Micron-scale mapping of megagauss magnetic fields using optical polarimetry to probe hot electron transport in petawatt-class laser-solid interactions

Gourab Chatterjee,J S Green,A P L Robinson,N Booth,L Labate,Amit D Lad,Prashant Kumar Singh,J Pasley,P Koester,P P Rajeev,L A Gizzi,D Blackman,G Ravindra Kumar,R J Dance,N C Woolsey,R J Gray,K L Lancaster,O Culfa

doi:10.1038/s41598-017-08619-1

Abstract

The transport of hot, relativistic electrons produced by the interaction of an intense petawatt laser pulse with a solid has garnered interest due to its potential application in the development of innovative x-ray sources and ion-acceleration schemes. We report on spatially and temporally resolved measurements of megagauss magnetic fields at the rear of a 50-μm thick plastic target, irradiated by a multi-picosecond petawatt laser pulse at an incident intensity of ~1020 W/cm2. The pump-probe polarimetric measurements with micron-scale spatial resolution reveal the dynamics of the magnetic fields generated by the hot electron distribution at the target rear. An annular magnetic field profile was observed ~5 ps after the interaction, indicating a relatively smooth hot electron distribution at the rear-side of the plastic target. This is contrary to previous time-integrated measurements, which infer that such targets will produce highly structured hot electron transport. We measured large-scale filamentation of the hot electron distribution at the target rear only at later time-scales of ~10 ps, resulting in a commensurate large-scale filamentation of the magnetic field profile. Three-dimensional hybrid simulations corroborate our experimental observations and demonstrate a beam-like hot electron transport at initial time-scales that may be attributed to the local resistivity profile at the target rear.

Highlights

An intense laser pulse focussed on a solid target can generate relativistic electron currents reaching mega-ampere levels[1,2,3]
This paper presents the first spatio-temporally resolved magnetic field measurements at the target rear for a multi-picosecond, petawatt driver laser pulse using optical pump-probe Cotton-Mouton polarimetry, yielding new insights into the principal characteristics of hot electron transport through solid targets under fast-ignition relevant irradiation conditions
The magnetic fields were inferred from a pump-probe polarimetric diagnostic[24,25,26], employing a linearly-polarized, time-delayed and frequency-doubled (λ = 526 nm) probe pulse, derived from the main interaction pulse, and focused to the rear of the target at near-normal incidence

Summary

Introduction

An intense laser pulse focussed on a solid target can generate relativistic electron currents reaching mega-ampere levels[1,2,3]. The spatial profile of the magnetic fields at the target rear is determined by the spatial profile of the hot electron currents evolved through their transport across the bulk of the target. To understand the dynamics of hot electron transport, a simultaneous spatio-temporal characterisation of the magnetic fields at the target rear is required. Studying the proton beam profile at various energies can shed light on the profile of the hot electron distribution at different instants of time, the temporal resolution of this technique may be limited by the duration of the proton pulse and its time of flight along with the energy resolution in its measurement

Methods

Results

Conclusion