation results are in good agreement with results from a coupled-mode solver when special relativity and the effects of finite size PIC simulation particles are accounted for. Linear gain spectra including both effects are discussed. Extending the PIC simulations past when the seed exits the simulation domain reveals bursts of large-amplitude scattering in many cases, which do not occur in simulations without the seed pulse. These bursts can have amplitudes several times greater than the amplified seed pulse, and an examination of the orbits of particles trapped in the wake illustrates that the bursts are caused by a reduction of Landau damping due to particle trapping. This large-amplitude scattering is caused by the seed inducing a wake earlier in the simulation, thus modifying the distribution function. Performing simulations with longer duration seeds leads to parts of the seeds reaching amplitudes several times more than the steady-state linear theory results, similarly caused by a reduction of Landau damping. Simulations with continuous seeds demonstrate that the onset of inflation depends on the seed wavelength and incident intensity, and oscillations in the reflectivity are observed at a frequency equal to the difference between the seed frequency and the frequency at which the inflationary SRS grows.

In the electron beam stopping studies, 3D PIC simulations are performed of relativistic electrons with a momentum of 10mec propagating in a cold FI core plasma. Some of the simulations use one simulation particle per real particle, and particle sizes much smaller than the interparitcle spacing. The wake made by a single electron is compared against that calculated using cold fluid theory assuming the phase velocity of the wake is near the speed of light. The results agree for the first wavelength of the wake. However, the shape of the wake changes for succeeding wavelengths and depends on the background plasma temperature, with the concavity pointing in the direction the electron is moving in cold plasmas and in the opposite direction as the plasma temperature increases. In the warm plasma the curvature is described by electrostatic Vlasov theory (for vparticle >> vth) and is due to the diffraction of the wave, while for cold plasmas the curvature is due to nonlinear radial oscillations of background electrons. Beams with multiple electrons exhibit correlation effects caused by electrons interacting through their wakes. Non-divergent beams are simulated, and a significant time-dependent increase in the stopping power is observed when the average electron spacing is 2c/&omegape or less. This increase is caused by beam-plasma-like instabilities including self-focusing and/or filamentation and the beam-plasma-like instability. The stopping power growth rate and peak value depend on the beam size and density. For long beams with dimensions of 10c/&omegape × 10c/&omegape × 80c/&omegape and an inter-particle separation of 0.25c/&omegape (nb/n0 ≈ 4× 10-3), the peak stopping power averaged over the electrons is (1 ± 3) × 103 times that of an uncorrelated electron. These results indicate that an enhanced energy-independent or weakly dependent correlated stopping may occur for Fast Ignition scenarios, even for interparticle spacings when discreteness effects are important. The dependence of correlation effects on beam electron separation in terms of c/&omegape also indicates that Fast Ignition may be possible with core densities below those designed using single-electron stopping powers. Target optimization to exploit correlated stopping in the target core may be possible once the effects of angular spread and energy spread are understood. Furthermore, this work begins to allow a connection from the discrete wakes effect to collective instabilities as the interparticle spacing is decreased relative to the size of the wake due to the use of denser beams, lower plasma densities, and the filamentation/self-focusing of the beam.

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