The LIFE minichamber experiment will investigate cooling of the strongly radiating Xe buffer gas protecting the LIFE
chamber wall. A theta pinch will inductively heat a few cc of Xe at ion density 2e16/cc to several eV. Thomson
scattering will be used to determine electron temperature and ionization state. Modeled is being done using the
magnetohydrodynamic code HYDRA with an external circuit model and inductive feedback from the plasma to the
external circuit. Coil stresses are being assessed using the 3D MHD code ALE3D. A major challenge to the design is the
paucity of opacity and conductivity data for Xe in the buffer gas regime. Results of the modeling will be presented.
*This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National
Laboratory under Contract DE-AC52-07NA27344.
ICF power plants, such as the LIFE scheme at LLNL, may employ a high-Z, target-chamber gas-fill to moderate the
first-wall heat-pulse due to x-rays and energetic ions released during target detonation.
To reduce the uncertainties of cooling and beam/target propagation through such gas-filled chambers, we present a
pulsed plasma source producing 2-5 eV plasma comprised of high-Z gases. We use a 5-kJ, 100-ns theta discharge for
high peak plasma-heating-power, an electrode-less discharge for minimizing impurities, and unobstructed axial access
for diagnostics and beam (and/or target) propagation studies. We will report on the plasma source requirements, design
process, and the system design.
The NIF, now under construction at LLNL, 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 X 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 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 across the crystal. A 37 cm X 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 is required. However, this 4 X 1 mechanical module will be comprised electrically of a pair of 2 X 1 sub-modules. Last year, we demonstrated full operation of a prototype 2 X 1 PEPC. In this PEPC, the plasma spans two KDP crystals. A major advance in the 2 X 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 X 1 NIF prototypes.
A large aperture optical switch based on plasma electrode Pockels cell (PEPC) technology is an integral part of the National Ignition Facility (NIF) laser design. This optical switch will trap the input optical pulse in the NIF amplifier cavity for four gain passes and then switch the high-energy output optical pulse out of the cavity. The switch will consist of arrays of plasma electrode Pockels cells working in conjunction with thin-film, Brewster's angle polarizes. The 192 beams in the NIF will be arranged in 4 X 2 bundles. To meet the required beam-to-beam spacing within each bundle, we have proposed a NIF PEPC design based on a 4 X 1 mechanical module (column) which is in turn comprised of two electrically independent 2 X 1 PEPC units. In this paper, we report on the design a single 2 X 1 prototype module and experimental tests of important design issues using our single, 32 cm aperture PEPC prototype. The purpose of the 2 X 1 prototype is to prove the viability of a 2 X 1 PEPC and to act as engineering test bed for the NIF PEPC design.
We use a 32 cm plasma electrode Pockels cell (PEPC) prototype at Lawrence Livermore National Laboratory to determine switching performance in the presence of external magnetic fields. The interaction with external magnetic fields is important because of the B-fields generated by the high current flow through amplifier flashlamp arrays, and their proximity to the PEPC. We have experimentally determined what is the maximum allowable magnetic induction for good PEPC operation, and then we calculate the magnetic induction generated by a flashlamp array to determine the minimum PEPC to amplifier spacing. We have also experimentally determined the effect of a tandem PEPC placement. We consider several cathode designs. We revisit the hollow cathode design and we investigate the tradeoffs between the hollow cathode and planar magnetron. The recent development of a metallic body for the future 1 X 2 PEPC has led us to do experiments with a biased boundary in the PEPC. Experimental results of various biasing potentials and dielectric coating materials for the PEPC body are discussed.
We describe a model which gives the effects of magnetic fields on a plasma electrode Pockels cell. The fields arise from the return currents to the cathode as well as from neighboring devices such as amplifier flashlamps. In effect, electrons are treated as a static, planar fluid moving under the influence of magnetic fields, the electric field of the discharge, electron pressure gradients, and electron-atom elastic collisions. This leads to a closed 2D equation for the electron density, which is solved subject to appropriate boundary collisions. The model is applied to four cases: the baseline NIF configuration with magnetic fields due to balanced return currents; a case with unbalanced return currents; the reverser configuration containing an external field parallel to the main plasma current; and a configuration with a field perpendicular to both the current and the optical direction.
In a plasma-electrode Pockels cell (PEPC), plasma discharges serve as transparent electrodes on each side of an electro-optic crystal such as KDP. These plasmas facilitate rapid and uniform charging and discharging of the crystal. We describe PEPC technology deployed on Beamlet and envisioned for the National Ignition Facility. Performance on Beamlet is discussed in detail. We also discuss models which have shed light on PEPC operation. These models describe both the high-voltage sheath that forms near the crystal surface and the characteristics of the bulk plasma column.
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