Vlasov Simulations of Trapping and Inhomogeneity in Raman Scattering

We study stimulated Raman scattering (SRS) in laser-fusion conditions with the Eulerian Vlasov code ELVIS. Back SRS from homogeneous plasmas occurs in sub-picosecond bursts and far exceeds linear theory. Forward SRS and re-scatter of back SRS are also observed. The plasma wave frequency downshifts from the linear dispersion curve, and the electron distribution shows flattening. This is consistent with trapping and reduces the Landau damping. There is some acoustic ($\omega\propto k$) activity and possibly electron acoustic scatter. Kinetic ions do not affect SRS for early times but suppress it later on. SRS from inhomogeneous plasmas exhibits a kinetic enhancement for long density scale lengths. More scattering results when the pump propagates to higher as opposed to lower density.Comment: 4 pages, 6 figures. Submitted to "Journal of Plasmas Physics" for the conference proceedings of the 19th International Conference on Numerical Simulation of Plasma

Threshold for Electron Trapping Nonlinearity in Langmuir Waves

We assess when electron trapping nonlinearity is expected to be important in Langmuir waves. The basic criterion is that the inverse of the detrapping rate nu_d of electrons in the trapping region of velocity space must exceed the bounce period of deeply-trapped electrons, tau_B = (n_e/delta n)^{1/2} 2pi/omega_pe. A unitless figure of merit, the "bounce number" N_B = 1/(nu_d tau_B), encapsulates this condition and defines a trapping threshold amplitude for which N_B=1. The detrapping rate is found for convective loss (transverse and longitudinal) out of a spatially finite Langmuir wave. Simulations of driven waves with a finite transverse profile, using the 2D-2V Vlasov code Loki, show trapping nonlinearity increases continuously with N_B for transverse loss, and is significant for N_B ~ 1. The detrapping rate due to Coulomb collisions (both electron-electron and electron-ion) is also found, with pitch-angle scattering and parallel drag and diffusion treated in a unified manner. A simple way to combine convective and collisional detrapping is given. Application to underdense plasma conditions in inertial confinement fusion targets is presented. The results show that convective transverse loss is usually the most potent detrapping process in a single f/8 laser speckle. For typical plasma and laser conditions on the inner laser cones of the National Ignition Facility, local reflectivities ~3% are estimated to produce significant trapping effects.Comment: 16 pages, 15 figures, accepted for publication in Phys. Plasma

Vlasov simulations of kinetic enhancement of Raman backscatter in laser fusion plasmas

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 151-156).Stimulated Raman scattering (SRS) is studied in plasmas relevant to inertial confinement fusion (ICF). The Eulerian Vlasov-Maxwell code ELVIS was developed and run for this purpose. Plasma waves are heavily Landau damped in the regimes of interest, and coupled-mode theory predicts back-scattered SRS is a convective instability. Simulations in a finite length, homogeneous plasma show electron trapping drastically elevates the reflected light over convective gain values ("kinetic enhancement"). Average reflectivities are [approx.] 10%, while the instantaneous reflectivity is chaotic and does not reach a steady state. Trapping reduces the plasma-wave Landau damping and downshifts the observed frequencies from their linear values. Two longitudinal acoustic (? ? k) features and light from possible stimulated electron acoustic scattering (SEAS) are present. The phase-matched SEAS plasmon lies on the observed acoustic mode with phase velocity 1.3(Te/me)1/2. As the pump laser intensity is increased or the electron temperature is decreased, SRS transitions sharply from the coupled-mode steady state to kinetically enhanced levels. Enhancement happens for different back SRS seed levels and monochromatic or broadband seeds. Simulations with a Krook relaxation operator to mimic speckle sideloss display enhancement when resonant electrons complete a bounce orbit before escaping, with a sharp onset as the relaxation rate varies. The sudden development of kinetic enhancement as parameters change suggests trapping makes SRS absolutely unstable.(cont.) Simulations with mobile ions give kinetic enhancement until a burst of activity occurs near the laser entrance, after which back SRS is low. The burst contains several Brillouin and Raman re-scatters and subsequent Langmuir decay instability (LDI), although no LDI of back SRS is seen. SRS runs in a density gradient show kinetic enhancement for long scale lengths and coupledmode convective levels for shorter ones. The reflectivity is higher when the pump propagates toward higher, rather than lower, density. The amplitude of externally-driven plasma waves in a density gradient is also enhanced over linear levels and displays a similar directional asymmetry. These results imply kinetic enhancement of SRS may be a concern in hohlraum plasmas for ICF experiments such as the National Ignition Facility.by David J. Strozzi.Ph.D

Kinetic Enhancement of Raman Backscatter, and Electron Acoustic Thomson Scatter

1-D Eulerian Vlasov-Maxwell simulations are presented which show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73, 025401 (2006)]. For the first time, a low phase velocity electron acoustic wave (EAW) is seen developing from the self-consistent Raman physics. Backscatter of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single hot spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is not produced by it. The beating of different beam acoustic modes is proposed as the EAW excitation mechanism, and is called beam acoustic decay. Supporting evidence for this process, including bispectral analysis, is presented. The linear electrostatic modes, found by projecting the numerical distribution function onto a Gauss-Hermite basis, include beam acoustic modes (some of which are unstable even without parametric coupling to light waves) and a strongly-damped EAW similar to the observed one. This linear EAW results from non-Maxwellian features in the electron distribution, rather than nonlinearity due to electron trapping.Comment: 15 pages, 16 figures, accepted in Physics of Plasmas (2006

Fast-ignition design transport studies: realistic electron source, integrated PIC-hydrodynamics, imposed magnetic fields

Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell [PIC] code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum, and a divergent angle spectrum (average velocity-space polar angle of 52 degrees). Transport simulations with the PIC-based divergence do not ignite for > 1 MJ of fast-electron energy, for a modest 70 micron standoff distance from fast-electron injection to the dense fuel. However, artificially collimating the source gives an ignition energy of 132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields. Uniform fields ~50 MG are sufficient to recover the artificially collimated ignition energy. Experiments at the Omega laser facility have generated fields of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such imploded fields are however more compressed in the transport region than in the laser absorption region. When fast electrons encounter increasing field strength, magnetic mirroring can reflect a substantial fraction of them and reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite radius, is presented as one field configuration which circumvents mirroring.Comment: 16 pages, 17 figures, submitted to Phys. Plasma