Simulation study of ionization-induced injection in sub-terawatt laser wakefield acceleration

By using a thin, high-density gas cell, subterawatt laser wakefield acceleration (sub-TW LWFA) of electrons can be driven by few tens of megajoule pulses from diode-pumped lasers operated at high repetition rates. When a 0.5-TW, 1030-nm pulse interacts with a dense plasma, the self-focusing effect and the self-modulation instability are induced to enhance the pulse intensity to a level capable of exciting plasma bubbles. Through particle-in-cell simulations, this study investigates the sub-TW LWFA in which a H2-N2 mixture is applied for the gas target; in this fashion, the nitrogen doping ratio ρN can be varied to improve the output energy and the charge of accelerated electrons with the addition of ionization-induced injection. The results show that the acceleration efficiency is limited when using a pure hydrogen target, since the self-injection of electrons rarely occurs in the first plasma bubble having the highest accelerating field. By doping the hydrogen target with nitrogen, free electrons generated when the pulse peak ionizes the N 5 + and N 6 + ions can be injected into the first bubble. The optimal performance of sub-TW LWFA can be acquired with a nitrogen doping ratio between ρN = 1% and 3%, from which electrons can be produced with a maximum energy of > 40 MeV and a total charge ∼6 pC for the high-energy component (>20 MeV). Using a relatively high doping ratio, ρ N ≥ 5% will significantly degrade the properties of the output electrons, primarily because of the manifest ionization defocusing encountered by the driving pulse.