LLE Review 139

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Highlights

This volume of the LLE Review, covering April–June 2014, features “Magnetic Reconnection Between Colliding Magnetized, Laser-Produced Plasma Plumes.” This article reports on the first demonstration of magnetic reconnection between colliding plumes of externally magnetized laser-produced high-energy-density plasmas. Two counter-propagating plasma flows are created by IR-radiating oppositely placed plastic targets with 1.8-kJ, 2-ns laser beams on the OMEGA EP Laser System. The two plumes are magnetized by an externally controlled magnetic field. The interaction region is prefilled with a low-density background plasma. The counter-flowing plumes sweep up and compress the magnetic field and the background plasma into a pair of magnetized ribbons, which collide, stagnate, and reconnect at the midplane, allowing for the first detailed observation of a stretched current sheet in laser-driven reconnection experiments.

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

• Two approaches to increase the ablation pressure are demonstrated that can help to achieve implosion performance on the OMEGA laser that is hydrodynamically scalable to ignition on the National Ignition Facility. A target design that uses a Be ablator is shown to increase the hydrodynamic efficiency, resulting in a ~10% increase in the ablation pressure, compared to the standard CH ablator. Reducing the beam size is shown to recover all of the ablation pressure lost to cross-beam energy transfer, but the illumination uniformity reduces the integrated target performance. The hydrodynamic efficiency is measured for the current cryogenic design, multiple ablator material design, and various beam focal-spot sizes.
• Measurements of pressure and fuel–shell mix in compressed isobaric hydrogen implosion cores using x-ray continuum are described. The x-ray emissivity depends almost entirely on the pressure when measured within a restricted spectral range. In this way, the measured free-free emissivity profile becomes a direct measure of the hot-core pressure at the time of peak emission. A simple scaling of the total filtered x-ray emission as a constant power of the total neutron yield is explained. The hot-spot “fuel–shell” mix mass can be inferred by attributing the excess emission to the higher emissivity of shell carbon mixed into the implosion central hot spot.
• A narrowband x-ray imager with an astigmatism-corrected bent quartz crystal for the Si He_α line was developed to record backlit images of cryogenic direct-drive implosions. With backlighter laser energies of ~1.25 kJ at a 10-ps pulse duration, the radiographic images show a high signal-to-background ratio of >100:1 and a spatial resolution of the order of 10 µm. The backlit images can be used to assess the symmetry of the implosions close to stagnation and the mix of ablator material into the dense shell.
• Picosecond time-resolved, monochromatic 8-keV x-ray radiographic measurements of imploded cone-in-shell targets on OMEGA are reported. They provide for the first time a detailed quantitative study of the hydrodynamic evolution of nonsymmetrically imploded, high-density matter up to peak compression. This work is an important step forward for fast ignition because it demonstrates that sufficient areal density can be compressed in nonspherical implosions to stop that part of the fast-electron spectrum (~mega-electron volt) that is relevant for fast ignition.
• A programmable liquid crystal beam-shaping system was installed for a 200-mJ optical parametric chirped-pulse–amplification system front end and was applied to dramatically improve the beam uniformity in the subsequent amplifier. A highly nonuniform beam profile caused by gain inhomogeneity in the amplifier was precompensated by the beam-shaping system. The issues of running a liquid crystal device with a high-energy, ultrashort-pulse laser, such as damage risk and temporal contrast degradation, are addressed.
• A new neutron time-of-flight (nTOF) detector was installed on the OMEGA Laser System for fuel-areal-density measurements in cryogenic DT implosions. Four gated photomultiplier tubes (PMT’s) with different gains are used to measure primary DT and D₂ neutrons, down-scattered neutrons in nT, and nD kinematic edge regions, and to study tertiary neutrons. The design details of the nTOF detector, PMT optimization, and test results on OMEGA are presented.