Method of producing 100 keVs ion beams from a gas jet using two intense laser pulses

Experiments have been undertaken using the VEGA-3 petawatt laser system at the Centro de Láseres Pulsados (CLPU) facility in Salamanca to investigate electron and ion acceleration in under-dense plasma. The respective longitudinal and transverse fields of the ‘bubble’ structure of a laser wakefield accelerator (LWFA) simultaneously accelerates electrons to GeV energies, and ions to 100s keV/u to MeV/u energies. The laser is configured to produce two ultra-intense laser pulses, each with a minimum pulse duration of 30 fs and a variable inter-pulse delay up to 300 fs. The double pulses can superpose or resonantly excite the LWFA bubble to increase the accelerating fields. By focusing the laser beam into a 2.74 mm diameter supersonic jet of He gas, using an F/10.4 parabola, an initial intensity of up to ≈1019 Wcm−2 can be realized at focus. This ionises the gas to produce plasma and the imposes a ponderomotive force that creates the LWFA accelerating structures. For backing pressures of 30 – 60 bar, corresponding to plasma densities of 1–4×1019 cm−3, the fields of the LWFA can exceed 200 MV/m, which is sufficient to accelerate electrons to GeV energies, and ions to 100s keV/u. This study focuses on ion acceleration in the transverse direction. He+1 and He+2 ion spectra have been measured using a Thompson parabola spectrometer and a multi-channel plate detector. He ions with energies up to a few hundred keV/u are observed for both single pulses (5.0 J) and double pulses (5.0 J and 3.6 J, respectively), where the inter-pulse delay is varied between 0 fs and ± 300 fs. The measured spectra are consistent with numerical simulations. Ions are observed to undergo electron exchange in the neutral surrounding gas, which produces different charge states ions and neutral atoms.