LLE Review 115

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Highlights

This volume of the LLE Review, covering April–June 2008, features a report on the development and implementation of large-aperture, tiled-grating compressors for the OMEGA EP Laser System. These units are critical to temporally compress the laser pulses on OMEGA EP from nanosecond to subpicosecond durations. There are two large (1.5-m) grating compressors on the OMEGA EP laser. Each of these consists of four sets of tiled-grating assemblies. The article provides details on the techniques used for tiling individual tiled-grating assemblies and for optimizing the overall performance of the compressor. Both assemblies achieved subpicosecond-pulse duration without tiling-induced temporal degradation.

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

• A comprehensive scientific program is being pursued at LLE to investigate the fast-ignitor concept for inertial confinement fusion. The OMEGA EP laser, completed in April 2008, is key to this program. Two of the OMEGA EP beams can operate in short-pulse mode with up to 2.6 kJ at a 10-ps duration. These beams can be routed into either the OMEGA EP chamber or combined collinearly into the existing OMEGA target chamber for integrated fast-ignition experiments. Fuel-assembly experiments have already started on OMEGA and have achieved an areal density of ~200 mg/cm²—sufficient to stop MeV electrons produced by the short-pulse laser.
• A focal-spot diagnostic for high-energy petawatt-class power lasers. Accurate measurements at full energy are demonstrated using high-resolution wavefront sensing, in combination with techniques to calibrate on-shot measurements with low-energy sample beams. Results are shown for full-energy activation shots on OMEGA EP.
• Suprathermal electrons generated by the two-plasmon-decay instability, gas-filled hohlraums. The OMEGA experiments focused on studies of this instability in the exploding laser entrance hole window in gas-filled hohlraums. This instability results in the generation of high-energy electrons. A threshold laser intensity of 5 × 10¹⁴ W/cm² was measured. As a result of this research, the initial overlapped intensity laser incident on the entrance hole window of the ignition target for the NIF has been set below the measured intensity threshold to retain ignition margin by staggering the turn-on time of the inner and outer cones of beams.
• Effectiveness of silicon as a laser shinethrough barrier for 351-nm laser light. Silicon (Si) has been found to be a promising candidate for a direct-drive cryogenic target shinethrough-barrier material. Several cryogenic targets have been coated with Si, permeation filled with either deuterium or deuterium–tritium, and subsequently layered and optically characterized. Experiments have shown that a 200-µm-thick coating of Si is sufficient to mitigate shinethrough in cryogenic targets.
• Mitigation of self-pulsing in watt-level, dual-clad, ytterbium-doped fiber lasers. High-power, high-beam quality, stable cw fiber lasers are desired in sensing, ranging, telecommunications, and spectroscopy. Although high-output powers have been achieved in many high-power fiber-laser systems, self-pulsing often occurs under specific conditions and cause catastrophic damage to the fiber laser. Self-pulsations are caused by the dynamic interaction between the photon population and population inversion. The addition of a long section of passive fiber in the laser cavity makes gain recovery faster than the self-pulsation dynamics, allowing only stable continuous-wave lasing.
• Resolving dark pulses from photon pulses in NbN superconducting single-photon detectors (SPD’s). Some applications of SPD’s include quantum communications, quantum key distributions, and satellite communications. A desirable feature of an ideal SPD is its photon-number-resolution (PNR) capability. InGaAs avalanche photodiodes work at telecommunications wavelengths and are commercially available; however, they suffer severe after-pulsing and require time gating, limiting their maximum count rate. A scheme is presented using an NbN superconducting SPD (SSPD) that uses a low-noise cryogenic high-electron mobility transistor and a high-load resistor directly integrated with the detector to achieve amplitude resolution of dark and photon counts. This scheme makes it possible to study the physical origin of dark counts in SSPD’s and may enable both photon-number resolving and energy-resolving capabilities of the standard, meander-type SSPD.