Electron transport dependence on target surface conditions and laser spot shape

The interaction of intense short pulse radiation with thick Al foils is studied as a function of the absorption region density profile and the laser spot shape. Absorption of the light generates relativistic hot electrons near the critical surface. When the density profile is steep with micron scale lengths, and a small spot (8 mu m FWHM) the hot electrons are retained near the surface while undergoing strong lateral surface transport through intense thermoelectric magnetic fields. Alternatively, with mild, similar to 30 mu m, scale length initial profiles the light beam bores a hole in the corona, wraps it with B-field, extinguishes the lateral hot electron flow, and sends a reduced fraction of hot electrons forward in filaments. Finally, broadening the spot to 40 mu m and totally flattening it gives strong forward-directed hot electron penetration, which, if achievable, could serve effectively for Fast Ignition and radiography.