On the dynamics of a laser-produced Be plasma and its implication for a Be/Ne photopumped x-ray laser scheme

Abstract

A photoresonant-pumped x-ray laser scheme using Be and Ne is considered. In this scheme the Be iv 2p-1s transition at 75.93 Å is used to pump the Ne vii 2s-4p level at 75.77 Å. The wavelength mismatch can be decreased by either a bulk Doppler shift of the Be plasma produced by hydrodynamic expansion which requires a velocity of 6×107 cm/s, or by opacity broadening of the Be iv pump line, or by a combination of both. To investigate the viability of this particular scheme, preliminary studies were made of the Be plasma by focusing 14 to 24-J, 3-ns pulses of 1054-nm laser light onto a planar Be target. Using a time-of-flight technique, it is found that the velocity needed to Doppler shift the Be iv pump line into resonance with the Ne vii 2s-4p transition does not develop until at least 10 ns after the peak of the driving laser pulse. Spatially resolved time-integrated spectral measurements indicate that by this time the Be iv emissions have greatly decreased in intensity and this suggests that the needed bulk Doppler shift is difficult to achieve while still maintaining sufficient pump power. However, the observed opacity broadening of the Be iv pump line (0.7 Å FWHM) is found to be sufficient to create significant spectral overlap between the nearly resonant lines to allow photopumping to occur. It is also observed that the temporal duration of the opacity-broadened Be iv emission is roughly 5 ns (FWHM), which is significantly longer than the 3-ns driving laser pulse, and this fact can limit the efficiency of this particular x-ray laser approach.