ABSTRACT
We present target design for the inner–shell photo–ionization (ISPI) xray laser scheme and discuss the requirements for the ionizing x–ray source in rise time and flux. An investigation of the rapid rise time of x–ray emission from targets heated by an ultra–short pulse (USP) high–intensity optical laser was conducted for use as the x–ray source for ISPI x–ray lasing. Modeling using the hydrodynamic/atomic kinetics code LASNEX of a 45 fs USP driving laser with energy of order 1 J incident on a structured Au target composed of vertical rods with diameter of 500 Å predicts sufficient x rays to produce a gain–length product of order 10 in C at 45Å. Collisional ionization to the lower lasing level limits the duration of lasing giving a x–ray laser pulse duration of order 60 fs FWHM. Results of x–ray rise times from instantaneously heated Au rod targets show little benefit in using optical pulse widths less than 15 fs. Our calculations for a constant energy source varying the pulse width show significant increased gain for decreasing pulse widths down to approximately 15 fs. However, for a constant intensity source we see a decrease in gain for shorter pulse widths.
