Studies of Laser Guiding and Electron Injection in a High Power Laser Wakefield Accelerator

Abstract

This thesis describes experimental findings related to Laser Wakefield Acceleration (LWFA)- the efficient acceleration of electrons to relativistic energies using an ultraintense laser pulse to drive nonlinear waves in an underdense plasma. In order to extract the highest possible energy from the accelerating structure, the accelerator should be operated at the lowest possible density. To date the most successful LWFA experiments have relied on self-injected, rather than externally-injected electrons. The mechanism of self-injection mandates a minimum threshold density for injection. This self-injection requirement limits the achievable acceleration. Ad ditionally, a means must be found to keep the intense laser focused, or guided, over the entire length of the accelerator. In this thesis, ablative capillaries were used to demonstrate guiding of 35 TW laser pulses. The experiment concluded that ablative capillaries can guide LWFA-relevant laser pulses to a sufficient degree to drive large amplitude relativistic plasma waves throughout 3 cm. However, capillary degradation and photoionization of the ablated material add complication to the accelerator. No accelerated electrons were detected because the density was too low for injection. Another potential solution to the guiding requirement is reliance on self-guiding. The nonlinear process of self-focusing can guide pulses in plasma without requiring external structures. LWFA experiments were conducted using an extended gas cell target. The transmitted laser mode was studied as a function of plasma density and focusing geometry. An appropriate range of P/Pcrit was found over which the laser can be guided for distances as long as 19 mm. Because of the impetus for operating at low density, any decrease in the density threshold for self-injection is beneficial. Ionization-induced injection, or injection of electrons born by ionization near the peak of the laser pulse, was found to significantly lower the injection threshold and increase the injected charge by as much as an order of magnitude compared to self-injection in a pre-ionized plasma at the same density. The research presented here will guide upcoming experiments at Michigan and elsewhere attempting to achieve and control multi-GeV electron acceleration. Some of these experiments will rely on ionization-induced injection and self-guiding in a gas cell.