Comparison of self-injection thresholds in He and N2 and role of self-focusing in LWFA

We present an experimental study of laser-plasma acceleration in which the injected charge was measured at self-injection threshold for He and N2. We use numerical particle-in-cell simulation to unfold the role of ionization in the self-injection process and to reconstruct the local electrons density from the atomic density and the ionization degree. Comparison of measured and calculated self-injection thresholds yields the dependence of injected charge upon the electron density and sheds light on the possible role of the picosecond pedestal of femtosecond laser pulses in setting the initial charge state of the plasma.