arXiv subjects

David J. Strozzi

Publications and source records attributed to David J. Strozzi.

4 recordsLinked to original sources

arXiv2021

Stopping Power Enhancement From Discrete Particle-Wake Correlations in High Energy Density Plasmas

Three-dimensional (3D) simulations of electron beams propagating in high energy density (HED) plasmas using the quasi-static Particle-in-Cell (PIC) code QuickPIC demonstrate a significant increase in stopping power when beam electrons mutually interact via their wakes. Each beam electron excites a plasma wave wake of wavelength $\sim2πc/ω_{pe}$, where $c$ is the speed of light and $ω_{pe}$ is the background plasma frequency. We show that a discrete collection of electrons undergoes a beam-plasma like instability caused by mutual particle-wake interactions that causes electrons to bunch in the beam, even for beam densities $n_b$ for which fluid theory breaks down. This bunching enhances the beam's stopping power, which we call "correlated stopping," and the effect increases with the "correlation number" $N_b \equiv n_b (c/ω_{pe})^3$. For example, a beam of monoenergetic 9.7 MeV electrons with $N_b=1/8$, in a cold background plasma with $n_e=10^{26}$ cm$^{-3}$ (450 g cm$^{-3}$ DT), has a stopping power of $2.28\pm0.04$ times the single-electron value, which increases to $1220\pm5$ for $N_b=64$. The beam also experiences transverse filamentation, which eventually limits the stopping enhancement.

physics.plasm-ph↗

arXiv2012

Convective Raman Amplification of Light Pulses Causing Kinetic Inflation in Inertial Fusion Plasmas

We perform 1D particle-in-cell (PIC) simulations using OSIRIS, which model a short-duration (~500/ω0 FWHM) scattered light seed pulse in the presence of a constant counter-propagating pump laser with an intensity far below the absolute instability threshold. The seed undergoes linear convective Raman amplification and dominates over fluctuations due to particle discreteness. Our simulation results are in good agreement with results from a coupled mode solver when we take into account special relativity and the use of finite size PIC simulation particles. We present linear gain spectra including both effects. Extending the PIC simulations past when the seed exits the simulation domain reveals bursts of large-amplitude scattering in many cases, which does not occur in simulations without the seed pulse. These bursts can have amplitudes several times greater than the amplified seed pulse, and we demonstrate that this large-amplitude scattering is the result of kinetic inflation by examining trapped particle orbits. This large-amplitude scattering is caused by the seed modifying the distribution function earlier in the simulation. We perform some simulations with longer duration seeds, which lead to parts of the seeds undergoing kinetic inflation and reaching amplitudes several times more than the steady-state linear theory results. Simulations with continuous seeds demonstrate that the onset of inflation depends on seed wavelength and incident intensity, and we observe oscillations in the reflectivity at a frequency equal to the difference between the seed frequency and the frequency at which the inflationary SRS grows.

physics.plasm-ph↗

arXiv2009

Nonlinear envelope equation and nonlinear Landau damping rate for a driven electron plasma wave

In this paper, we provide a theoretical description, and calculate, the nonlinear frequency shift, group velocity and collionless damping rate, $ν$, of a driven electron plasma wave (EPW). All these quantities, whose physical content will be discussed, are identified as terms of an envelope equation allowing one to predict how efficiently an EPW may be externally driven. This envelope equation is derived directly from Gauss law and from the investigation of the nonlinear electron motion, provided that the time and space rates of variation of the EPW amplitude, $E_p$, are small compared to the plasma frequency or the inverse of the Debye length. $ν$ arises within the EPW envelope equation as more complicated an operator than a plain damping rate, and may only be viewed as such because $(νE_p)/E_p$ remains nearly constant before abruptly dropping to zero. We provide a practical analytic formula for $ν$ and show, without resorting to complex contour deformation, that in the limit $E_p \to 0$, $ν$ is nothing but the Landau damping rate. We then term $ν$ the "nonlinear Landau damping rate" of the driven plasma wave. As for the nonlinear frequency shift of the EPW, it is also derived theoretically and found to assume values significantly different from previously published ones, assuming that the wave is freely propagating. Moreover, we find no limitation in $k λ_D$, $k$ being the plasma wavenumber and $λ_D$ the Debye length, for a solution to the dispertion relation to exist, and want to stress here the importance of specifying how an EPW is generated to discuss its properties. Our theoretical predictions are in excellent agreement with results inferred from Vlasov simulations of stimulated Raman scattering (SRS), and an application of our theory to the study of SRS is presented.

physics.plasm-ph↗

arXiv2006

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.

physics.plasm-ph↗