Focusing of laser-accelerated protons for fast ignition studies

Summary for only given. Proton focusing following the interaction of a short (tp<;l ps), ultra-intense (IL>;1018 Wcm"2) laser pulse with curved surfaces has been reported. However a thorough understanding of proton focusing in terms of focal position and achievable focal spot size is essential for applications such as isochoric heating, fast ignition and medical applications. In fast ignition, the required diameter of the focused proton beam should be ~ 40 μm and deposit ~ 20 kJ in ~ 20 ps in the high density DT core. Recent experiments performed at the Trident Laser Facility (EL ~ 80 J, tρ ~ 500 fs) have estimated the diameter of the proton beam focused from hemispherical shells enclosed in a conical structure to be within 50 μm, for proton energies between 5 and 15 MeV. In view of these results, in this talk we will describe general properties of proton beams accelerated when an ultra-intense laser beam is focused against cylindrical or partial hemispherical shell targets, by means of numerical simulations with the hybrid PIC code LSP. We will discuss the dependence of the proton focal spot size and position with laser focal spot size (FWHM = 10/100 mm) and radius of curvature of the target (ranging from 150 to 800 μm). This work is part of an ongoing effort in simulating and optimizing proton-driven fast ignition at ignition-scale level. For these studies, we assume an imploded configuration for the DT plasma, with a 30° half-angle cone inserted in the fuel assembly. A hemispherical target positioned inside the conical structure is the source for the multi-kJ igniting proton beam. This beam is focused to a narrow jet that can deliver more than 20% of its energy to the high density DT core. This makes proton fast ignition a viable option for IFE, provided that the necessary (>;30%) conversion efficiency from laser to protons is achieved.