Influence of spatial-intensity contrast in ultraintense laser\u2013plasma interactions

R Wilson,T P Frazer,S J Hawkes,Z E Davidson,C D Armstrong,P Mckenna,D Neely,M King,H Liu,R J Dance,N M H Butler,R J Clarke,J Jarrett,A Higginson,M J Duff,R J Gray,D C Carroll

doi:10.1038/s41598-022-05655-4

Abstract

Increasing the intensity to which high power laser pulses are focused has opened up new research possibilities, including promising new approaches to particle acceleration and phenomena such as high field quantum electrodynamics. Whilst the intensity achievable with a laser pulse of a given power can be increased via tighter focusing, the focal spot profile also plays an important role in the interaction physics. Here we show that the spatial-intensity distribution, and specifically the ratio of the intensity in the peak of the laser focal spot to the halo surrounding it, is important in the interaction of ultraintense laser pulses with solid targets. By comparing proton acceleration measurements from foil targets irradiated with by a near-diffraction-limited wavelength scale focal spot and larger F-number focusing, we find that this spatial-intensity contrast parameter strongly influences laser energy coupling to fast electrons. We find that for multi-petawatt pulses, spatial-intensity contrast is potentially as important as temporal-intensity contrast.

Highlights

In contrast to other beam properties, as discussed below, there is no measurable difference between the Epmax values for the two focusing geometries for a given equivalent intensity, indicating that the energy in the wings, which is substantial at the lower intensity end of the F/1 dataset, does not play a significant role in defining the maximum proton energy
Through measurement of the properties of the beam of accelerated protons, it is shown that the intensity in the focal spot wings of a tightly focused laser pulse, arising from effects such as focusing aberrations, strongly influences the overall laser energy coupling to fast electrons
Analytical modelling and 2D PIC simulations show that when the intensity in the wings is above the relativistic threshold, the spatial region at the front of the target over which fast electrons are generated is larger, which in turn increases the area at the target rear side over which a sheath electric field is formed and the overall proton source area

Summary

Introduction

By comparing proton acceleration measurements from foil targets irradiated with by a near-diffraction-limited wavelength scale focal spot and larger F-number focusing, we find that this spatial-intensity contrast parameter strongly influences laser energy coupling to fast electrons.

Results

Conclusion