Proton acceleration mechanisms in high-intensity laser interaction with thin foils

Abstract

International audienceThe interaction of short and intense laser pulses with plasmas or solids is a very efficient source of high-energy ions. This paper reports the detailed study, with particle-in-cell simulations, of the interaction of such a laser pulse with thin, dense targets, and the resulting proton acceleration. Depending on the laser intensity and pulse duration, the most energetic protons are found to come from the front, the core, or the back of the target. The main accelerating mechanisms discussed in this paper are plasma expansion acceleration, where proton acceleration is driven by the hot electron population, and shock acceleration, originating from the laser ponderomotive potential imposed at the front target surface. Three main regimes of proton acceleration are defined and the parameters for which each regime is dominant are obtained. For irradiances close to 10^20 W/cm^2, the highest proton energies are obtained from thin foils efficiently heated by relativistic transparency. At larger intensities, a complex interplay between collisionless shock acceleration and plasma expansion acceleration is evidenced

Investigations of Field Dynamics in Laser Plasmas with Proton Imaging

“…A complex interaction of laser pulse intensity, target thickness and temporal contrast (plasma scale length) causes the dominance of one of the mechanisms. In reference [74] a scenario is discussed where both mechanism can act together to accelerate ions to higher energies. The ions are first accelerated at the front, then they propagat through the target and are accelerated further by the electrostatic field at the rear side.…”

“…In the so-called transparent regime for example, a part of the laser pulse can be transmitted through thin (< 1 µm) targets causing a superior electron heating and proton acceleration at the target rear surface [74]. Effects like relativistic transparency [74] or Coulomb explosion [79] become important.…”

“…Effects like relativistic transparency [74] or Coulomb explosion [79] become important. Illuminating even thinner targets (tens of nanometers) with circularly polarized, intense laser pulses with a high contrast, reference [80] predicts the generation of highly energetic and mono-energetic ions.…”

“…As discussed in Chapter 3.1.3 the laser radiation pressure can accelerate the critical surface forward in laser direction (Hole Boring). This can launch an ion acoustic wave into the target which can evolve into an electrostatic shock [74] which accelerate the ions. One of the arguments for this mechanism is the observed ring structure in the proton beam which can be explained by magnetic fields which arise due to the propagation through the target [73].…”

“…Therefore the intensity has to be high enough to generate fast ions due to the shock. Additionally, the electrostatic field at the rear side has to last long enough so that the ions can still see a strong field when they exit the target rear side [74].…”