Return-current electron streams in high-intensity laser target interaction

X-ray streak photography of the interaction of a nanosecond CO2 laser pulse with large-diameter foil targets shows the presence of an annular x-ray emitting region which expands away from the focal spot with a veclocity up to 109 cm/sec. This region exhibits laser-polarization-dependent asymmetry, and it is postulated that it is formed by a return current of fast electrons bombarding the front of the target foil. It appears likely that this phenomenon is responsible in part for anomalous lateral energy transport and magnetic field generation in such experiments.