High energy electrons from interaction with a structured gas-jet at FLAME

In this paper we analyze the properties of the electron bunches produced in a laser-plasma acceleration experiment using a 10 mm helium gas-jet with a longitudinal density profile characterized by a double peak structure. Data were taken at three different gas-jet backing pressures of 5, 8 and 15 bars, corresponding to plasma densities of 1.2–3.6 1019 cm 3 in the peaks and 3.5–10 1018 cm 3 in the central plateau. The highest energy peak is recorded at more than 450 MeV, with average energies between 80 and 180 MeV. Bunch divergence and pointing stability have been measured and are found to be very sensitive to the density. Fully 3D PIC numerical simulations confirm that laser intensity and plasma density of our set up are in the range where electron acceleration takes place by self-injection in a bubble-like structure. Analysis shows that after the first density peak, accelerated electrons propagate through the plateau and the second density peak without the driver, undergoing non-linear interaction with the background plasma.