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 Iλ 2 ∼ = (0.3 − 1.2) × 10 21 Wcm −2 µm 2 in the plasma target with the electron density of n e λ 2 ∼ 10 23 cm −3 µm 2 , 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 Iλ 2 ∼ = (10 18 − 10 20 ) Wcm −2 µm 2 , a scaling in vague accordance with current published estimates in the range Iλ 2 ∼ = (0.14 − 3.5) × 10 21 Wcm −2 µm 2 , and again a distinct power increase beyond I = 3.5 × 10 21 Wcm −2 µm 2 . The low energy electrons penetrate normally to the target surface, the energetic electrons propagate in laser beam direction.

Electron heating in subpicosecond laser interaction with overdense and near-critical plasmas

“…However, this process is known to depend strongly on several experimental parameters. In experiments with ultraintense (I > 10 18 W/cm 2 ), ultrashort (sub * lorenzo.cialfi@polimi.it ps) pulses and sharp-edged micrometric plain solid targets (SLTs) two collisionless mechanisms dominate the electron heating [13,14]: the Brunel effect and j × B heating. In both cases, electrons are extracted from the skin layer and reinjected into the target by the action of the normal component of the electric field in the former and the oscillating component of the j × B force in the latter.…”

Amplification of Relativistic Electron Bunches by Acceleration in Laser Fields

“…In consequence, the target foil is pre-heated and starts to expand. It was shown in recent experiments for solid bulk targets and an oblique laser incidence angle, that a specifically prepared plasma density gradient increases the number of emitted fast electrons [2,3] either by an injection of slow electrons into the (reflected) transient laser field and a followed acceleration, or by a resonant process in the skin layer [22,33]. This observation gives an interesting insight to the concurrent laser ion acceleration, since the higher ion acceleration efficiency is obtained with the UHC condition for an optimized foil thickness [34 -36].…”

Anticorrelated Emission of High Harmonics and Fast Electron Beams From Plasma Mirrors

“…When the laser pulse reflects on this plasma mirror, for every oscillation of the laser field, some electrons are driven towards vacuum and sent back to the plasma [16,17]. These bunches of so-called Brunel electrons [18] impulsively excite collective high-frequency plasma oscillations in the density gradient that lead to the emission of XUV radiation through linear mode conversion [19].…”