Pump-Probe Experiments & Radiation Generation using Laser Wakefield Accelerators.

Abstract

This thesis describes pump-probe and radiation generation experiments using Laser Wakefield Acceleration (LWFA) as the driving source. LWFA systems generate highly relativistic electron beams in a compact geometry by driving a nonlinear plasma wave with an ultraintense laser pulse. These electrons beams, or the secondary radiation that they create, can be used to pump or probe interactions on $fs$-timescales due to inherent synchronization with the laser driver.

In this thesis, the first sub-$ps$ measurements of magnetic field dynamics in ultra-intense laser-solid interactions are presented. This experiment employed the LWFA electron beam to probe the laser-irradiated target at different time delays, and by measuring the subsequent beam deflections, the evolution of the magnetic field could be inferred. The effect of laser temporal contrast on the laser-target interaction was observed to play a crucial role in the magnetic field dynamics. High-contrast laser conditions were observed to rapidly evolve over the course of $ps$-timescale as electrons propagated radially along both the front and rear of the target, establishing an azimuthal field that was stronger on the front surface. On the other hand, low-contrast laser conditions allowed ablated plasma on the front surface of the target to limit magnetic field growth to only the rear of the target.

Using high-energy LWFA electron beams, bremsstrahlung radiation was created by interaction with various solid targets. Secondary processes generate high-energy electrons, positrons, neutrons, and pions, which can be measured using magnetic spectrometers, nuclear activation, bubble detectors, and Compton scattering. Presented in this thesis are proof-of-principle results from a high-resolution, high-energy gamma-ray spectrometer capable of single-shot operation as well as high repetition rate activation diagnostics. The first measurements of laser-generated neutral electron-positron plasma beams are also presented.

One promising application of LWFA is the compact implementation of nonlinear Thomson scattering (NLTS). Using high-energy LWFA electrons as a pump, a second, counter-propagating, ultraintense laser pulse can Thomson backscatter off these electrons and get upshifted to $sim MeV$ energies, yielding a high-brightness source of high-energy photons. Two experimental attempts of NLTS are presented here.