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We derive upper and lower bounds on the absorption of ultraintense laser light by solids as a function of fundamental laser and plasma parameters. These limits emerge naturally from constrained optimization techniques applied to a generalization of the laser-solid interaction as a strongly-driven, relativistic, two degree of freedom Maxwell-Vlasov system. We demonstrate that the extrema and the phase-space-averaged absorption must always increase with intensity, and increase most rapidly when $10^{18} < I_L lambda_L^2 < 10^{20}$ W $mu$m$^2/$cm$^{2}$. Our results indicate that the fundamental empirical trend towards increasing fractional absorption with irradiance therefore reflects the underlying phase space constraints.
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The interaction of intense laser beams with plasmas created on solid targets involves a rich non-linear physics. Because such dense plasmas are reflective for laser light, the coupling with the incident beam occurs within a thin layer at the interface between plasma and vacuum. One of the main paradigms used to understand this coupling, known as Brunel mechanism, is expected to be valid only for very steep plasma surfaces. Despite innumerable studies, its validity range remains uncertain, and the physics involved for smoother plasma-vacuum interfaces is unclear, especially for ultrahigh laser intensities. We report the first comprehensive experimental and numerical study of the laser-plasma coupling mechanisms as a function of the plasma interface steepness, in the relativistic interaction regime. Our results reveal a clear transition from the temporally-periodic Brunel mechanism to a chaotic dynamic associated to stochastic heating. By revealing the key signatures of these two distinct regimes on experimental observables, we provide an important landmark for the interpretation of future experiments.
Three-dimensional particle-in-cell simulation is used to investigate the witness proton acceleration in underdense plasma with a short intense Laguerre-Gaussian (LG) laser pulse. Driven by the LG10 laser pulse, a special bubble with an electron pillar on the axis is formed, in which protons can be well-confined by the generated transversal focusing field and accelerated by the longitudinal wakefield. The risk of scattering prior to acceleration with a Gaussian laser pulse in underdense plasma is avoided, and protons are accelerated stably to much higher energy. In simulation, a proton beam has been accelerated to 7 GeV from 1 GeV in underdense tritium plasma driven by a 2.14x1022 W/cm2 LG10 laser pulse.
Production of the huge longitudinal magnetic fields by using an ultraintense laser pulse irradiating a solenoid target is considered. Through three-dimensional particle-in-cell simulations, it is shown that the longitudinal magnetic field up to ten kilotesla can be observed in the ultraintense laser-solenoid target interactions. The finding is associated with both fast and return electron currents in the solenoid target. The huge longitudinal magnetic field is of interest for a number of important applications, which include controlling the divergence of laser-driven energetic particles for medical treatment, fast-ignition in inertial fusion, etc., as an example, the well focused and confined directional electron beams are realized by using the solenoid target.
Generation of relativistic electron (RE) beams during ultraintense laser pulse interaction with plasma targets is studied by collisional particle-in-cell (PIC) simulations. Strong magnetic field with transverse scale length of several local plasma skin depths, associated with RE currents propagation in the target, is generated by filamentation instability (FI) in collisional plasmas, inducing a great enhancement of the divergence of REs compared to that of collisionless cases. Such effect is increased with laser intensity and target charge state, suggesting that the RE divergence might be improved by using low-Z materials under appropriate laser intensities in future fast ignition experiments and in other applications of laser-driven electron beams.
The future applications of the short-duration, multi-MeV ion beams produced in the interaction of high-intensity laser pulses with solid targets will require improvements in the conversion efficiency, peak ion energy, beam monochromaticity, and collimation. Regimes based on Radiation Pressure Acceleration (RPA) might be the dominant ones at ultrahigh intensities and be most suitable for specific applications. This regime may be reached already with present-day intensities using circularly polarized (CP) pulses thanks to the suppression of fast electron generation, so that RPA dominates over sheath acceleration at any intensity. We present a brief review of previous work on RPA with CP pulses and a few recent results. Parametric studies in one dimension were performed to identify the optimal thickness of foil targets for RPA and to study the effect of a short-scalelength preplasma. Three-dimensional simulations showed the importance of ``flat-top radial intensity profiles to minimise the rarefaction of thin targets and to address the issue of angular momentum conservation and absorption.
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