Controlled electron injection from wake shaping using co-propagating laser pulses*

We introduce a novel method of controlled electron injection for Laser Wakefield Ac-

celeration (LWFA) operating in the high-intensity bubble regime. In this scheme, the

plasma acts to couple a high-intensity “driver” pulse to a phase controlled, low power

“satellite” pulse co-propagating off-axis. The satellite is tightly focused such that it

perturbs and drives a transient, asymmetric plasma wave before depleting. Doing so al-

lows for spatio-temporal manipulation or “shaping” of the wakefield to create a trigger

for overcoming the wave-breaking threshold and leads to efficient particle trapping and

acceleration. Supported by 2D and 3D Particle-in-Cell simulations using OSIRIS, we

demonstrate systematic investigation of the two-beam parameter space (e.g. temporal de-

lay, beam displacement, etc.) leads to control over beam pointing, charge, and emittance.

Results indicate this technique could be used to induce self-injection at plasma densities

and laser intensities well below theoretical predictions using satellites of less than 1% the

driver energy. Further scaling to additional co-propagating pulses proves to distort the

initial plasma wave formation in a predictable manner for near arbitrary wake-shaping.

This allows for an ad hoc spatiotemporal setup to control the momentum space of in-

jected electrons, leading to a route for enhanced and polarized betatron oscillations. The

results show promise for an all-optical knob to transition between a high charge, mono-

energetic, GeV accelerator and an enhanced x-ray source from betatron radiation through

independent tuning of the satellite.