R. C. Elton

Extension of 3p→3s Ion Lasers into the Vacuum Ultraviolet Region

The feasibility of extending existing near-ultraviolet ion lasers into the vacuum ultraviolet spectral region is analyzed with a simplified three-state model. Single-pass amplification in a laser-produced plasma of reasonable length, pump power, and rise-time requirements is predicted, especially for high electron temperatures. The results are intended to serve as a basis and incentive for detailed numerical modeling and for experiments.

Quasi-stationary population inversion on Kα transitions

In a search for an x-ray laser capable of operating in a quasi-cw mode, a suggested Kalpha-innershell scheme is examined using recently calculated rates. This scheme involves the decay of a K-shell vacancy followed by a more rapid (for certain elements) L-vacancy decay, which maintains the inversion. The present analysis indicates that the scheme is only marginally feasible unless a depletion of resonant absorbers is accomplished through line shifts associated with multiple ionization following K-vacancy production. The pumping requirements for overcoming photoionization losses in the beam and the associated gain conditions are estimated for three elements, namely silicon, calcium, and copper, and it is concluded that photoionization pumping in a selective energy band is required, with emission approaching the blackbody level. A multiline heavy-ion plasma source is suggested.

Seventh harmonic conversion of mode‐locked laser pulses to 38.0 nm

Seventh harmonic generation in helium is used to produce coherent light at 38 mm from laser pulses at 266.1 nm. The variation of both the fifth and seventh harmonic signals with helium pressure is described.

Short-wavelength laser calculations for electron pumping in carbonlike and heliumlike ions

A steady-state computer code has been used to compute population densities of excited terms in carbonlike and heliumlike ions. It is shown that an inversion between terms of the 1s22s22p3p and 1s22s22p3s configurations sufficient to produce observable laser gain can be obtained in high-density plasmas such as produced by laser irradiation of pellets or electromagnetically driven pinch discharges. Such inversions in elements of atomic number Z = 15–40 give rise to lasing in the 300–1200 A range. Shorter wavelength lasing (5–80 A) on 1s3s→1s2p transitions in heliumlike ions of Z = 10–40 is also modeled, but significant gain is probably not possible with current plasma devices.

X-ray laser experiments using laser-vaporized copper-foil plasmas

Recent exploratory experiments used laser-vaporized copper films in a search for gain on 3s–3p transitions in neonlike Cu xx plasmas. The Naval Research Laboratory Pharos iii laser was operated at 100–330 J with mostly 2-nsec pulses. The plasma length, the laser energy and pulse shape, and the copper thickness were varied. Vacuum-ultraviolet and x-ray spectroscopy and pinhole photography measured axial emission, homogeneity, front–rear symmetry, and electron temperature. A novel slotted-copper-foil target showed similar characteristics. Spectral features from a selenium target are described. A distinct dependence of plasma expansion and radiative transport on the rise-time characteristics of the laser pulse is observed. Various explanations for the lack of distinguishable gain in these initial tests are discussed.

X‐ray spectroscopic diagnostics of a copper plasma produced by a laser line focused onto a thin foil

A 500‐J, 2–6‐ns, Nd laser was focused to a 100‐μm‐wide line on a thin (375–1500 A) copper film of 4–23 mm length, supported by a 1500‐A thick Formvar substrate. Space‐resolved 8–16‐A wavelength x‐ray spectroscopy and pinhole photography were utilized. The main spectral features include intense neon‐like Cu xx lines, and somewhat less intense Cu xix and xxi lines. Two types of axial structure are observed, one characterized by a finely (≲100 μm) corrugated structure, and the other by a mm scale irregular nonuniformity. The former could well be due to breakups caused by a filamentation instability. The latter appears to be associated with a mismatch between the laser pulse rise time and a characteristic (thickness‐dependent) foil heating time, which makes it very sensitive to both laser‐beam and copper‐coating nonuniformities. The electron temperature in the Cu xx plasma, as estimated from intensity ratios of the 2p‐nd (n≥4) transition lines, increases with copper thickness.

Extended plasma source for short-wavelength amplifiers

An elongated plasma of length approximately 8 mm with an aspect ratio of ~6:1 has been formed by channeling a laser-produced plasma created in a point focus on a carbon surface. Densities of C(6+) ions near 10(16) cm(-3) were observed in a contracted region beyond the guiding channel, suggesting a use for short-wavelength amplification.

Solid neon sources for plasmas and x-ray lasers

A thin layer of frozen neon is condensed onto a cryogenically cooled substrate for use in generating a neon plasma for x-ray laser research. The freezing process and quasi-equilibrium state of the layer are discussed with a phase diagram. Spectral purity of a neon plasma formed by laser irradiation is demonstrated with grazing-incidence x-ray spectroscopy. Control of the layer thickness is possible to the point that a substrate spectrum is also obtained.

Gain-verification problem in extreme ultraviolet lasing.

The verification of gain on extreme ultraviolet ion lines in elongated plasmas using optically thin intercombination lines from similar ions is proposed. New impurity-line identifications in laser-produced plasmas suggest that recent reports of gain at 58.23 A may be inconclusive.

X‐ray damage in optical coatings

Thin film coatings are susceptible to high intensity x‐ray damage. The PHAROS III laser was utilized to generate a point source of x‐ray emission used to determine the damage threshold of AR‐coated space optics. Thin filters coupled with magnets were used to shield the specimens from thermal radiation and plasma debris. Grids supporting the thin filters could be patterned into the coatings. The surface morphology of damaged specimens has been examined with SEM and AFM microscopes to determine the nature of the damage in multilayer AR coatings. Microscopic techniques were used to measure the depths of coating damage and edge sharpness in the patterned region.