Collisionless absorption, hot electron generation, and energy scaling in intense laser-target interaction

Abstract

Among the various attempts to understand collisionless absorption of intense and superintense ultrashort laser pulses, a whole variety of models and hypotheses has been invented to describe the laser beam target interaction. In terms of basic physics, collisionless absorption is understood now as the interplay of the oscillating laser field with the space charge field produced by it in the plasma. A first approach to this idea is realized in Brunel's model the essence of which consists in the formation of an oscillating charge cloud in the vacuum in front of the target, therefore frequently addressed by the vague term “vacuum heating.” The investigation of statistical ensembles of orbits shows that the absorption process is localized at the ion-vacuum interface and in the skin layer: Single electrons enter into resonance with the laser field thereby undergoing a phase shift which causes orbit crossing and braking of Brunel's laminar flow. This anharmonic resonance acts like an attractor for the electrons and leads to the formation of a Maxwellian tail in the electron energy spectrum. Most remarkable results of our investigations are the Brunel like spectral hot electron distribution at the relativistic threshold, the minimum of absorption at Iλ2≅(0.3−1.2)×1021 Wcm−2μm2 in the plasma target with the electron density of neλ2∼1023cm−3μm2, the drastic reduction of the number of hot electrons in this domain and their reappearance in the highly relativistic domain, and strong coupling, beyond expectation, of the fast electron jets with the return current through Cherenkov emission of plasmons. The hot electron energy scaling shows a strong dependence on intensity in the moderately relativistic domain Iλ2≅(1018−1020) Wcm−2μm2, a scaling in vague accordance with current published estimates in the range Iλ2≅(0.14−3.5)×1021 Wcm−2μm2, and again a distinct power increase beyond I=3.5×1021 Wcm−2μm2. The low energy electrons penetrate normally to the target surface, the energetic electrons propagate in laser beam direction.