Divergence control with optimized targets for laser proton acceleration

O. Deppert1, K. Harres1, F. Nürnberg1, G. Schaumann1, M. Schollmeier2, and M. Roth1 1Institut für Kernphysik, Technische Universität Darmstadt, Germany; 2Sandia National Laboratories, Albuquerque NM, USA The irradiation of thin metal foils by an ultra-intense, relativistic laser pulse leads to the generation of a highly laminar and intense proton beam accelerated from the target rear side by a mechanism called “Target-NormalSheath-Acceleration” [1]. This kind of acceleration mechanism strongly depends on the geometry of the target. A Gaussian-like electron sheath at target rear-side generates an electric field on the order TV/m which is responsible and formative for the acceleration of the protons. This sheath adds an energy dependent divergence to the local proton beam profile on the order of 50◦ full opening angle for a flat foil. For further applications, like the transport of the beam, it is essential to reduce the divergence in the beam profile already from the “source” of the acceleration process. Therefore, numerical simulations were performed with the Particle-in-Cell code PSC [2] in order to analyze and optimize the proton acceleration process with respect to beam divergence. The results led to the design of different target geometries, manufactured at the target lab at TU Darmstadt. The targets consist of a hemispherical, proton focusing part and a cone-like top part for collimation, illustrated in figure 3.