Laser wakefield acceleration at reduced density in the self-guided regime

Experiments conducted using a 200 TW 60 fs laser have demonstrated up to 720 MeV electrons in the self-guided laser wakefield regime using pure helium gas jet targets. The self-trapped charge in a helium plasma was shown to fall off with decreasing electron density with a threshold at 2.5x1018 cm-3, below which no charge is measured above 100 MeV. Self-guiding, however, is shown to continue below this density limitation over distances of 14 mm with an exit spot size of 25 μm. Simulations show that injection of electrons at these densities can be assisted through ionization induced trapping in a mix of helium with 3% oxygen.