LLE Review 111

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Highlights

This volume of the LLE Review, covering July–September 2007, features the analysis and the experimental results of a study of pump-induced, temporal-contrast degradation in optical parametric chirped-pulse amplification (OPCPA). In OPCPA systems, the temporal fluctuations of the pump pulse are coupled to the spectrum of the chirped signal by the instantaneous parametric gain and lead to a reduction in the temporal contrast of the recompressed amplified signal. The authors derive equations describing the contrast degradation in an OPCPA system due to the pump amplified spontaneous emission. They also quantify the reduction of the contrast in the amplified pulse both analytically and via simulations for an OPCPA system. The placement of a Bragg grating in the regenerative amplifier produces a simple and an efficient pump-intensity reduction, demonstrating contrast improvements up to 30 dB.

Additional highlights of research presented in this issue include the following:

• Observations of the collapse of strong convergent shocks at the center of spherical capsules filled with D₂ and ³He gas, which induces both D-D and D-³He nuclear production. Temporal and spectral measurements of products from both reactions verify data reliability and allow efficient and insightful alterations in ICF simulations.
• Work on equation-of-state measurements in Ta₂O₅ aerogel. Highly porous samples of tantalum pentoxide (Ta₂O₅) aerogel were compressed from initial densities of 0.1, 0.15, and 0.25 g/cm³ by shock waves with strengths between 0.3 and 3 Mbar. A comparison of the compression measurements and an available high-energy-density equation-of-state (HED-EOS) model found that the model underestimates the level of compression achieved by shock loading below a Mbar. The thermal measurements also indicate less significant heating than models predict.
• Discussion of EXAFS (extended x-ray absorption fine structure) measurements used to determine the temperature and compression in a vanadium sample quasi-isentropically compressed to pressures of up to ~0.75 Mbar. VISAR (velocity interferometer system for any reflector) measurements, with aluminum substituting for the vanadium, are used to calibrate the drive pressure. The experimental results obtained by EXAFS and by VISAR agree with each other and with the simulations of a hydrodynamic code. The role of a shield to protect the sample from impact heating and the role of radiation heating from the imploding target and the laser-absorption region are also studied.
• Report on the effect of resonance absorption in OMEGA direct-drive designs and experiments. Simulations demonstrate an important contribution of the resonance absorption during both the short laser picket (~100 ps) and the first 200 to 300 ps in the long laser pulse. Planar reflection light experiments on OMEGA were conducted to validate the theoretical results.
• Presentation of the diagnosing of direct-drive, shock-heated, and compressed plastic planar foils with noncollective spectrally resolved x-ray scattering. Plastic (CH) and Br-doped CH foils were driven with six beams, having an overlapped intensity of ~1 × 10¹⁴ W/cm² and generating ~15-Mbar pressure in the foil. The uniformly compressed portion of the target was probed with 9.0-keV x rays from a Zn He_α backlighter created with 18 additional tightly focused beams.
• An examination of the scattered x-ray spectra reveals that an upper limit of Z ~ 2 and T_e = 20 eV can be inferred, since low average ionizations (i.e., Z < 2) cannot be accurately diagnosed in this experiment.