Effects of laser prepulse on proton generation: active manipulation of the distribution of laser accelerated proton beams

Laser pre-pulse is a major issue in experiments on laser-generation of protons, often limiting the performances of laser sources. In this paper, we show how we can actively use a low intensity prepulse (<10 ¹³W/cm², ns duration) to manipulate the proton beam direction or spatial energy distribution. The prepulse is focused onto the front surface of a thin foil before the arrival of the high intensity pulse (≈10¹⁹W/cm², ps duration). Under oblique high-intensity irradiation and for low prepulse intensities, the proton beam is directed away from the target normal. Deviation is towards the laser forward direction, with an angle that increases with the level and duration of the ASE pedestal. Also, for a given laser pulse, beam deviation increases with proton energy. The observations are discussed in terms of Target Normal Sheath Acceleration, in combination with a laser-controllable shock wave locally deforming the target surface. Results obtained with an annular intensity distribution of the prepulse show smooth proton beams with a sharp circular boundary at all energies. Potential mechanisms to explain the observations are discussed.