LLE Review 129

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Highlights

This volume of LLE Review, covering October–December 2011, features “Crossed-Beam Energy Transfer in Direct-Drive Implosions.” Direct-drive implosion experiments on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1995)] have shown discrepancies between simulations of the scattered (non-absorbed) light levels and measured ones that indicates the presence of a mechanism that reduces laser coupling efficiency by 10% to 20%. The authors of the feature article attribute this degradation in laser coupling to crossed-beam energy transfer (CBET), which is electromagnetically seeded, low-gain stimulated Brillouin scattering. CBET scatters energy from the central portion of the incoming light beam to outgoing light, reducing the laser absorption and hydrodynamic efficiency of implosions. One-dimensional hydrodynamic simulations including CBET show good agreement with all observables in implosion experiments on OMEGA. Three strategies to mitigate CBET and improve laser coupling are considered: the use of narrow beams, multicolor lasers, and higher-Z ablators. Experiments on OMEGA using narrow beams have demonstrated improvements in implosion performance.

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

• The hot-electron equilibration dynamics in high-intensity laser interactions with picosecond pulses and thin-foil solid targets is inferred using time-resolved K_α spectroscopy. The measured K_α-emission pulse width increases from ~3 to 6 ps for laser intensities from ~10¹⁸ to 10¹⁹ W/cm². Collisional energy-transfer model calculations suggest that hot electrons with mean energies from ~0.8 to 2 MeV are contained inside the target. The inferred mean hot-electron energies are broadly consistent with ponderomotive scaling over the relevant intensity range
• The hot-electron generation by the two-plasmon-decay (TPD) instability in plasmas relevant to direct-drive inertial confinement fusion is measured. Density scale lengths of 400 µm at n_cr = 4 in planar CH targets allows the TPD instability to be driven to saturation for vacuum intensities above ~3.5 × 10¹⁴ W/cm². In the saturated regime, ~1% of the laser energy is converted to hot electrons. The hot-electron temperature is measured to increase rapidly from 25 to 90 keV as the laser beam intensity is increased from 2 to 7 × 10¹⁴ W/cm². This increase in the hot-electron temperature is compared with predictions from nonlinear Zakharov models.
• A design of an ultra-intense optical parametric chirped-pulse–amplification (OPCPA) system at 910 nm is presented. Technologies are being developed for large-scale systems based on deuterated potassium dihydrogen phosphate (DKDP) optical parametric amplifiers that could be pumped by kilojoule-class Nd:glass lasers such as OMEGA EP. Results from a prototype white-light–seeded chain of noncollinear optical parametric amplifiers (NOPA’s) are reviewed. The development of a cylindrical Öffner stretcher that has advantages over standard stretchers for ultra-intense, high-contrast systems is described. The front-end development will culminate in demonstrating a mid-scale optical parametric amplifier line (OPAL) that will use scalable technologies to produce 7.5-J, 15-fs pulses with a temporal contrast exceeding 10¹⁰.
• A narrowband x-ray imager for a Cu K_α line at ~8 keV using a spherically bent quartz crystal is designed and implemented on the OMEGA EP laser. The quartz crystal is cut along the 2131 (211) planes for a 2d spacing of 0.3082 nm, resulting in a Bragg angle of 88.7°, very close to normal incidence. An optical system is used to remotely align the spherical crystal without breaking the vacuum of the target chamber. The images show a high signal-to-background ratio of typically >100:1 with laser energies ≥1 kJ at a 10-ps pulse duration and a spatial resolution of less than 10 µm.
• A new operation regime of NbN superconducting single-photon detectors (SSPD’s) by integrating them with a low-noise cryogenic high-electron-mobility transistor and a high-load resistor is proposed. The new SSPD operating scheme makes it possible to distinguish dark pulses from actual photon pulses in SSPD’s and therefore gain a better understanding of the origin of dark counts generated by the detector. A statistical analysis of amplitude distributions of recorded trains of the SSPD photoresponse transients is used to obtain information on the spectral characteristics of incident photons and demonstrates that meander-type SSPD’s exhibit some photon-number–resolving capability.
• The thermal conductivity of solid D₂ is measured by the 3ω method, in which a wire embedded in the medium serves as both a heater and a temperature sensor. Accurate values of conductivity are obtained for solid D₂ in the temperature range 13.4 K to 18.6 K. In this temperature range, normal and ortho D₂ are found to have the same conductivity.