Interaction of intense laser pulses with plasma gradients at fast-ignition scales*

Fast ignition depends on the efficient conversion from picosecond-scale intense laser pulses into MeV electrons to heat the compressed core of an inertial confinement fusion target. We study the interaction of multi-picosecond, intense laser pulses with plasma density gradients, using full-scale particle-in-cell simulations. We find that, in contrast to the diffraction-limited sub-ps laser pulses used in experiments today, the long-energetic-large-spot pulses relevant to fast ignition create over a few picoseconds a plasma profile and a hot electron spectrum that are nearly independent of the initial conditions. We discuss scaling properties with laser intensity, spot size and plasma scale length.