LAUSR

The distribution function of electrons accelerated by intense laser pulses at steep vacuum-plasma interfaces is investigated by using the Fokker-Planck equation and methods from extreme statistics. The energy spectrum of electrons penetrating into the dense plasma after being accelerated at the interface and in the preplasma shows a systematic cutoff-like decrease in the momentum component p x / m e c along the laser propagation axis. While the distribution associated with the kinetic energy spectrum (Ekin) is often approximated by a thermal distribution, F ( E kin ) ∝ exp ( − E kin / T h ), with a hot particle temperature Th, the nature of the distribution close to the cutoff is clearly nonthermal. Electron distributions are analyzed here from two-dimensional Particle-in-Cell simulations. Via a comparison with solutions derived from a Fokker-Planck equation and based on Chirikov's standard map models, we find that the electron distributions show a clear signature of stochastic heating, due to repeated acceleration in the standing wave in the preplasma. Further analysis of the solutions to the Fokker-Planck equation allows us to describe the cutoff seen in the momentum p of the distributions F(p), which can be expressed as a function of time τ in the form F ( p , τ ) ∝ [ ( p max − p ) / δ p ] exp ( − 2 p 3 / 9 τ ), portraying a time-dependent cutoff at p → p max. This implies that the energetic tail of the distribution belongs to the maximum domain of attraction of the Weibull law, which means that the probability to find high-energy electrons varies abruptly near pmax. The variance of physical observables sensitive to the high-energy tail is consequently considerably higher than when assuming thermal distribution.The distribution function of electrons accelerated by intense laser pulses at steep vacuum-plasma interfaces is investigated by using the Fokker-Planck equation and methods from extreme statistics. The energy spectrum of electrons penetrating into the dense plasma after being accelerated at the interface and in the preplasma shows a systematic cutoff-like decrease in the momentum component p x / m e c along the laser propagation axis. While the distribution associated with the kinetic energy spectrum (Ekin) is often approximated by a thermal distribution, F ( E kin ) ∝ exp ( − E kin / T h ), with a hot particle temperature Th, the nature of the distribution close to the cutoff is clearly nonthermal. Electron distributions are analyzed here from two-dimensional Particle-in-Cell simulations. Via a comparison with solutions derived from a Fokker-Planck equation and based on Chirikov's standard map models, we find that the electron distributions show a clear signature of stochastic heating, d...