Ionization induced trapping in a laser wakefield accelerator

Experimental measurements of electrons accelerated in a laser wake?eld accelerator indicate trapping initiated by an ionization mechanism. By using a range of noble gases and nitrogen as a very small percentage contaminant in helium gas, it is found that tunneling ionization of inner shell electrons plays an important role in determining the trapped charge. Although there is a small increase in electron number density due to the higher Z contaminant, it is shown to be insufficient to account for the increase in charge in the trapped bunch. The behavior of the trapping with laser intensity is also consistent with the tunnel-ionization thresholds for the gases used. This mechanism is likely to allow lower power laser systems to be used to generate comparably high-energy and low-emittance monoenergetic electron beams than in pre-ionized gas configurations. 2D particle-in-cell simulations including an ADK ionization model were used to correlate with experimental results.