Observation Of Soft-X-Ray Emission In The Corona Of 0.35-Mu-M Laser-Irradiated Gold Disks

The Phebus laser was used to irradiate gold targets at lambda = 0.35 mu-m; the laser pulse was 0.7 or 1.3 ns and the energy on target 1 to 1.8 kJ. The incident intensity was varied from 3 x 10(13) to 3 x 10(15) W/cm2. The time history of x-ray emission was monitored in the range 0.2-2.5 keV with a streak camera coupled with a transmission grating. Numerical simulations cannot replicate the temporal evolution of soft x-ray emission (hv < 800 eV), although the total conversion efficiency is correctly reproduced. Some observations, including time and space-resolved images, show that unexpected soft x-ray emission is produced in the corona at large distances from the target surface. A possible explanation is that cold and dense jets are generated towards the underdense plasma by an instability.