Synchronized Ion Acceleration by Ultraintense Slow Light

Abstract

An effective scheme of synchronized laser-triggered ion acceleration and the corresponding theoretical model are proposed for a slow light pulse of relativistic intensity, which penetrates into a near-critical-density plasma, strongly slows, and then increases its group velocity during propagation within a target. The 3D PIC simulations confirm this concept for proton acceleration by a femtosecond petawatt-class laser pulse experiencing relativistic self-focusing, quantify the characteristics of the generated protons, and demonstrate a significant increase of their energy compared with the proton energy generated from optimized ultrathin solid dense foils.With the rapid development of laser technology, several acceleration concepts using short relativistically intense laser pulses are applied for generating high-energy ions [1,2]. Most mechanisms of laser-triggered ion generation are tailored to a forward acceleration of ions from solid targets, but a new trend has recently appeared in this field based on using low-density targets [3,4], which could be related to advanced materials such as aerogels, nanoporous carbon, etc. The hope in using such targets are that they may increase the energy of the accelerated ions compared with solid targets, particularly when ions are accelerated by lasers that are now available with ∼1 PW power. One of the challenging ways for accelerating ions up to ∼1 GeV by such lasers is ion wake-field acceleration [5,6], but this is a difficult task for existing high-power laser systems because heavy particles cannot be pre-accelerated and trapped by the wake field as easily as electrons can [7,8].For an effective acceleration in a rare plasma, similarly to electron wake-field acceleration, ions must be preaccelerated up to a velocity close to the speed of light. Based on simplified 1D and 2D numerical simulations, several two-stage schemes for ion acceleration have been proposed [9][10][11][12] to test the idea using an additional target (thin foil or micro-droplets) to pre-accelerate ions in the radiation-pressure-dominated regime [13]. These ions can then be trapped and accelerated in a gas plasma. For the proposed scenario to be viable, an Exawatt-class laser is required. The further development in this direction involved the laser snowplow effect in a near-critical density plasma, where the electrostatic potential generated by the laser pulse accelerates and reflects ions [5,14,15] similarly to collisionless electrostatic shock acceleration [16]. Specifically, a near-critical-density plasma, which reduces the laser group velocity, was proposed [15] for a second stage of accelerating ions initially pre-accelerated from a thin target by a circularly polarized super-Gaussian pulse. Based on the simulation results, a proton acceleration to an energy of hundreds of MeV was reported. It is important that the effect of relativistically induced transparency plays a key role in ion acceleration with a near-critical density plasma [14].

Laser-triggered proton acceleration from hydrogenated low-density targets

“…5). This is similar to what was demonstrated for hypothetical CH 2 targets [13]. We also studied the role of a more deeply distributed hydrogen contamination (gray dashed curve in Fig.…”

“…Nanostructured targets are also discussed as a way of increasing the efficiency of lasertarget coupling and proton acceleration [16,17]. In this paper, we extend recently published results of SASL simulations [13] with hypothetical low-density targets to targets that are available in practice. In Sec.…”

“…A principally new concept for using low-density targets for proton acceleration was recently proposed based on the synchronized propagation of the laser pulse and proton bunch [13]. The key points of this acceleration mechanism are to stop the laser pulse at the front of the target and then accelerate the infiltrating intense part of the pulse inside the plasma at the same rate as the proton energy increase in the ponderomotive potential to achieve synchronized acceleration by slow light (SASL).…”

“…The 3D particle-in-cell (PIC) simulations in support of the SASL mechanism were performed with the example of a low-density CH 2 target [13]. Such a material does not yet exist, and a suitable hydrogenated material is highly desirable as a substitute from the experimental standpoint.…”

Parametric investigation of laser interaction with uniform and nanostructured near-critical plasmas

“…Relativistic laser interaction with NCPs is characterized by a rich physics and has been investigated for a wide variety of applications: laser-driven particle sources (electrons [9], positrons [10] and especially ions [11,12,13,14,15,16,17,18,19]),laboratory astrophysics (the proposed schemes to observe collision-less shock acceleration of ions rely on slightly overcritical plasmas [20,21]), advanced strategies for Inertial Confinement Fusion [22,23] and ultra-intense, ultra-short gamma sources [24,25,26,27,28]. Some recent results include enhanced high-order harmonic emission in NCPs [29] .…”