Reduction in hot electron transport on dielectric targets at high laser irradiance

Conducting properties of the target material are shown to affect both the penetration depth and lateral extent of hot electron transport in experiments with nsec-CO2-laser pulses incident on various planar layered targets. At an irradiance around 5×1013 W/cm2 on metal targets most hard x-rays are emitted outside the focal spot. The hard x-ray emission area is reduced by a factor of ten when the conducting target material is replaced by a dielectric of similar Z. The inward transport is likewise reduced. Strong electric fields produced by a local negative charge accumulation in a dielectric may explain the transport reductions.