DEPENDENCE OF KEV X-RAY GENERATION ON THE TEMPORAL AND SPATIAL SEPARATION OF 2 KRF LASER-PULSES

The generation of keV x-ray emission from plasma produced by a 15 mJ 90 ps KrF laser pulse focused to an 8 mu m spot on a copper target preceded by a similar 8 mJ pulse is studied experimentally. The change in efficiency of generation of keV x rays by the second pulse due to the presence of plasma produced by the preceding laser pulse on target is investigated as a function of the spatial and temporal separation of the laser pulses. When closely overlapped in space and time the pair of pulses behave like a single higher intensity pulse. When separated in space by more than 100 mu m the two pulses behave independently generating x rays at the same rate,as they would as two separate pulses. However, when overlapped in space and separated in time the x-ray generation by the second pulse is enhanced for small temporal separations and then suppressed for larger temporal separations due to the plasma created by the first pulse. These results can be explained by the initial production of hot plasma which enhances x-ray generation followed by the production of an extended colder plasma from the unloading of the shock heated target surface which suppresses keV x-ray production by the second pulse. The observed time of transition from enhancement to reduction of x-ray generation agrees with the time for the hot initial plasma due to the first laser pulse to expand and cool significantly.