Electron injection and trapping in a laser wakefield by field ionization to high-charge states of gases

A scheme for electron injection into a laser wakefield is presented, which makes use of two orthogonally directed laser pulses and a gaseous medium with a moderate or high atomic number such as neon. A pump laser pulse ionizes the medium to its midcharge states to form underdense plasma and meanwhile excites a high amplitude wakefield firstly. Another ultrashort laser pulse with higher intensity is then injected transversely, which further ionizes the medium to high-charge states to produce new free electrons with certain energy. Part of these new-born electrons can be trapped and accelerated by the laser wakefield to high energies. Numerical simulations using a particle-in-cell code with field ionization included are conducted to verify the scheme.