Modelling collisionally pumped x-ray lasers in optically field ionised noble gases

This thesis presents a proof of principle investigation into the main physical processes following optical field ionisation (OFI) of noble gases. A high intensity optical laser of circular polarisation can produce a hot plasma which is capable of lasing at soft X-ray wavelengths through electron collisional excitation. A numerical model of the driving laser which includes OFI with above threshold ionisation (ATI) and inverse bremsstrahlung (IB) heating mechanisms is used to determine the initial state of a plasma for a range of target densities and pulse polarisations. A time dependant atomic / hydrodynamics code is used to calculate gain coefficients and saturated irradiances. Large gains of the order of 100 - 1000 cm"-"1 are predicted for the valence shell lines in the range of 30-40 nm for eight times ionised argon, krypton and xenon. Gains of around 20 cm"-"1 are predicted for lines at 15.3 and 15.7nm on the inner sub shell lines of argon. The saturated irradiance is found to be low at around 10"7 - 10"8 W/cm"2. Propagation of the driving laser pulse into the gas / plasma is examined with a code which models the ionisation induced refraction. Refraction is found to be severe and will restrict the use of these schemes to target densities of less than 10"1"8 cm"3. A possible solution by using a pre-formed plasma as a wave-guiding structure is examined and it is found that plasma channels of several cm may be formed. Finally, an examination of the non-thermal nature of the electrons within the plasma is made. For the case of the argon target it is found that a degree of elliptical polarisation in the driving laser may result in optimal pumping of the laser. (author)Available from British Library Document Supply Centre- DSC:DXN054464 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo