Hollow microspheres as targets for staged laser-driven proton acceleration

TL;DR

This paper introduces a novel hollow microsphere target for staged laser-driven proton acceleration, demonstrating experimentally how its geometry enhances proton energy via charge wave refocusing and transient fields.

Contribution

The study presents a new target design that combines TNSA with staged acceleration using a simple, cost-effective spherical geometry, supported by experimental and simulation evidence.

Findings

Feasibility of using hollow microsphere targets demonstrated experimentally.

Proton energy spectra show redistribution consistent with simulations.

Transient charge separation fields effectively post-accelerate protons.

Abstract

A coated hollow core microsphere is introduced as a novel target in ultra-intense laser-matter interaction experiments. In particular, it facilitates staged laser-driven proton acceleration by combining conventional target normal sheath acceleration (TNSA), power recycling of hot laterally spreading electrons and staging in a very simple and cheap target geometry. During TNSA of protons from one area of the sphere surface, laterally spreading hot electrons form a charge wave. Due to the spherical geometry, this wave refocuses on the opposite side of the sphere, where an opening has been laser micromachined. This leads to a strong transient charge separation field being set up there, which can post-accelerate those TNSA protons passing through the hole at the right time. Experimentally, the feasibility of using such targets is demonstrated. A redistribution is encountered in the experimental proton energy spectra, as predicted by particle-in-cell simulations and attributed to transient fields set up by oscillating currents on the sphere surface.