LARGE ANISOTROPY OF THE ELECTRON-DISTRIBUTION FUNCTION IN THE HIGH-DENSITY PLASMA PRODUCED BY AN ULTRASHORT-PULSE UV LASER

Anisotropy of the electron distribution function in a high-density pla sma was measured with x-ray polarization spectroscopy. A prepulse cont rolled KrF laser was used to produce a plasma with a density of up to 10(22) cm(-3). In the high-density plasma, a large beamlike anisotropy was observed when the collision time was less than the laser pulse le ngth. The direction of this anisotropy was aligned with the polarizati on of the incident laser field. The amplitude of the anisotropy was re duced when the polarization of the laser was changed from p to a polar ization. These results show that the observed anisotropy was introduce d by the applied laser field and that x-ray polarization spectroscopy is a good tool for diagnosing high-density plasmas created by ultrasho rt-pulse lasers.