Paving the way for a revolution in high repetition rate laser-driven ion acceleration

Abstract

Exceptionally strong TV m −1 fields generated during the interaction of high intensity lasers with plasma targets are capable of accelerating ions to MeV energies over micron-scale distances. This offers exciting possibilities for the development of compact accelerators. It has been demonstrated that the maximum achievable ion energy scales with the laser intensity and consequently much of the work in this field has utilised lasers at the cutting edge of development with multi-J energies and short (<ps) duration. The peak fluxes of laser-plasma generated ion beams are extremely high, but owing to the low repetition rate (=1Hz) of most high power laser systems, the average flux is very low. A significant increase in the average particle flux is required for many applications of these beams, including hadrontherapy. The challenge in producing these sources lies not only in the development of stable, high energy, high repetition rate laser systems but also of suitable targetry and diagnostics. In the paper of Morrison et al (2018), the authors demonstrate a novel planar target compatible with kHz production of MeV proton beams and kHz-compatible beam diagnostics, which represent important steps in the development of laser-driven ion sources for applications.While energetic ions were measured during laser-plasma interactions in the mid-1980s, the development of chirped pulse amplification, allowing amplification of short laser pulses to higher energy, lead to a dramatic enhancement in acceleration to tens MeV energies. When the high intensity (I L >10 18 W cm −2 ) laser pulse is incident on a target, typically a few microns in thickness, a dense plasma is formed and the newly freed electrons are rapidly accelerated to relativistic energies in the laser fields. These 'hot' electrons escape the target, from the front and rear surface, which is left with a net positive charge leading to TV m −1 'charge-separation' fields that can accelerate ions. Ion beams produced in this way are typically directed along the surface normal, have exceptionally low emittance, divergences of tens of degrees and a broad energy spread with the maximum energy depending on multiple factors including the laser intensity.While the low repetition rate of high energy laser facilities has limited development of these ion sources, several multi-Joule, multi-Hz laser systems will become available over the next 5 years. However, exploiting these presents many technical challenges. The high laser intensities required for maximal electron heating lead to destruction of the targets that must then be replaced with micron level precision in position and ablation of solid target material rapidly coats surrounding optics with debris.Over the last decade, many groups have been exploring potential high repetition rate targetry and diagnostics using low energy laser systems. Tape-drive targets with thicknesses of 10sμm have produced few-Hz, MeV-level proton beams (McKenna et al 2002, Noaman-ul-Haq et al 2017, although as target thickness is reduced to inc...

Combined laser-based X-ray fluorescence and particle-induced X-ray emission for versatile multi-element analysis

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