LLE Review 132

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Highlights

This volume of LLE Review, covering July–September 2012, features “Laser–Plasma Interactions in Direct-Drive–Ignition Plasmas,” which reviews the current understanding of multibeam laser–plasma instabilities including cross-beam energy transfer (CBET) and two-plasma decay (TPD) for direct-drive ignition. CBET is driven by multiple laser beams and can significantly reduce the hydrodynamic efficiency in direct-drive experiments on OMEGA. The combination of zooming and dynamic bandwidth reduction will provide a 30% effective increase in the drive energy on OMEGA direct-drive implosions. TPD instability can also be driven by multiple laser beams. Both planar and spherical experiments were performed to study the hot electrons generated by TPD at the Omega Laser Facility. At ignition-relevant intensities, the fraction of laser energy converted to hot electrons is measured to decrease by an order of magnitude when the ablator material is changed from carbon-hydrogen to aluminum.

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

• The investigations of stress-radiation–induced swelling in plastic capsules are reported. The process of filling targets with DT for cryogenic experiments on the OMEGA laser induces small-scale features on the inner surface of the plastic capsules. Each feature was a cluster of low-level domes (<0.1 µm high) with individual lateral dimensions smaller than 5 µm that collectively covered lateral dimensions of up to 300 µm². These features were observed only when a high-radiation dose was combined with high stress in the plastic wall, as occurs when the capsules are permeation filled and transferred at cryogenic temperatures. No porosity or void structure was observed in or below these domes. It is speculated that the domes are a swelling caused by radiation-induced bond scission and chemical restructuring that reduces the plastic density in localized regions.
• Studies on time-resolved carrier dynamics in Si-on-glass absorbers for photovoltaic cells are presented. A femtosecond pump–probe spectroscopy method was used for characterizing amorphous and microcrystalline silicon films grown on glass substrates as absorbers for photovoltaic applications and collected corresponding transmissivity change (ΔT/T) waveforms. Depending on their growth process, optically excited, ~300-nm-thick Si absorbers exhibited a bi-exponential carrier relaxation with characteristic times varying from picoseconds to nanoseconds. Experimental data were interpreted using a three-rate–equations model, and the relaxation was identified as electron–phonon cooling, followed by carrier trapping and/or Shockley–Read–Hall recombination. An excellent fit between the model and the ΔT/T transients was obtained, indicating a correlation between the hydrogen content in the film and the trap concentration.
• Measurements of energetic protons in cone-in-shell fast-ignitor experiments are performed at Omega. In these experiments, charged-particle spectrometers were used to measure a significant population (>10¹³) of energetic protons (7.5 MeV maximum), indicating the presence of strong electric fields. These energetic protons, observed in directions both transverse and forward relative to the direction of the short-pulse laser beam, have been used to study aspects of coupling efficiency of the fast ignitor. Forward-going protons were less energetic and showed no dependence on laser intensity or whether the cone tip was intact when the short-pulse laser was fired. Maximum proton energies transverse to the cone-in-shell target scale with incident on-target laser intensity (2 to 6 × 10¹⁸ W/cm²), as described by the ponderomotive scaling (~I^1/2). It is shown that these protons are accelerated from the entire cone surface, possibly due to return currents, rather than from the cone tip alone. The proton-inferred lower bound on the hot-electron temperature was hotter than the ponderomotive scaling by a factor of 2 to 3.
• R. D. Petrasso’s (Plasma Science and Fusion Center, MIT) report on the Fourth Omega Laser Facility Users’ Group Workshop is presented.
• This volume concludes with a summary of LLE’s Summer High School Research Program, the FY12 Laser Facility Report, and the National Laser Users’ Facility and External Users’ Programs.