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].

Based on multidimensional particle-in-cell simulations of a short laser pulse interaction with a homogeneous planar target, we perform an optimization study to find the best design parameters for maximizing the number of high-energy electrons generated by a sub-petawatt class laser system for deep gamma radiography purposes. We find that a low-density target with an electron density of 10% of the critical density and a thickness of 240 μm irradiated by a 30 fs 4 J laser pulse can generate 7-nC electron bunches with a mean characteristic energy of 100 MeV.

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