Pump pulse-width dependence of grazing-incidence pumped transient collisional soft-x-ray lasers

Abstract

The output energy dependence of high repetition rate grazing incidence pumped Ni-like Mo, Ni-like Ag, and Ne-like Ti transient collisional soft x-ray lasers on the duration of the pump pulse was studied combining experiments and model simulations. Lasing is observed to occur for a wide range of pump pulse widths (e.g., $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}18\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for Ni-like Ag), with maximum output occurring for pump pulses of $4--6\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ FWHM, corresponding to pump intensities of $1.4--2.0\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. Moderately short pump pulses are observed to be optimum for lasers that make use of preplasmas in which the mean degree of ionization approaches the charge of the lasing ion, while long pump pulses produce over-ionization and weaker lasing. However, long pump pulses are capable of producing gain and lasing in low $Z$ ions even when the preplasma has a very low degree of ionization after expanding for $5\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. As the duration of the pump pulse shortens the optimum delay respect to the prepulse is observed to decrease. The physics that determines these and other measured trends is discussed.