The Measurement and Optimization of Direct Laser Acceleration

Abstract

This thesis presents experimental measurements and numerical modeling of electron acceleration optimization and laser channel formation in relativistic laser interactions with underdense plasma, as well as scintillator characterization for proton imaging applications.

The Direct Laser Acceleration (DLA) of electrons during a high-energy, picosecond laser interaction with a gas jet target was studied experimentally and numerically. Experiments using the OMEGA EP facility demonstrate that the electron maximum energy and mean energy are significantly enhanced by controlling the laser focusing geometry. Energetic electron beams with maximum energy (∼ 400 MeV) exceeding 20 times the ponderomotive energy of the laser pulse were measured under certain focusing, pulse energy, and plasma density conditions. 2D particle-in-cell simulations demonstrate that the laser focusing condition will change the laser pulse evolution, channel field generation, and electron oscillations, all of which contribute to the final electron energy. Through this observation, a simple model was developed to calculate the optimal laser focal spot size in more general conditions and is validated by experimental data.

DLA was further optimized experimentally by tilting the gas nozzle toward the laser source, providing a plasma density with a sharp up ramp and gentle down ramp profile. Using a picosecond pulse with energy of 63 J and a plasma with a peak density of 8 × 10^18 cm−3, a 30◦ tilted nozzle produced 4.5 times the electrons of a straight nozzle. 2D simulations reveal that the plasma density gradient affects the final number and energy of electrons by changing the laser self-focusing, electron injection position, and the sheath field evolution. Electrons start to be accelerated from a lower density in the sharp upward ramp region and lose less energy while traveling through a sheath field formed in a gentle downward ramp region.

The channel formation and filamentation evolution in the DLA process were diagnosed using proton deflectometry and optical probing methods. Through comparing proton images from shot to shot, the channel width at the stable stage is found to be dependent on the laser focal spot size, and the final channel width is related to the pulse duration. Simulations show that a pulse with a duration longer than one picosecond may form new modes as it propagates in the channel.

The final part of this thesis characterized the spatial resolution and imaging properties of plastic scintillators. Laser-driven proton beams with broad energy spectra were used to illuminate the scintillators. Different types and thicknesses of Eljen Technology scintillators are compared to determine their intrinsic point spread function. Point projection imaging of a mesh is used to compare the imaging resolution of the scintillator to the frequently used but single-shot imaging detector, radiochromic film, and was found to be reasonably comparable and sufficient for many experimental applications.