Abstract
The target normal sheath acceleration is a robust mechanism for proton and ion acceleration from solid targets when irradiated by a high power laser. Since its discovery extensive studies have been carried out to enhance the acceleration process either by optimizing the laser pulse delivered onto the target or by utilizing targets with particular features. Targets with different morphologies such as the geometrical shape (thin foil, cone, spherical, foam-like, etc.), with different structures (multi-layer, nano- or micro-structured with periodic striations, rods, pillars, holes, etc.) and made of different materials (metals, plastics, etc.) have been proposed and utilized. Here we review some recent experiments and characterize from the target point of view the generation of protons with the highest energy.
Highlights
The record high electric fields (~ 106 V μm−1) produced by ultra-high power lasers is a key feature that is being exploited for acceleration of sub-atomic particles [1]
The acceleration mechanism at work was identified as the target normal sheath acceleration (TNSA) in which the electrons accelerated by the laser pulse pass trough the target and create a spatial charge very close to its rear side [4]
A survey of the main proton acceleration results published in the literature is presented, in support of the commissioning of experimental areas at ELI-NP, where solid targets will be irradiated with the 1 and 10 PW pulses
Summary
The record high electric fields (~ 106 V μm−1) produced by ultra-high power lasers is a key feature that is being exploited for acceleration of sub-atomic particles [1]. The focused laser in a spot of the order of a few microns with intensity above 1018 W cm−2 ionizes the target and accelerates the electrons to relativistic energies (in the 100’s keV to MeV range) via the ponderomotive force These “hot” electrons cross the target, exit from the opposite side and create a spatial charge near the back surface. If electrons with energy 1 MeV flow through a simple Al foil (with no laser irradiation), their combined radiative and collisional stopping power is of order 0.4 keV μm−1 [18] This situation changes as the target becomes a fully ionized plasma during laser irradiaton and the dynamics of electron acceleration is affected by the creation of the opposed sheath electric field at the back of the target which leads to the generation of return currents [19]. The prepulse can be used to good advantage to enhance ion acceleration if it is produced in a controllable manner, with defined duration and intensity [15]
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