INVESTIGATION OF ZN AND CU PREPULSE PLASMAS RELEVANT TO COLLISIONAL EXCITATION X-RAY LASERS

This paper presents the results of a comparative experimental study of low-temperature Zn and Cu line plasmas created on slab targets by 400 -ps laser pulse producing irradiance from 4x10(9) to 10(11) W cm(-2). The aim was to examine the nanosecond-scale postpulse evolution of pla smas created in conditions equivalent to those produced by prepulses i n collisional x-ray lasers, of elements that have neighboring atomic n umbers but very different material properties. The plasmas were interf erometrically probed at 4 and 10 ns next to the driving pulse, using g eometry that made it possible to obtain an authentic two-dimensional ( 2D) electron density pattern in the plane perpendicular to the plasma axis. VIS-IR spectroscopy and imaging were used to provide an indicati on of the electron temperature and volume of the plasma layer near the target. We observe that over the whole range of the applied irradianc es the characteristics and/or the expansion history of the Zn and Cu p lasmas are very different. For irradiance exceeding a threshold specif ic to each element the density patterns exhibit an unexpected structur e characterized by symmetrical flanks strongly localized in space, sug gesting plasma is generated in addition to that produced within the la ser pulse duration. The results imply that during the postpulse time t he energy coupling between the plasma and the target is substantial fo r the plasma flow that exhibits a complex 2D character. A comparison o f the data and results of a 1.5D hydrodynamic simulation for 10(11) W cm(-2) is made, indicating reasons for problems of such models in the treatment of the plasmas in question, and thus in the treatment of sma ll-prepulse action in some x-ray laser systems. [S1050-2947(97)07011-X ].