Optimizing two stage Laser Wakefield accelerator with hybrid of ionization and density transition injection

The injection process in a Laser Wakefield Accelerator (LWFA) is a critical determinant of the resulting electron beam properties like the emittance, charge, and electron spectra. Separating the injection and acceleration process into stages enables separate control of both so that electron beam parameters can be tuned for applications. However, the interaction between the laser and the plasma in the injection stage also affects the acceleration process in a subsequent stage. Notably, the pulse can be self-compressed temporally and spatially in a high density injector stage. We conducted 2.5D3V particle-in-cell (PIC) simulations to study how the coupling between an injection and acceleration stage changes the properties of the electron beam. Using Bayesian optimization with Gaussian processes, we are able to efficiently use computational resources to find optima. By adjusting two parameters, one for the density and another for the length scale of the injection stage, we find distinct regimes based on different injection mechanisms and electron properties. Our work shows how to use Gaussian process surrogate models to optimize for desired electron beam parameters and analyze the underlying physical processes.