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 targets produced by optical-field ionisation from plane polarised high intensity laser radiation. Methods for controlling the explosion, such that plasma suitable for recombination X-ray lasers may be produced are devised. A numerical model of breakdown by high intensity pulses is used to calculate optical-field ionisation, above-threshold ionisation and inverse bremsstrahlung electron heating, and Coulomb ion heating mechanisms. A 1D Lagrangian hydrodynamics and atomic kinetics model is used as a postprocessor to simulate the recombination phase. Both optimised nitrogen and nitrogen doped with hydrogen mixtures are examined. Electron temperatures are found to be significantly lower in mixtures, yielding higher gains. Single and double (pre- and main) pulses are investigated with double pulses found to give good control of the ion temperature resulting from the Coulomb ion repulsion. An optimised nitrogen hydrogen mixture ionised by a double pulse produce a gain of 60 cm−1 at 247 Å with a low saturation intensity of ∼1×107 W cm−2. A two-dimensional pulse propagation model, which solves the paraxial wave equation and includes optical-field ionisation, refraction and diffraction effects, is used to study nitrogen hydrogen mixtures with double pulse configurations. A loosely focused pre-pulse of vacuum peak intensity 4×1014 W cm−2 is applied to produce a wide and 4 mm long channel of singly ionised dissociated gas mixture. The plasma is further ionised to the He-like N state by a tighter main pulse of peak intensity 6×1016 W cm−2, which is suitable for achieving the gain referred to above.