Effects of back surface degradation on TNSA protons for Fast Ignition*

When an intense short laser pulse (>10^18W/cm2, of order picosecond pulse duration) irradiates the surface of a thin (<100 micron) foil, laser-accelerated electrons generate strong electrostatic fields on the rear surface that will accelerate ions to multi-MeV energies; this mechanism is referred to as target normal sheath acceleration (TNSA) [1]. The energetic ions typically originate from a sub-micron thick layer of hydrocarbons on the surface of the foil. We study the effect of finite (i.e., not top hat shaped) rear-surface density profiles that could be caused [2], for example by laser pre-pulses or radiative heating of the foil, on the TNSA generated ion spectrum, using Particle-in-Cell simulations [3].* This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and the LLNL LDRD with tracking number 24-SI-003 and was supported by the Livermore Computing (LC) Grand Challenge Program.