Proton radiography investigations of plasma dynamics driven by intense lasers

Abstract

The electromagnetic fields in high-energy-density (HED) plasmas are able to be mapped with high temporal and spatial resolution via proton radiography. The interaction of high-intensity (>1018 W/cm2) laser pulses with thin (∼µm) foils generates a population of hot electrons which propagate through the foil, expand into vacuum and establish a quasi-electrostatic sheath field on the non-irradiated surface of the foil. The electric field preferentially accelerates protons to multi-MeV energies in a process known as target normal sheath acceleration (TNSA). The short duration, laminarity and high particle flux of such proton beams make them ideal for imaging electromagnetic fields. The protons suffer small deflections, due to Lorentz forces, and the resultant profile is imaged using a suitable detector, such as radiochromic film. The work presented in this thesis aims to study various electromagnetic fields associated with laser-plasma interactions using proton radiography. The expansion of a laser-driven, high density, blast wave plasma into a lower density, ambient plasma generates collisionless shock waves (CSW) which are relevant to several astrophysical phenomena. The effects of introducing an external magnetic field or increasing the ambient plasma density on the CSW were investigated using proton radiography. The propagation of a hot electron current through a resistivity-varying material, such as foam, can result in filamentation of the current. The degree of filamentation determines the amount of energy that is deposited into the fuel in the fast ignitor approach to inertial confinement fusion. Proton radiography imaged the electromagnetic fields of electron filaments and revealed that the resistivity instability was responsible for the onset of filamentation.T he interaction of a laser pulse with a solid target generates an azimuthal magnetic field via the Biermann battery mechanism. The interaction of two such fields, which had parallel field lines when interacting, was investigated using proton radiography to observe magnetic field compression.

Date of Award	Dec 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Atomic Weapons Establishment
Supervisor	Marco Borghesi (Supervisor), Thomas Hodge (Supervisor) & Satyabrata Kar (Supervisor)