Beamlet Laser Research Articles

Scaling laser preheat for MagLIF with the Z-Beamlet laser

Optimizing the performance of the Magnetized Liner Inertial Fusion (MagLIF) platform on the Z pulsed power facility requires coupling greater than 2 kJ of preheat energy to an underdense fuel in the presence of an applied axial magnetic field ranging from 10 to 30 T. Achieving the suggested optimal preheat energies has not been experimentally achieved so far. In this work, we explore the preheat design space for cryogenically cooled MagLIF targets, which represent a viable candidate for increasing preheat energies. Using 2D and 3D HYDRA MHD simulations, we first discuss the various physical effects that occur during laser preheat, such as laser energy deposition, self-focusing, and filamentation. After identifying the changes that different phase plates, gas-fill densities, and magnetic fields bring to the aforementioned physical effects, we, then, consider higher laser energies that are achievable with modest upgrades to the Z Beamlet laser. Finally, with a 6.0-kJ upgraded laser, 3D calculations suggest that it is possible to deliver 4.25 kJ into the MagLIF fuel, resulting in an expected deuterium neutron yield of YDD≃1.5×1014, or roughly 50 kJ of DT equivalent yield, at 20-MA current drive. This represents a 10-fold increase in the currently achieved yields for MagLIF.

Demonstration of thermonuclear conditions in magnetized liner inertial fusion experimentsa)

The magnetized liner inertial fusion concept [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)] utilizes a magnetic field and laser heating to relax the pressure requirements of inertial confinement fusion. The first experiments to test the concept [M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)] were conducted utilizing the 19 MA, 100 ns Z machine, the 2.5 kJ, 1 TW Z Beamlet laser, and the 10 T Applied B-field on Z system. Despite an estimated implosion velocity of only 70 km/s in these experiments, electron and ion temperatures at stagnation were as high as 3 keV, and thermonuclear deuterium-deuterium neutron yields up to 2 × 1012 have been produced. X-ray emission from the fuel at stagnation had widths ranging from 50 to 110 μm over a roughly 80% of the axial extent of the target (6–8 mm) and lasted approximately 2 ns. X-ray yields from these experiments are consistent with a stagnation density of the hot fuel equal to 0.2–0.4 g/cm3. In these experiments, up to 5 × 1010 secondary deuterium-tritium neutrons were produced. Given that the areal density of the plasma was approximately 1–2 mg/cm2, this indicates the stagnation plasma was significantly magnetized, which is consistent with the anisotropy observed in the deuterium-tritium neutron spectra. Control experiments where the laser and/or magnetic field were not utilized failed to produce stagnation temperatures greater than 1 keV and primary deuterium-deuterium yields greater than 1010. An additional control experiment where the fuel contained a sufficient dopant fraction to substantially increase radiative losses also failed to produce a relevant stagnation temperature. The results of these experiments are consistent with a thermonuclear neutron source.

Plasma Electrode Pockels Cell for the National Ignition Facility

The National Ignition Facility (NIF), now under construction at Lawrence Livermore National Laboratory, will be the largest laser fusion facility ever built. The NIF laser architecture is based on a multi-pass power amplifier to reduce cost and maximize performance. A key component in this laser design is an optical switch that closes to trap the optical pulse in the cavity for four gain passes and then opens to divert the optical pulse out of the amplifier cavity. The switch is comprised of a Pockels cell and a polarizer and is unique because it handles a beam that is 40 cm × 40 cm square and allows close horizontal and vertical beam spacing. Conventional Pockels cells do not scale to such large apertures or the square shape required for close packing. Our switch is based on a Plasma-Electrode Pockels Cell (PEPC).In a PEPC, low-pressure helium discharges (1–2 kA) are formed on both sides of a thin slab of electro-optic material. Typically, we use KH2PO4 crystals (KDP). The discharges form highly conductive, transparent sheets that allow uniform application of a high-voltage pulse (17 kV) across the crystal. A 37 cm × 37 cm PEPC has been in routine operation for two years on the 6 kJ Beamlet laser at LLNL. For the NIF, a module four apertures high by one wide (4×1) is required. However, this 4×1 mechanical module will be comprised electrically of a pair of 2×1 sub-modules.Last year (FY 97), we demonstrated full operation of a prototype 2×1 PEPC. In this PEPC, the plasma spans two KDP crystals. A major advance in the 2×1 PEPC over the Beamlet PEPC is the use of anodized aluminum construction that still provides sufficient insulation to allow formation of the planar plasmas. In this paper, we discuss full 4×1 NIF prototypes.

Characterization of Third-Harmonic Target Plane Irradiance on the National Ignition Facility Beamlet Demonstration Project

The Beamlet laser is a single-aperture prototype for the National Ignition Facility (NIF). We have recently installed and activated a 55 m{sup 3} vacuum vessel and associated diagnostic package at the output of the Beamlet that we are using to characterize target plane irradiance at high power. Measurements obtained both with and without a kinoform diffractive optic are reported. Dependences on critical laser parameters including output power, spatial filtering, and wavefront correction are discussed and compared with simulations.

Performance of large-aperture optical switches for high-energy inertial-confinement fusion lasers

We describe the design and performance of large-aperture (>30 cm × 30 cm) optical switches that have demonstrated, for the first time to our knowledge, active switching of a high-energy (>5 kJ) optical pulse in an inertial-confinement fusion laser. These optical switches, which consist of a plasma-electrode Pockels cell (PEPC) and a passive polarizer, permit the design of efficient, multipass laser amplifiers. In a PEPC, plasma discharges on the faces of a thin (1-cm) electro-optic crystal (KDP or KD*P) act as highly conductive and transparent electrodes. These plasma electrodes facilitate rapid (<100 ns) and uniform charging of the crystal to the half-wave voltage and discharging back to 0 V. We discuss the operating principles, design, optical performance, and technical issues of a 32 cm × 32 cm prototype PEPC with both KDP and KD*P crystals, and a 37 cm × 37 cm PEPC with a KDP crystal for the Beamlet laser. This PEPC recently switched a 6-kJ, 3-ns pulse in a four-pass cavity.

Plasma Electrode Pockels Cells for the Beamlet and NIF Lasers

We describe Plasma Electrode Pockels Cells (PEPC) for the Beamlet laser and the proposed National Ignition Facility (NIF) laser. These PEPCs, together with passive polarizers, function as large ape...

Serrated-aperture apodizers for high-energy laser systems

Spatial beam apodization is a critical part of the design of high-energy solid-state laser systems. Standard methods of making apodizers include photographic and metal-vapor-deposition techniques. Apodizers fabricated with these methods are subject to damage and deterioration from high-intensity laser pulses. An alternative approach is to use a serrated-edge aperture in conjunction with the spatial filter. This system can produce beams with smooth edge profiles. We present the theory of operation of the serrated aperture along with some useful design rules and describe the successful application of a serrated-aperture apodizer in the Beamlet laser system.