Influence of transverse laser beam profiles on proton acceleration

The irradiation of foil targets with ultra-intense short laser pulses leads to the generation of an MeV ion beam from the non-irradiated target rear side [1]. The accelerating mechanism was identified to be the build up of a dense sheath of energetic electrons at the non-irradiated, rear surface of the target with an electric field strength exceeding 10 V/m. The acceleration was found to take place within a few ps only. The beam always is directed normal to the rear surface of the target with an emittance being superior compared to conventional accelerator beams [2, 3]. The laser pulse accelerates electrons to relativistic energies in the front side preplasma formed by the prepulse. They penetrate the target and form the sheath at the rear side. Assuming a quasineutral, electrostatic expansion the proton emission follows the gradient of the electron sheath. Compared to the acceleration gradients achieved in conventional accelerator technology this gradient turns out to be about 6 orders of magnitude larger. For further investigations of the acceleration mechanism of the ions the virtual laboratory VIPBUL was formed, including GSI as the participating Helmholtz center [4]. It was found in earlier experiments that the transverse shape of the laser beam strongly influences the transverse shape of the proton beam [5]. There the source size of the protons had to be estimated by the angular broadening of the electrons, transported through the target. It was fitted to 25 , a value that roughly fits the angle resulting from multiple Coulomb scattering. We have done experiments at the TRIDENT laser facility at LANL, Los Alamos, to investigate this influence of the laser beam profile on the proton beam with the knowledge of the source size. The targets were thin (10 50) m gold foils, that had a micro-structured rear side with equally spaced grooves of 5 or 10 m distance and less than 1 m depth. This micro-structure leads to a micro-focussing of the protons that follow the local surface normal in the beginning of the acceleration [3]. While in former experiments by our group the laser was focussed to a nearly diffraction limited spot with a radially symmetric beam profile [2], we have changed the laser to form a line focus (see the left image in fig. 1). The target was a 13 m thick Au-foil with 10 m spaced lines at the rear side. The laser was