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 9 0 ps KrF laser pulse focused to an 8 mu m spot on a copper target prec eded 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 t he 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 gener ating x rays at the same rate,as they would as two separate pulses. Ho wever, when overlapped in space and separated in time the x-ray genera tion by the second pulse is enhanced for small temporal separations an d then suppressed for larger temporal separations due to the plasma cr eated by the first pulse. These results can be explained by the initia l 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.