Chinese Acad Sci, Inst Phys, Lab Opt Phys, Beijing 100080, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100076, Peoples R China
; Univ Oxford, Dept Phys, Oxford OX1 3PU, England
Effects of a prepulse on gamma-ray radiation have been investigated experimentally using 150-fs laser pulses at an irradiance of I lambda(2) similar to 5X10(15) W cm(-2)mu m(2) focused on copper targets. The fraction of high energy photons (>100 keV) has been found to be greatly enhanced by introducing an 8% prepulse at 70 ps before the main pulse. Measurements have shown that a hot electron temperature as high as 83 keV has been produced at such a modest irradiance.