USING LOW AND HIGH PREPULSES TO ENHANCE THE J=0-1 TRANSITION AT 19.6 NM IN THE NE-LIKE GERMANIUM XUV LASER

We report a study of the effect of prepulses on XUV lasing of Ne-like germanium for an irradiation geometry where approximate to 20 mm long germanium slab targets were irradiated at approximate to 1.6 x 10(13) W cm(-2) using approximate to 0.7 ns (1.06 mu m) pulses from the VULCA N glass laser. Prepulses were generated at fractional power levels of approximate to 2 x 10(-4) (low) and approximate to 2 x 10(-2) (high) a nd arrived on target 5 and 3.2 ns respectively in advance of the main heating pulse, For both the low and high prepulses the output of the 3 p-3s, J = 0-1, line at 19.6 nm was enhanced such that the peak radiant density (J/st) for this line became greater than that for the normall y stronger J = 2-1 lines at 23.2 and 23.6 nm. The J = 0-1 line, whose FWHM duration was reduced from approximate to 450 ps to approximate to 100 ps, delivered approximate to 6 x more power (W) than the average for the combined J = 2-1 lines, whose FWHM duration was approximate to 500 ps for both levels of prepulse, The higher prepulse was more effe ctive, yielding approximate to 2 x more radiant density and approximat e to 7 x more power on both the J = 0-1 and J = 2-1 transitions compar ed to the low prepulse case, The most dramatic observation overall was the approximate to 40 x increase of power in the J = 0-1 line for the high prepulse (approximate to 2%) case compared with the zero prepuls e case. These observations, coupled with measurements of beam divergen ce and beam deviation through refractive bending, as well as general a greement with modelling, lead us to conclude that, for germanium, the main influence of the prepulse is (a) to increase the energy absorbed from the main pulse, (b) to increase the volume of the gain zone and ( c) to relax the plasma density gradients, particularly in the J = 0-1 gain zone.