Ion acceleration in short-laser-pulse interaction with solid foils

Abstract

We discuss the physical processes, which take place in a multi-component plasma set in expansion by a minority of energetic electrons. The expansion is in the form of a collisionless rarefaction wave associated with three types of electrostatic shocks. Each shock manifests itself in a potential jump and in the spatial separation of plasma species. The shock front associated with the proton-electron separation sets the maximum proton velocity. Two other shocks are due to the hot-cold electron separation and the light-heavy ion separation. They result in the light ion acceleration and their accumulation in the phase space. These structures open possibilities for control of the number and the energy spectrum of accelerated ions. Simple analytical models are confirmed in numerical simulations where the ions are described kinetically, and the electrons assume the Boltzmann distribution.

Laser-based Particle Acceleration

“…Similar to the TNSA process at the proton front, the light protons from the low energy part of the spectrum are efficiently accelerated across the carbon boundary by the second sheath field, whereupon they immediately enter the zerofield region and remain in a state of motion of uniform ballistic flow. Note that the potential of charge separation for the spectral modulation of laser-accelerated ion beams had been identified in earlier works already (Tikhonchuk et al, 2005;Robinson et al, 2006). The confined TNSA scheme has been confirmed for the first time experimentally by our group Pfotenhauer et al, 2008).…”

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

“…Interestingly, secondary ions (rather than electrons) have recently been argued to contribute to ultrafast collisional plasma heating by electrostatic shocks [57]. The role of a minority secondary ion population on plasma expansion was investigated earlier in a number of studies [31,[58][59][60][61]. Our work at hand aims at generalizing those earlier models by considering a nonthermal (non-Maxwellian) plasma environment and a finite (arbitrary) admixture of ion components.…”

“…violation of the charge neutrality hypothesis (plasma approximation), numerically [25,30], for electron-ion plasmas. Interestingly, a bi-Maxwellian (two electron temperature) approach was considered in a number of works [31][32][33], described the effect of the co-existence of two thermal electron populations. Other analytical studies have focused on kinetic-theoretical considerations for the electron distribution [34], the effect of instabilities [35] and even magnetic field generation [36], among other effects.…”

“…Our work at hand aims at generalizing those earlier models by considering a nonthermal (non-Maxwellian) plasma environment and a finite (arbitrary) admixture of ion components. From first principles, the presence of secondary ions in an expanding plasma is manifested in the appearance of spectral peaks, which are of interest both in experimental diagnostics and for application purposes [31,59]. As a simplifying hypothesis, one may assume that the plasma doesn't change its shape during the expansion process and that there is no charge separation.…”

Pic simulations of femtosecond interactions with mass-limited targets

“…There are many different applications for such fast ions [1,2]. Various regimes have been discussed in the framework of this concept: the plasma thermal expansion into vacuum (including two electron temperature case) [3], the Coulomb explosion of a strongly ionized cluster [4], transverse explosion of a selffocusing channel [5] and two ion species acceleration [6]. At the front side, ions are accelerated into the target mostly by ponderomotive force induced electrostatic field [7], while on the rear side of thin targets, ions are prevalently accelerated in the sheath formed by fast electrons escaping out of the target rear side [2], this mechanism is called TNSA (target normal sheath acceleration).…”