Simulations of stable compact proton beam acceleration from a two-ion-species ultrathin foil

TL;DR

This paper demonstrates stable laser-driven proton acceleration using ultrathin two-ion-species foils, with simulations showing effective species separation and suppression of instabilities, leading to high-quality proton bunches.

Contribution

It introduces a simple three-interface model explaining instability suppression and validates it through multi-dimensional simulations of various compound foils.

Findings

Stable proton bunches achieved without RT instability.

Species separation driven by radiation pressure.

Carbon ion charge state affects proton energy spectrum.

Abstract

We report stable laser-driven proton beam acceleration from ultrathin foils consisting of two ion species: heavier carbon ions and lighter protons. Multi-dimensional particle-in-cell (PIC) simulations show that the radiation pressure leads to very fast and complete spatial separation of the species. The laser pulse does not penetrate the carbon ion layer, avoiding the proton Rayleigh-Taylor-like (RT) instability. Ultimately, the carbon ions are heated and spread extensively in space. In contrast, protons always ride on the front of the carbon ion cloud, forming a compact high quality bunch. We introduce a simple three-interface model to interpret the instability suppression in the proton layer. The model is backed by simulations of various compound foils such as carbon-deuterium (C-D) and carbon-tritium (C-T) foils. The effects of the carbon ions' charge state on proton acceleration are also investigated. It is shown that with the decrease of the carbon ion charge state, both the RT-like instability and the Coulomb explosion degrade the energy spectrum of the protons. Finally, full 3D simulations are performed to demonstrate the robustness of the stable two-ion-species regime.