Optimization of ionization injection in laser wakefield acceleration

Laser-plasma accelerators are capable to produce GeV beam energies over only a few centimeter, making them interesting as driver for compact light sources. However, electron beam parameters have to be matched to the beam optics. Online tunability of energy, energy spread and charge are therefore mandatory for efficient electron beam transport. Controlling the injection mechanism of the electrons into the wakefield is one promising way to optimize these parameters. Here, we present the influence of the dopant concentration on electron beams generated with ionization injection. Using the 200 TW peak power laser system ANGUS at the LUX facility in Hamburg, we accelerated electron beams up to a few hundred MeVs in a gas mixture of nitrogen and hydrogen. We precisely scanned the nitrogen concentration online between 0.1 and 5% while keeping the background plasma density forming the wakefield constant. Changes in beam properties can therefore directly be related to the injection mechanism. We found that the amount of trapped charge is increased significantly for larger nitrogen concentrations, while energy gain is reduced.