Effects of laser prepulse on proton generation

Low-intensity laser prepulse (<10¹³ W/cm², ns duration) are a major issue in experiments on laser-generation of protons, often limiting the performances of proton sources produced by high-intensity lasers (≈10¹⁹ W/cm², ps or fs duration). Several effects are associated to the prepulse and are discussed in this contribution:i)Destruction of thin foil targets by the shock generated by the laser prepulseii)Creation of preplasma on target front side affecting laser absorptioniii)Deformation of target rear sideiv)Whole displacement of thin foil targets affecting focusing condition. In particular, we show that 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.