The effect of Coulomb explosion ion heating in optical-field ionised nitrogen X-ray laser schemes

This paper examines the effect of Coulomb ion heating in various nitrogen t argets produced by optical-field ionisation from plane polarised high inten sity laser radiation. Methods for controlling the explosion, such that plas ma suitable for recombination X-ray lasers may be produced are devised. A n umerical model of breakdown by high intensity pulses is used to calculate o ptical-field ionisation, above-threshold ionisation and inverse bremsstrahl ung electron heating, and Coulomb ion heating mechanisms. A 1D Lagrangian h ydrodynamics and atomic kinetics model is used as a postprocessor to simula te the recombination phase. Both optimised nitrogen and nitrogen doped with hydrogen mixtures are examined. Electron temperatures are found to be sign ificantly lower in mixtures, yielding higher gains. Single and double (pre- and main) pulses are investigated with double pulses found to give good co ntrol of the ion temperature resulting from the Coulomb ion repulsion. An o ptimised nitrogen hydrogen mixture ionised by a double pulse produce a gain of 60 cm(-1) at 247 Angstrom with a low saturation intensity of similar to 1 x 10(7) W cm(-2). A two-dimensional pulse propagation model, which solve s the paraxial wave equation and includes optical-field ionisation, refract ion and diffraction effects, is used to study nitrogen hydrogen mixtures wi th double pulse configurations. A loosely focused pre-pulse of vacuum peak intensity 4 x 10(14) W cm(-2) is applied to produce a wide and 4 mm long ch annel of singly ionised dissociated gas mixture. The plasma is further ioni sed to the He-like N state by a tighter main pulse of peak intensity 6 x 10 (16) W cm(-2), which is suitable for achieving the gain referred to above. (C) 2000 Elsevier Science B.V. All rights reserved.