Laser-Driven Electron Accelerators as a Broadband Radiation Source - from Infrared to X-Rays

Abstract

This dissertation explores the acceleration of electrons and associated production of radiation through the interaction of intense laser pulses with underdense plasma. For femtosecond duration pulses, where the pulse duration is shorter than the plasma period, the dominant electron acceleration mechanism is Laser Wakefield Acceleration (LWFA), wherein electrons gain energy by ``surfing'' electron plasma waves. For high-intensity picosecond pulses the transverse laser field becomes the dominant acceleration mechanism, giving electrons longitudinal momentum via} the v x B force. This is known as Direct Laser Acceleration (DLA).

Experiments on the OMEGA EP laser facility demonstrated the existence of an optimal density for acceleration of electrons up to 600 MeV via DLA using a high-energy, picosecond duration pulse. Two-dimensional particle-in-cell (PIC) simulations conducted using the EPOCH code confirm DLA as the dominant acceleration mechanism and elucidate that dynamic role of quasi-static channel fields on electron energy enhancement. Electron beams generated by this scheme could be used to obtain brilliant, spatially coherent X-rays with the capability to be accurately synchronized to short pulse laser-initiated events.

During LWFA, the formation of a plasma bubble results in a co-moving refractive index gradient that produces time dependent frequency shifts in the driving laser pulse, generating wavelengths extending into the mid-infrared. High-resolution spectral measurements of this radiation and its dependence on laser and plasma parameters were investigated using the HERCULES laser system. The use of a tailored plasma density was shown to produce wavelengths extending to 2.5 micrometer, and containing up to 15 mJ of energy. Further experimental studies on HERCULES yielded the first measurements of backward Stimulated Raman Scattering (BSRS) in the strongly coupled regime of LWFA, which were found to be highly modulated and broadened in cases where electrons were accelerated. Simulations and experiments indicate that measurement of BSRS may be used as a diagnostic of bubble formation and trapped electron charge within the bubble. Finally, X-ray imaging results of an Al-Si alloy obtained using betatron radiation from a LWFA indicate that these sources can be competitive with conventional synchrotron radiation.