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Register a new userEnhanced laser-driven ion acceleration by superponderomotive electrons generated from near-critical-density plasma
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We report on the experimental studies of laser driven ion acceleration from double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer thin diamond-like carbon foil. A significant enhancement of proton maximum energies from 12 to ~30 MeV is observed when relativistic laser pulse impinge on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.
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We demonstrate laser-driven Helium ion acceleration with cut-off energies above 25 MeV and peaked ion number above $10^8$ /MeV for 22(2) MeV projectiles from near-critical density gas jet targets. We employed shock gas jet nozzles at the high-repetition-rate (HRR) VEGA-2 laser system with 3 J in pulses of 30 fs focused down to intensities in the range between $9times10^{19}$ W/cm$^2$ and $1.2times10^{20}$ W/cm$^2$. We demonstrate acceleration spectra with minor shot-to-shot changes for small variations in the target gas density profile. Difference in gas profiles arise due to nozzles being exposed to a experimental environment, partially ablating and melting.
Ion acceleration driven by superintense laser pulses is attracting an impressive and steadily increasing effort. Motivations can be found in the potential for a number of foreseen applications and in the perspective to investigate novel regimes as far as available laser intensities will be increasing. Experiments have demonstrated in a wide range of laser and target parameters the generation of multi-MeV proton and ion beams with unique properties such as ultrashort duration, high brilliance and low emittance. In this paper we give an overview of the state-of-the art of ion acceleration by laser pulses as well as an outlook on its future development and perspectives. We describe the main features observed in the experiments, the observed scaling with laser and plasma parameters and the main models used both to interpret experimental data and to suggest new research directions.
During the interaction of intense femtosecond laser pulses with various targets, the natural mechanisms of laser energy transformation inherently lack temporal control and thus commonly do not provide opportunities for a controlled generation of a well-collimated, high-charge beam of ions with a given energy of particular interest. In an effort to alleviate this problem, it was recently proposed that the ions can be dragged by an electron bunch trapped in a controllably moving potential well formed by laser radiation. Such standing-wave acceleration (SWA) can be achieved through reflection of a chirped laser pulse from a mirror, which has been formulated as the concept of chirped-standing-wave acceleration (CSWA). Here we analyze general feasibility aspects of the SWA approach and demonstrate its reasonable robustness against field structure imperfections, such as those caused by misalignment, ellipticity and limited contrast. Using this we also identify prospects and limitations of the CSWA concept.
Laser-driven collisonless electrostatic shock formation and the subsequent ion acceleration have been studied in near critical density plasmas. Particle-in-cell simulations show that both the speed of laser-driven collisionless electrostatic shock and the energies of shock-accelerated ions can be greatly enhanced due to fast laser propagation in near critical density plasmas. However, a response time longer than tens of laser wave cycles is required before the shock formation in a near critical density plasma, in contrast to the quick shock formation in a highly overdense target. More important, we find that some ions can be reflected by the collisionless shock even if the electrostatic potential jump across the shock is smaller than the ion kinetic energy in the shock frame, which seems against the conventional ion-reflection condition. These anomalous ion reflections are attributed to the strongly time-oscillating electric field accompanying laser-driven collisionless shock in a near critical density plasma.
Magnetic Vortex Acceleration (MVA) from near critical density targets is one of the promising schemes of laser-driven ion acceleration. 3D particle-in-cell simulations are used to explore a more extensive laser-target parameter space than previously reported on in the literature as well as to study the laser pulse coupling to the target, the structure of the fields, and the properties of the accelerated ion beam in the MVA scheme. The efficiency of acceleration depends on the coupling of the laser energy to the self-generated channel in the target. The accelerated proton beams demonstrate high level of collimation with achromatic angular divergence, and carry a significant amount of charge. For PW-class lasers, this acceleration regime provides favorable scaling of maximum ion energy with laser power for optimized interaction parameters. The mega Tesla-level magnetic fields generated by the laser-driven co-axial plasma structure in the target are prerequisite for accelerating protons to the energy of several hundred MeV.
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