Longitudinal instabilities affecting the moving critical-layer laser-plasma ion accelerators

In this work we analyze the longitudinal instabilities of the propagating acceleration structures that are driven by a relativistically intense laser at the moving plasma critical layer [1]. These instabilities affect the energy-spectra of the accelerated ion-beams in propagating critical layer schemes [2][3]. Specifically, using analytical theory and PIC simulations we look into three fundamental physical processes and their interplay, which are critical to the understanding of spectral control by making the laser-plasma ion accelerators stable. The interacting processes are (i) Doppler-shifted ponderomotive bunching [4] (ii) potential quenching by beam-loading and (iii) two-stream instabilities. These phenomena have been observed in simulations analyzing these acceleration processes [5][6][7]. From the preliminary models and results we present in this work, we can infer measures by which these instabilities can be controlled [8] to improve the energy-spread of the beams.