Self-injection Process in Laser-wakefield Accelerator Driven by CO₂ Laser Pulses

The use of long-wavelength infrared laser drivers for laser wakefield acceleration (LWFA) is a promising alternative to traditional near-infrared laser drivers. This is because the ponderomotive force strength is proportional to the square of the laser wavelength, thus more efficient LWFAs can be achieved using longer wavelength lasers. In this study, we used PIC simulations to investigate the mechanisms of self-injection in CO₂ laser-driven wakefields in the blowout regime. We considered a range of parameters, including laser amplitudes, spot size, pulse durations, and plasma densities, to determine the injection threshold based on peak laser amplitude a₀*. We found a parameter range, specifically a_o 4, that suppresses self-injection in fully blown-out bubbles, which is crucial for the controlled injection in the LWFA experiments. Our results suggest that the accelerating bubble structure is dynamic and highly sensitive to the local laser and plasma properties. Additionally, we found that the velocity at the back of the bubble plays a crucial role in self-injection physics and that comparing the velocity of the electron with the velocity at the back of the bubble can provide an accurate criterion for self-injection.