A. A. Offenberger

Phase conjugation and pulse compression of KrF-laser radiation by stimulated Raman scattering

An experimental investigation of phase conjugation and pulse compression by stimulated Raman scattering in methane gas using a narrow-linewidth KrF-laser pump is presented. Good spatial reproduction of the pump beam is observed at reflectivities of up to 80%. With a pump-pulse duration of 25 nsec, backward Stokes pulses of 5-nsec duration with 40% of the 100-mJ pump energy were obtained.

Saturation of stimulated Brillouin scattering by ion wave decay in a dissipative plasma

Secondary decay of an ion‐acoustic wave is shown to provide an efficient saturation mechanism for stimulated Brillouin scattering in underdense plasma (n<0.5ncr). Phase effects and linear damping of ion waves are included in the analysis. In the weak damping regime, secondary decay dominates linear damping providing efficient saturation of Brillouin scattering. Heavy linear damping of ion waves increases the threshold for secondary decay considerably and very small densities are required to obtain saturation.

Stimulated Raman backscattering in the presence of ion‐acoustic fluctuations

Threshold conditions are analyzed for stimulated Raman backscattering (SRS) from an underdense plasma in the presence of ion‐acoustic fluctuations. For typical ion‐acoustic spectra which could result from a Brillouin instability occurring simultaneously with SRS, a reduced collisional threshold due to induced bremsstrahlung is found for the Raman instability. Similarly, for inhomogeneous plasma the phase mismatch created by a density gradient can be compensated for by an inhomogeneity in the ion‐acoustic spectrum; as a consequence, the inhomogeneity threshold for absolute instability can be substantially reduced. Comparisons are made with experimental observations of stimulated Raman scattering in the presence of strong stimulated Brillouin scattering for CO2 laser‐plasma interaction. It is found theoretically that for fluctuation levels typical of that accompanying SBS, induced bremsstrahlung growth rates for Langmuir waves can be of the order of collisional damping rates.

Electrically triggered multimodule KrF laser system with narrow‐linewidth output

A narrow‐linewidth multimodule KrF discharge laser system is described which is designed to be the front end of an electron‐beam pumped amplifier–Raman compressor high‐power UV laser facility. The discharge laser system consists of an intracavity triple etalon‐narrowed master oscillator followed by an injection‐locked slave oscillator and two single‐pass amplifiers. The master oscillator has 30 cm of gain length while the three remaining devices utilize 90 cm of gain length. The units are electrically triggered from a high‐voltage multitrigger pulse generator. The pertinent electrical parameters have been determined and are reported. Narrowband output (0.1 cm−1) of 1.6 J can be obtained in 20‐ns FWHM pulses. The small signal gain and saturation intensity of the 90‐cm amplifier modules were measured giving values of 0.115 cm−1 and 1.7 MW/cm2, respectively.

Multiline injection mode locking of a transversely excited atmosphere CO2 laser

A multiline injection mode locked transversely excited atmosphere CO2 laser has been operated for use in high intensity laser‐plasma interaction experiments. Laser pulses of 390 MW and 2‐ns duration with up to three equal power lines were produced by installing intracavity SF6 cells in both the injection and slave oscillators to tune them to the desired laser transition. A simple model defines the laser system requirements to achieve simultaneous oscillation on several lines.

Temperature and x‐ray intensity scaling in KrF laser plasma interaction

Experimental measurements are presented of space and time integrated x‐ray emission from aluminum plasma produced by 2 ns (full width at half‐maximum), 268 nm Raman compressed KrF laser pulses. A single temperature spectrum was measured giving temperatures of 75–440 eV over the range of intensities 9×1011–2×1013 W/cm2. This scaling agrees well with a simple self‐regulating model. Conversion of laser energy into x rays above 1 keV energy was found to scale as I2.37L reaching 0.35% at 3.5×1013 W/cm2.

Low‐jitter, low‐inductance, electrically triggered spark gap

This investigation is concerned with the development of a low‐inductance, low‐jitter, electrically triggered, high‐voltage annular rail spark gap. Three trigger electrode geometries—blade, dull, and sawtooth edge—are tested for the effects of different field stresses on the spark gap switching performance. All possible polarity combinations are applied to the high‐voltage and trigger electrodes of the rail gap, and the switching inductance and jitter is measured for each case. Finally, the effects of photopreionization from bare sparks on the spark gap switching performance is explored. A switch inductance of 4 nH and jitter of 0.6 ns have been achieved. A practical application of the rail gap is also described.

Experimental study of an SF6 Brillouin amplifier pumped by KrF laser radiation

Experimental measurements are presented of the gain for a backward Brillouin amplifier using SF6 gas, in the pressure range of 3–10 atm, pumped by KrF laser radiation. A small signal gain of 6×10−4 cm MW−1, pump depletion of 20%, and an intensity gain of 2.5 have been demonstrated.

Kinetic model for plasmas in a strong high-frequency field

A kinetic model for stable, underdense plasma in the presence of a strong electromagnetic pump wave is presented. Ion dynamics are properly treated to account for enhancement in electrostatic fluctuations. Such fluctuations are important even for pump strength below threshold for the oscillating two‐stream and parametric decay instabilities in the underdense region. Thresholds of these instabilities are numerically determined. The results of calculations for the static form factor S(k) electron collision terms, and the absorption coefficient are presented for stable, homogeneous plasmas. For moderate pump values (below instability threshold) enhanced fluctuation levels and increased absorption are found, which have relevance to a variety of experimental situations involving high‐intensity laser, microwave, or radio wave interaction with plasmas.

Reduced Brillouin backscatter in CO2 laser-target interaction

Abstract Substantially reduced Brillouin reflection has been found for CO 2 laser irradiated high density gas targets. In contrast to high reflectivity (60%) previously observed for underdense hydrogen plasma, total backscatter (stimulated plus specular) is found to peak at 30% for incident intensity 5 × 10 12 W/cm 2 and decrease thereafter to 18% at 10 13 W/cm 2 . Ponderomative effects are postulated to account for these observations.