Collisionless plasma expansion into a vacuum

Abstract

Particle simulations of the expansion of a collisionless plasma into vacuum are presented. The cases of a single-electron-temperature plasma and of a two-electron-temperature plasma are considered. The results confirm the existence of an ion front and verify the general features of self-similar solutions behind this front. A cold electron front is clearly observed in the two-electron-temperatures case. The computations also show that for a finite electron-to-ion mass ratio, me/mi, the electron thermal velocity in the expansion region is not constant, but decreases approximately linearly with ξ=x/t, where x is distance and t is time. A self-similar solution, derived from the relation Ten1−γe=const, where Te is the electron temperature, ne is the electron density, and γ is a constant (instead of the isothermal assumption made in earlier theories), yields a linearly decreasing ion acoustic speed, c≃c0−(γ−1) ξ/2, and comparison with computer simulation results show that the constant γ−1 is proportional to (Zme/mi)1/2, where Z is the ion charge number.

Monoenergetic proton beams from laser driven shocks

“…The red vertical line at c/ω p = 600 shows where the initial boundary between plasma 1 and 2 was at t = 0. In both cases, the characteristic rarefaction wave associated with plasma expansion into vacuum can be seen at x = 470 (c/ω p ) traveling in the negative x direction at 0.0275c (∼C s ), in good agreement with theory [25]. In the case of Γ = 2 where the plasmas have similar densities, plasma 2 has impeded the expansion of plasma 1 into vacuum at a high density (n ∼ 0.75n cr ) where the expansion velocity is ∼0.009c (0.31C s ).…”

Electron, photon, and ion beams from the relativistic interaction of Petawatt laser pulses with solid targets

“…The overall intensity I 0 shows order of magnitude shot-to-shot variation for the same laser energy, although the upper envelope of the data appears to increase with laser energy. We note that choosing a value of 5x10 11 for I 0 and 4 MeV for T gives 1.3 J/sr when integrated across the whole spectrum and >5J when integrated over all directions assuming a 100º FWHM. Figure 3 shows the forward hemisphere average rads-at-1-meter seen by the TLD array plotted against laser energy for a number of shots.…”

Multispecies plasma expansion into vacuum: The role of secondary ions and suprathermal electrons

“…Physically speaking, this assumption is justified if the system's scale is much larger than the characteristic Debye length, and the flow of both ions and electrons is sufficient smooth (laminar) on the ionic timescale. As a matter of fact, the shape of the expanding plasma front has been investigated via numerical simulation in a number of studies in the past [25,34,62,63], which corroborated the general features of the self-similar solution. As a matter of fact, the aforementioned studies have addressed the expansion of a collisionless cold-ion fluid against an electron cloud obeying either a step-like or a Maxwell-Boltzmann distribution.…”

“…The self similar solution is worthy and useful in its own merit for the description of the plasma expansion in a simple and analytically tractable way. Numerical simulations suggest that, with the passage of time, the expanding plasma profile approaches the form provided by the self-similar methodology [25,34,62,63]. Furthermore, we have assumed that the electron temperature remains constant throughout the plasma expansion process.…”