B. L. Whitten

Observation of soft x‐ray amplification in neonlike molybdenum

Thin molybdenum coated foils have been irradiated in line focus geometry with from 3 to 8×1014 W cm−2 of 0.53‐μm light at the Nova laser. The resulting exploding foil plasma has demonstrated x‐ray laser gain at four wavelengths (106.4, 131.0, 132.7, and 139.4 A), identified as 3s‐3p transitions in neonlike Mo. The J=0–1, a 3s–3p transition at 141.6 A has been identified, but does not show evidence of significant gain in disagreement with the theory.

Scaling of neonlike lasers using exploding foil targets

We present a set of calculations for laser-gain predictions in a neonlike collisional excitation scheme using laser-driven exploding foil targets. The calculation includes three steps: the ionization balance, the neonlike excited-state kinetics, and the hydrodynamics of the exploding foil target. The ionization-balance model solves steady-state rate equations, including excited states, using scaled hydrogenic atomic physics. The model for the neonlike excited-state kinetics is also steady state and includes the ground state and the 36 n = 3 excited states, with radiative and collisional transitions connecting these states. The plasma conditions in the exploding foil targets are calculated by using the similarity model of London and Rosen [ Phys. Fluids29, 3813 ( 1986)]. For selected elements in the range 20 < Z < 56, we predict the gain for the two most prominent 2p53p to 2p53s (J = 2–1) transitions seen in experiments, the plasma conditions necessary to maximize the gain, and the specifications for the laser driver and target required to reach those plasma conditions. Our predicted gains are larger than those measured in experiments, for reasons we discuss, but our calculations agree qualitatively with the observed trends; the gain peaks for elements around selenium and falls off for both lighter and heavier ions. Neglected effects, such as time-dependent kinetics and radiation trapping, are also discussed.

Mutual neutralization in rare gas halides

The cross sections for mutual neutralization in Ne++F−, Ar++F−, and K++Cl are obtained from two‐state close coupling calculations. The Landau–Zener approximation gives adequate results in each case. The rate of neutralization is studied as a function of ambient gas density. Near atmospheric pressure, mutual neutralization is more likely than excimer formation of Ne++F− collisions, but less likely in Ar++F− collisions.

Lawrence Livermore National Laboratory X-Ray Laser Research: Recent Results

Since the successful demonstration of gain in neon-like selenium using an exploding foil amplifier, the x-ray laser group at Lawrence Livermore National Laboratory has investigated further the exploding foil amplifier concept for use in XUV lasers. Results are reported of the characteristics of selenium amplifiers up to 50 mm in length. Observation of at least 16 gain lengths for the 206 Å line of selenium is reported. Output powers in excess of 1 MW have been measured in pulses of approximately 200 picoseconds. The effects of refraction on the performance of long amplifiers have been studied. The occurrence time of the x-ray laser output relative to the input heating pulse has been measured and found to be in disagreement with a recent model that suggests three-body recombination driven by rapid radiative cooling as the inversion process in the selenium plasma.

No pain—no gain: The complex art of soft x‐ray laser target design and analysis

We review our methodologies in the design and analysis of soft x‐ray laser experiments. We convolve large scale 2‐D hydro code output with detailed atomic data bases in a kinetics code with 1‐D or 2‐D line transfer. The time and space dependent level population data is then post processed further with a beam transport code, including refraction, to predict actual experimental results. While mysteries do remain, we present many examples that show how this complex modeling procedure is crucial in explaining experimental results.

Theory and Analysis of Soft X-Ray Laser Experiments

The atomic modeling of soft x-ray laser schemes presents a formidable challenge to the theorists -- a challenge magnified by the recent successful experiments. A complex plasma environment with many ion species present must be simulated. Effects such as turbulence, time dependence, and radiation transport, which are very difficult to model accurately, may be important. We shall describe our efforts to model the recently demonstrated soft x-ray laser in collisionally pumped neon-like selenium, with emphasis on the ionization balance and excited state kinetics. The relative importance of various atomic processes, such as collisional excitation and dielectronic recombination, on the inversion kinetics will be demonstrated. We shall compare our models with experimental results and evaluate the success of this technique in predicting and analyzing the results of x-ray laser experiments.

Time‐resolved x‐ray line intensities for diagnostics of laser‐produced plasmas (abstract)

We have developed a streaked crystal spectrograph for time‐resolving x‐ray lines from laser‐produced plasmas. The spectrograph combines an ellipsoidally curved crystal and x‐ray streak camera to optimize spectral coverage and resolution, even though the instrument is located a meter from the source. The instrument has been used at Novette to measure S lines doped in concentrations of 4% in the center of CH foils. Time histories of the temperature and density of the CH plasma have been derived from relative intensities of the S lines. Details of the experiment and analysis will be presented along with the instrument design.

Soft X‐ray laser experiments at the Novette laser facility

We discuss the results of and future plans for experiments to study the possibility of producing an x‐ray laser. The schemes we have investigated are all pumped by the Novette Laser, operated at short pulse (τL∼100 psec) and an incident wavelength of λL∼0.53 μm. We have studied the possibility of lasing at 53.6, 68.0−72.0, 119.0, and 153.0 eV, using the inversion methods of resonant photo‐excitation, collisional excitation, and three‐body recombination.