Accurate target-plane focal-spot characterization in high-energy laser systems using phase retrieval.

Target-plane intensities on the short-pulse beamlines of OMEGA EP, a petawatt-class laser, are characterized on-shot using the focal-spot diagnostic (FSD), an indirect wavefront-based measurement. Phase-retrieval methods are employed using on-shot and offline camera-based far-field measurements to improve the wavefront measurements and yield more-accurate, repeatable focal-spot predictions. Incorporation of these techniques has improved the mean cross-correlation between the FSD predictions and direct far-field fluence measurements in the target chamber from 0.78 to 0.94.

A transmitted-beam diagnostic for the wavelength-tunable UV drive beam on OMEGA.

A transmitted-beam diagnostic (P9TBD) was developed as part of a new experimental platform used to study laser-plasma interactions on OMEGA. Located in the opposing port to the wavelength-tunable (350 nm to 353 nm) UV drive beam, the P9TBD characterizes the beam after it propagates through an undersense plasma. The instrument consists of a large-aperture window that allows light to exit the target chamber and project onto a thin sheet of semi-transparent diffuser material. Light transmitted through the diffuser is recorded using a time-integrated camera and a fiber-optically coupled streaked spectrometer, providing measurements of the energy, power, fluence, polarization, and spectrum of the transmitted beam. The diagnostic enables direct observation of a variety of cross-beam energy transfer phenomena, such as wavelength detuning, polarization effects, and gain saturation.

Scaling hot-electron generation to high-power, kilojoule-class laser-solid interactions.

Thin-foil targets were irradiated with high-power (1 ≤ P(L) ≤ 210 TW), 10-ps pulses focused to intensities of I>10(18) W/cm(2) and studied with K-photon spectroscopy. Comparing the energy emitted in K photons to target-heating calculations shows a laser-energy-coupling efficiency to hot electrons of η(L-e) = 20 ± 10%. Time-resolved x-ray emission measurements suggest that laser energy is coupled to hot electrons over the entire duration of the incident laser drive. Comparison of the K-photon emission data to previous data at similar laser intensities shows that η(L-e) is independent of laser-pulse duration from 1 ≤ τ(p) ≤ 10 ps.

High-contrast plasma-electrode Pockels cell.

A plasma-electrode Pockels cell (PEPC) has been developed for use on the OMEGA extended performance (EP) laser system that can be used in a high-contrast optical switch, as required for isolation of the system from retroreflected pulses. Contrast ratios reliably exceeded 500:1 locally everywhere in the clear aperture. The key to achieving this improvement was the use of circular windows simply supported on compliant O rings, which is shown to produce very low stress-induced birefringence despite vacuum loading. Reliable operation was achieved operating at a relatively high operating pressure, low operating pressures being found to be strongly correlated to occurrences of local loss of plasma density.