H. Guennou

Soft-x-ray amplification by lithium like ions in recombining hot plasmas

This paper describes calculations and experiments about soft-x-ray amplification by lithiumlike ions in recombining laser-produced plasmas. Time- and space-dependent population inversion densities calculated with a collisional-radiative model used as the postprocessor of a hydrodynamic code are reported. Amplification diagnostic accuracy in plasma experiments is discussed. Time-integrated and time-resolved measurements of gain are presented, especially at 105.7 A in lithiumlike aluminum. It is shown that, in a plasma produced by a 3-nsec laser pulse, the peak of amplified radiation occurs about 7 nsec after the top of the pulse. The maximum gain–length product measured previously was 2–2.5. A short description of a future experiment designed for producing a much larger gain is presented.

Amplification of soft-X-ray spontaneous emission in aluminium and magnesium plasmas

Amplification of spontaneous emission at 105.7 and 127.9 AA is reported for Li-like aluminium and magnesium in recombining laser-produced plasmas. Evidence of gain is deduced from the variation of radiation intensity according to the length of a line-shaped plasma. Detailed calculations of rate equations with a suitable plasma model, associated with numerical simulations of plasma hydrodynamics, predict population inversions between 3d and 5f levels, in accordance with these experiments.

Effective rates for li-like ions; calculated XUV gains in Al10+

This work concerns the modelling of the Li-like recombination X-ray laser. We have improved the description of the ionization dynamics and the gain production in a recombining laser-produced plasma. The contribution of the Li-like excited levels (up to n=30), which are assumed to follow a quasi-steady state evolution with respect to the ionic populations, is now incorporated in the ionization and recombination rates connecting He-like and Li-like ionic species. Simple expressions for these effective rates are derived. The number of Li-like excited levels to be included in the model, as well as the determination of the lower limit of the thermal band are examined. Predicted 3d-4f gain values (3 cm−1 at λ=154.6 Å) when simulating fibre targets cylindrically illuminated by a short laser pulse are now in good agreement with experiments. Persistent discrepancies in other cases (3d-5f inversion or slab targets) are discussed.

Soft–X-Ray Amplification in Recombining Aluminum Plasma

This paper reports time-resolved measurements of gain at 105.7 A in an aluminum plasma produced by a laser. It is shown that the amplification occurs during the plasma cooling, at the end of the laser pulse. The observed time-variation of the gain agrees with the prediction of a plasma recombination model used for calculating the level populations of the lithium-like Al10+ ions.

New spectroscopic results in the 2s2p5-2s2p43s transition array of Mg4+, Al5+ and Si6+ ions

Experimental and theoretical results on far UV transitions between levels of configurations 2s2p43s and 2s2p5 in Mg V, Al VI and Si VII spectra are reported. Theoretical analysis by means of ab initio and semi-empirical methods leads to a new identification of a number of lines, nine of which pertain to the Si VII spectrum. These are situated in the 70-90 AA range, and have not been observed previously. The energy levels of the upper configurations of Al5+ and Su6+ ions are calculated in the intermediate coupling scheme. The correlation effects expressed by configuration interactions do not greatly influence the relative energy position of the analysed lines but they do have significant consequences on their relative intensities.

Lasing near 200 Å with neon-like zinc and lithium-like sulfur

In this paper we present the recent results obtained on X-ray lasers with the new facility of LULI (Palaiseau, France). As regards the study of collisional-excitation pumping we show that the J=0–1 line of neon-like zinc, at 212 Å, exhibits a surprisingly large gain coefficient of 4.9 cm−1. A detailed comparison of the time-dependent intensities of the J=0–1 and the J=2−1 line at 267 Å leads to the conclusion that these two lines are not emitted in the same region of the plasma. On the other hand we performed a theoretical and experimental study of 5g-4f lasing line of lithium-like sulphur at 206.5 Å. For the first time, in the case of plasma recombination pumping, we did not observe any reduction of the gain coefficient when the plasma length is raised from 1 cm to 2 cm. From numerical simulation this is likely due to radiation trapping and similar processes having much less influence on 5g-4f population inversion than on previously observed lithium-like lasing transitions. These results show that large gain-length values should be obtained with ≈0.5 kJ driving laser pulses.

RADIATION TRAPPING AND X-RAY LASING IN AL10+ IN THE RECOMBINATION SCHEME

Abstract A self-consistent model for radiation trapping in recombining plasmas is presented in view of X-ray laser gain calculations. We obtain the population density of the lasing levels in the presence of resonance photons which are emitted from the lower level of the lasing transition and which may be reabsorbed elsewhere in the plasma. The resulting gain reduction at the time and in the cell of maximum gain is about 25% and 10% for the Li-like Al transitions at 154.7 and 105.7 A, respectively. Our results for the transition at 154.7 A agree to within 10% with the most recent measured gain, in contrast with the up-to-date calculations on the escape probability which predict much smaller gains.

Calculated Population Inversions in Lithiumlike Cu26+ Ions in Recombining Plasmas

In the attempt to obtain amplification of soft-X-ray emission at shorter wavelengths, the lithiumlike ions are today considered with a particular interest. An experimental result on the 4f/3d inversion in Cu26+, the wavelength of the corresponding transition being 25.6 A, has been recently reported. Using a collisional-radiative model, we have calculated the population inversions which may be expected according to the conditions of temperature and density of the plasma concerned. Our results are compatible with the experimental observations. A comment on the comparison between calculated and observed results is presented.

Frequency redistribution in the radiative transfer problem of amplifying media. Application to Ge22+ and C5+ x-ray lasers

This paper is devoted to the effect of frequency redistribution on x-ray laser beams. The redistribution function, calculated by extending the frequency fluctuation model to two-photon processes, shows coherent scattering and a redistributed contribution. The spectral emissivity, corrected in order to account for this effect, is calculated for lasers using neon-like germanium and hydrogen-like carbon as amplifiers. The radiative transfer equation is solved, and the intensity of the x-ray laser output is given for two 3p-3s lines in a neon-like amplifier. We also study the variation in the emissivity of the C5+ Balmer-α lasing line. The physical conditions of the calculation are derived from modelling of experiments conducted at the Rutherford Appleton Laboratory.

Simulations of X‐UV gain in a plasma pumped by a pulse‐train laser

We present numerical simulations of X‐UV gain in plasmas produced by irradiating a target with a train of 100 ps‐pulses. Our predictions based on a 1.5D hydrocode do not account for the experimental gains reported on the A1XI 3d‐5f by Hara and his co‐workers. We suggest that the simulation of multiple‐pulse irradiated plasmas may require a bidimensional description and propose an experimental verification of this assumption.