A review of X-ray laser development at Rutherford Appleton Laboratory

Recent experiments undertaken at the Rutherford Appleton Laboratory to produce X-ray lasing over the 5-30 nm wavelength range are reviewed. The efficiency of lasing is optimized when the main pumping pulse interacts with a preformed plasma. Experiments using double 75-ps pulses and picosecond pulses superimposed on 300-ps background pulses are described. The use of travelling wave pumping with the approximately picosecond pulse experiments is necessary as the gain duration becomes comparable to the time for the X-ray laser pulse to propagate along the target length. Results from a model taking account of laser saturation and deviations from the speed of light c of the travelling wave and X-ray laser group velocity are presented. We show that X-ray laser pulses as short as 2-3 ps can be produced with optical pumping pulses of approximate to1-ps

Recombination and population inversion in plasmas generated by tunneling ionization

Above-threshold ionization (ATI) ionization by linearly polarized light has been proposed by several authors as a means of driving recombination lasers in the soft x-ray spectral region. The pump radiation generates a cold electron plasma with ions in a single ionization stage, which is an ideal starting condition for strong recombination. Population inversions form during the recombination cascade to the ground state of the next ionization stage. In the absence of any relaxation the electron distribution is strongly peaked near zero energy. However, a number of different processes all heat the cold electrons towards Maxwellian, and may thereby reduce the recombination rate in the higher levels. Using numerical models we investigate these relaxation processes and their effect on recombination. We show that the recombination can be well described by the standard cascade model, provided an appropriate temperature is used. We examine two cases in detail, hydrogen-like lithium where the inversion is with respect to the ground state, and lithium-like nitrogen where it is with the first excited state. The two cases differ markedly in the degree of relaxation achieved, and in the duration of the population inversion

Optimizing the performance of nickel-like collisionally pumped x-ray lasers. III. Exploding foil lasers for the wavelength range below 50 Å

In previous work, it was found that heat conduction limited the development of laser action at wavelengths below 50 Å. In this paper, we discuss the use of thin foils to limit these effects, and generate a high-density and -temperature plasma with low refractive index gradient. By using a relatively short (~100 ps) first pulse and short (~1 ps) second, it is shown that effective high-gain systems may be developed

MODELLING STUDIES OF XUV LASERS

A discussion of the general requirements of conditions necessary to generate high gain in expansion cooled recombination lasers will be given. Numerical modelling allows scaling laws for the conditions of optimum gain to be derived, and hence matched to plasma generated from laser heated carbon fibres. Comparison between experiment and theory is good. These scaling relations may be used to optimise gain. Generalisation of this approach using foils and composite target allows a range of wavelengths to be generated

Optimizing the performance of nickel-like collisionally pumped x-ray lasers. II. Lasers for the wavelength range 50-100 Å

Soft x-ray lasers operating in the super- 100 Å regime and using grazing incidence pumping methods are now established as efficient sources of radiation in this waveband. The concepts underlying this approach are to separate the ionization and excitation phases of the laser, and to match the pumping density of the latter to the optimal for gain generation. It is therefore of considerable interest to examine whether these ideas can be successfully applied to sub-100 Å lasers. Three problems arise: first the adverse scaling of ionization with temperature for high atomic number ions, second the strong thermal conduction at these temperatures leads to a large hot zone upstream of the absorption, and third the optimum pumping density is greater than the critical density of 1 µm wavelength, solid state pump lasers. Using analytic models and simulation we identify a strategy to overcome these problems using a pre-pulse of a mixed harmonic and fundamental radiation of Nd-glass laser radiation followed by the main pumping pulse of the fundamental normally incident. Due to the large upstream thermal zone and the high ionization temperature, we find that the energy required in the pre-pulse is much (~3 times) larger than that in the main, and that the energy needed consequently scales rapidly with the atomic number and therefore decreasing x-ray wavelength. Systems generating output energies of a few tens of µJ are examined at wavelengths between 50 and 70 Å

Computational modelling of short pulse X-ray laser experiments

The basic theory of amplified spontaneous emission (ASE) for long lived lasers was developed several years ago, allowing a clear understanding of the effects of saturation on the output irradiance to be achieved. The development of ultra-short lasers has necessitated the development of the model to include time dependence. Extending a simple ASE model developed earlier, we have investigated in detail the features of time dependent ASE laser action. The model is compared to more detailed computational simulation, and shows good agreement, whilst revealing the limitations of the model. Simulation shows that the beam is broadened transverse to the propagation direction. Good agreement is shown between simulation and experimental values

Computational modelling of a Ne-like-Ar collisionally pumped O.F.I. X-ray laser

Schemes in which a hot plasma is formed through optical field ionisation (OFI) of a noble gas lasing in the X-UV and pumped by collisional excitation have been proposed for some time [1]. However experimental evidence has been published only once [2] to date and only recently been reproduced [3]. We have performed detailed computational simulations for OFI produced neon like argon (Ar8+) which is predicted to lase at 46.9 nm on the [MATH] transition. Simulations of the atomic and hydrodynamic process within the plasma are used to calculate the temporal evolution of the gain and saturation intensities. A separate code is used to examine the propagation of the driving laser into the plasma and calculates the dispersion of the beam and the resulting degree of ionisation. We find that although high gains are predicted, the saturated output is relatively low at 107 – 108 W/cm2. Furthermore we find that propagating the driving laser into the gas is difficult due to severe diffraction effects and will be a limiting factor in the usefulness of such schemes

An introduction to computer simulation

X, 311 tr.; 24 cm