Prepulse effects on the generation of high energy electrons in fast ignition scheme

The energy distribution of the produced high energy electrons in the interaction of ultraintense picosecond laser pulses with high-Z solid targets is shown to be sensitive to the preformed plasma created by the prepulse and the amplified spontaneous emission pedestal. The created preformed plasmas, which are obtained by radiation hydrodynamic simulations for the present heating laser system at ILE, Osaka University, are seen to extend up to 30-100 mu m just before the arrival of the main pulse. The dependences of the coupling efficiency of the laser energy to high energy electrons, and the energy spectra of these accelerated electrons, on this preformed plasma, are studied via a two-dimensional particle-in-cell simulation code. It is found that in a small preformed plasma case, JxB heating is dominant and the produced electron temperature agrees well with Haines' scaling law [Haines , Phys. Rev. Lett., 102, 045008 (2009)]. While in a large preformed plasma case, in addition to JxB heating and/or vacuum heating, other acceleration mechanisms, such as stochastic heating, can accelerate electrons to very high energies, carrying a significant fraction of input laser energy. Even after several picoseconds, the number of high energy electrons (0.5 MeV < E < 5 MeV) generated in a small preformed plasma case can be several times larger than that of a large preformed plasma case.