Controllable Electron Injection Using Co-Propagating Laser Pulses in the Bubble Regime

We introduce a novel method of controlled electron injection for Laser Wakefield Acceleration (LWFA) operating in the high-intensity "bubble" regime. In this scheme, a fraction of a high-intensity "driver" pulse is diverted and compressed into a low power, few-cycle "satellite" pulse co-propagating alongside the driver. This satellite is tightly focused off-axis where it acts to perturb bubble formation and drive an asymmetric plasma wave before stabilizing on-axis. Doing so allows for manipulation of the particle separatrix; creating a trigger to overcome the wave-breaking injection threshold and lead to efficient particle trapping and acceleration. 2D and quasi-3D Particle-in-Cell (PIC) simulations support this concept, demonstrating that systematic investigation of the two-beam parameter space (e.g. temporal delay, beam displacement) leads to controllable variance in the electron beam phase space. Results indicate this method could be used to induce self-injection in wakefields at plasma densities and driving laser intensities well below theoretical predictions. The results show promise for an all-optical route to high charge, mono-energetic particle acceleration to GeV energies or enhancement of electron betatron radiation through independent tuning of the satellite pulse.