Congratulations to Philip Bradford, a York Fusion CDT student who successfully defended his thesis at viva in March 2021. Phil’s thesis is entitled “Laser-driven discharges and electromagnetic fields” and his supervisor is Professor Nigel Woolsey . An abstract from Phil’s thesis is below.
“When high intensity lasers interact with solid targets, hot electrons are produced that can exit the material and leave behind a positive electric charge. As this accumulated charge is neutralised by a cold return current, radiation is emitted with characteristics dependent on laser and target properties. This thesis examines how electromagnetic radiation is emitted in experiments with long-pulse and short-pulse lasers.
Radiofrequency electromagnetic pulses emitted during ps-duration laser inter-actions can disrupt scientific measurements and damage electronic equipment close to the target. A study of electromagnetic pulses produced by the Vulcan laser is presented. Strong fields exceeding 100 kV/m and 0.1 mT were measured 1.5 m from the target using conducting probes. Scaling of the EMP field with laser and target parameters shows qualitative agreement with target charging models. A novel EMP mitigation scheme is presented using a dielectric spiral target holder. Experimental
results are used to benchmark a frequency-domain dipole antenna model of EMP emission that connects charging physics to EMP fields measured at an arbitrary distance from the target.
In a separate experiment, coil targets were driven with three ns laser beams from the Vulcan laser, generating multi-tesla quasi-static magnetic fields. Dual-axis proton deflectometry was used to measure electric and magnetic fields around the coils. Results suggest that wire electric fields of order 0.1 GV/m develop on a 100 ps timescale. Maximum currents of 10 kA were observed towards the end of the laser drive for 1 mm- and 2 mm-diameter loop targets, corresponding to an axial magnetic field of B ≈ 12 T in the 1 mm loops. Deflectometry results agree well with
a plasma diode model, whereas B-dot probe measurements of the magnetic field were approximately 10× larger. Analytic and computational modelling of charged particle motion in electric and magnetic fields is presented. Prospects for an all-optical platform for magnetized high energy density physics experiments are discussed”.