X-ray laser pulses at the Fourier transform limit

The temporal output of a Ni-like Ag x-ray laser of wavelength 13.9 nm has been recorded using a streak camera with ultrashort (700 fs) temporal resolution. We present a model to calculate the degree of coherence and Fourier transform limit of x-ray laser pulses produced by amplified spontaneous emission and relate the results from the model to previous interferometric measurements of the coherence length of the same Ni-like Ag x-ray laser and our measured duration of temporal output. Our modeling shows that the interferometer and streak camera results are consistent and close to the Fourier transform limit at longer gain medium lengths

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

Efficient calculation of degenerate atomic rates by numerical quadrature on GPUs

The rates of atomic processes in cold, dense plasmas are governed strongly by effects of quantum degeneracy. The electrons follow Fermi-Dirac statis- tics and their high density limits the number of quantum states available for occupation after a collision. These factors preclude a direct solution to the usual rate coefficient integrals. We summarise the formulation of this problem and present a simple, but efficient method of evaluating collisional rate coefficients via direct numerical integration. Numerical quadrature has an intrinsically high level of parallelism, ideally suited for graphics processor units. GPUs are particularly suited to this problem because of the large number of integrals which must be carried out simultaneously for a given atomic model. A CUDA code to calculate the rates of signicant atomic processes as part of a collisional-radiative model is presented and discussed. This approach may be readily extended to other applications where rapid and repeated evaluation of many integrals is required

The creation of radiation dominated plasmas using laboratory extreme ultra-violet lasers

Ionization in experiments where solid targets are irradiated by high irradiance extreme ultra-violet (EUV) lasers is examined. Free electron degeneracy effects on ionization in the presence of a high EUV flux of radiation is shown to be important. Overlap of the physics of such plasmas with plasma material under compression in indirect inertial fusion is explored. The design of the focusing optics needed to achieve high irradiance (up to 1014 Wcm−2) using an EUV capillary laser is presented

Plasma scale length effects on protons generated in ultra-intense laser–plasmas

The energy spectra of protons generated by ultra-intense (1020 W cm−2) laser interactions with a preformed plasma of scale length measured by shadowgraphy are presented. The effects of the preformed plasma on the proton beam temperature and the number of protons are evaluated. Two-dimensional EPOCH particle-in-cell code simulations of the proton spectra are found to be in agreement with measurements over a range of experimental parameter

RECOMBINATION AND PHOTO-PUMPING MECHANISMS FOR PRODUCING GAIN ON HYDROGEN-LIKE BALMER ALPHA LINES

Two of the most promising classes of schemes for X-ray laser action are the recombination approach, in which population inversions are produced by the decay cascade following electron recombination into high-lying quantum states of a recombining ion, and the photo-pumping approach, in which population inversions are produced by direct pumping of the upper quantum state by photo-excitation from the ground state. In this paper, the recombination and photo-pumping approaches for producing x-ray lasing on the hydrogen-like Balmer alpha line are compared and the effects of the hydrogen-like fine structure on the plasma and other conditions required to achieve Balmer alpha gain are examined. For electron densities Ne ≤ 1012 Z7 cm -3 (where Z is the atomic number of the hydrogen-like ion), it is shown that Balmer alpha gain can be much more difficult to produce than is predicted by simple calculations assuming all sub-levels are populated statistically. It is shown, for example, that Balmer alpha gain produced by 1s-3p photo-pumping becomes impossible for electron densities Ne ≤ 1010 Z7 cm-3 because the 2s1/2 sub-level becomes metastable

Analytical modeling of group-velocity effects in saturated soft-x-ray lasers pumped with a picosecond traveling-wave excitation

A monodimensional amplified spontaneous emission model of traveling-wave pumped soft-x-ray lasers in transient collisional excitation (TCE) is presented. The model explicitly includes the influence of the local gain on the group velocity of the x-ray pulses propagating in the active medium and the saturation of the amplified x-ray signal. Complete analytical solutions of the model in a number of physically interesting cases are derived. The important parameters governing the behavior of these lasers are identified, their influence on the output is investigated, and the optimum configuration required in experiments for efficient traveling-wave pumping is discussed for conditions typical of soft-x-ray lasers in the TCE regime

A model for opacity measurements across expanding X-ray laser media

A theoretical model has been developed for opacity calculations across an expanding laser produced plasma used as an x-ray laser medium. The assumptions of planar geometry, exponentially decreasing emissivity and absorption coefficient with distance with a step rise at a particular point and a linear with distance plasma bulk velocity profile have been made. The hydrodynamic coupled to the atomic 'EHYBRID' code has been modified to take into account the radiation trapping across the plasma using the net radiative bracket predicted by the above model. The result of the theoretical model as well as the output of the modified EHYBRID code have been employed to solve the radiation transfer equation and to simulate 123 Ne-like and 399 F-like Germanium resonance lines both in time resolved and time integrated modes. We obtain agreement between the simulated spectra and experimental spectra measured during x-ray laser experiments

Group speed effects in saturated soft X-ray lasers in transient collisional excitation regime driven with travelling wave pumping

A monodimensional uni-directional Amplified Spontaneous Emission (ASE) model for application to the description of travelling wave pumped soft X-ray lasers in the Transient Collisional Excitation (TCE) scheme is presented. The model includes explicitly the influence of the local gain on the group velocity of the X-ray pulses propagating in the active medium and the saturation of the amplified X-ray signal. Complete analytical solutions of the model in a number of physical interesting cases are derived. The important parameters governing the behaviour of this ASE mirrorless lasers are identified and their influence on the output is investigated for conditions typical of soft X-ray lasers in the TCE regime