A. S. Wan

Ignition target design and robustness studies for the National Ignition Facility

Recent results are presented from two‐dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasmas Phys. Controlled Thermonucl. Fusion 2, 51 (1975)] calculations of the indirectly driven hohlraum and ignition capsules proposed for the National Ignition Facility (NIF). The calculations concentrate on two capsule designs, the baseline design that has a bromine‐doped plastic ablator, and the beryllium design that has a copper‐doped beryllium ablator. Both capsules have a cryogenic fuel layer. Primary emphasis in these calculations is placed upon robustness studies detailing various sensitivities. Because of computer modeling limitations these studies fall into two categories: those performed with integrated modeling where the capsule, hohlraum, and laser rays all are modeled simultaneously with the laser power levels as the only energy input; and those performed in a capsule‐only mode where an externally imposed radiative flux is applied to the exterior of the capsule, and only the capsule performan...

Fringe formation and coherence of a soft-x-ray laser beam illuminating a Mach--Zehnder interferometer

We investigated the fringe visibility produced by a Mach-Zehnder interferometer illuminated by a collisionally pumped yttrium x-ray laser operating at 15.5 nm. Fringe visibility varied as a function both of relative path delay and of relative spatial overlap of the beams. This visibility information was extracted quantitatively from several interferograms and analyzed to produce a characterization of the temporal coherence, yielding a gain-narrowed linewidth of 1.3 pm for the 15.5-nm laser transition and spatial coherence consistent with an effective source size of approximately 220 microm +/- 50% at the x-ray laser output.

Simultaneous measurement of local gain and electron density in X-ray lasers

X-ray lasers (XRLs) have experimental average gains that are significantly less than calculated values and a persistently low level of spatial coherence. An XRL has been used both as an injected signal to a short XRL amplifier and as an interferometer beam to measure two-dimensional local gain and density profiles of the XRL plasma with a resolution near 1 micrometer. The measured local gain is in agreement with atomic models but is unexpectedly spatially inhomogeneous. This inhomogeneity is responsible for the low level of spatial coherence observed and helps explain the disparity between observed and simulated gains.

Extreme-ultraviolet interferometry at 15.5 nm using multilayer optics

The development of multilayer mirror technology capable of operating in the range of 3-30 nm and the construction of thin membranes with excellent uniformity and strength have made it possible to design and implement a Mach-Zehnder interferometer operating at 15.5 nm. We have tested this interferometer by using a soft x-ray laser as a source, and we show its use in probing high-density plasmas.

Detailed radiative transport modeling of a radiative divertor

Abstract An effective radiative divertor maximizes the utilization of atomic processes to spread out the energy deposition to the divertor chamber walls and to reduce the peak heat flux. Because the mixture of neutral atoms and ions in the divertor can be optically thick to a portion of radiated power, it is necessary to accurately model the magnitude and distribution of line radiation in this complex region. To assess their importance we calculate the effects of radiation transport using CRETIN, a multi-dimensional, non-local thermodynamic equilibrium simulation code that includes the atomic kinetics and radiative transport processes necessary to model to complex environment of a radiative divertor. We also include neutral transport to model radiation from recycling neutral atoms. This paper presents a case study of a high-recycling radiative divertor with a typical large neutral pressure at the divertor plate to estimate the impact of H line radiation on the overall power balance in the divertor region with consideration for line opacities and atomic kinetics.

X-ray lasers for high density plasma diagnostics (invited)

Collisionally pumped soft x‐ray lasers now operate over a wavelength range extending from 35 to 300 A. These well‐characterized sources have high peak brightness (GeV blackbody temperature) and narrow bandwidth making them ideal for x‐ray imaging and interferometry. We will describe experiments which have used the yttrium neon‐like x‐ray laser operating at 155 A to probe plasmas at electron densities exceeding 4×1021 cm−3. The short pulse duration of this x‐ray laser (∼150 ps) has made it possible to image directly driven thin foils with 1–2 μm spatial resolution. Advances in multilayer mirrors and beam splitters have now also made it possible to develop x‐ray laser interferometers. We will describe initial experiments to probe plasmas relevant to ICF using x‐ray laser interferometry. The progress in the development of short pulse x‐ray lasers (∼30 ps) which are ultimately necessary to extend x‐ray laser diagnostic techniques to higher densities will also be presented.

The role of hot spot mix in the low-foot and high-foot implosions on the NIF

Hydrodynamic mix of the ablator into the DT fuel layer and hot spot can be a critical performance limitation in inertial confinement fusion implosions. This mix results in increased radiation loss, cooling of the hot spot, and reduced neutron yield. To quantify the level of mix, we have developed a simple model that infers the level of contamination using the ratio of the measured x-ray emission to the neutron yield. The principal source for the performance limitation of the “low-foot” class of implosions appears to have been mix. Lower convergence “high-foot” implosions are found to be less susceptible to mix, allowing velocities of >380 km/s to be achieved.

Impact of detailed radiation transport on volume recombination

Abstract Recently both the Alcator C-Mod and DIII-D tokamaks observed significant recombination of major ion species in the divertor region during detachment. For sufficiently low temperatures the mixture of neutral atoms and ions can be optically thick to line radiation. The optical depth of the recombined region to Lyα radiation can be very large and opacity effects and radiation trapping can dramatically change the heat flux to the divertor walls. This paper presents an analysis of the effect of line radiation on volume recombination using CRETIN, a multi-dimensional, non-local thermodynamic equilibrium (NLTE) simulation code that includes the atomic kinetics and radiative transport processes necessary to model this complex environment.

Application of x-ray-laser interferometry to study high-density laser-produced plasmas

Collisionally pumped soft-x-ray lasers now operate over a wavelength range extending from 4 to 40 nm. With the recent advances in the development of multilayer mirrors and beam splitters in the soft-x-ray regime, we can utilize the unique properties of x-ray lasers to study large, rapidly evolving laser-driven plasmas with high electron densities. Using a neonlike yttrium x-ray laser, which operates at a wavelength of 15.5 nm, we have performed a series of radiography, moire deflectometry, and interferometry experiments to characterize plasmas relevant to inertial confinement fusion. We describe experiments using a soft-x-ray laser interferometer, operated in the Mach–Zehnder configuration, to study CH plasmas. The two-dimensional density profiles obtained from the interferograms allow us to validate and benchmark our numerical models used to study the physics of laser–plasma interactions.

Characterization of germanium stripe x-ray lasers

One method of improving the transverse spatial coherence of x-ray lasers (XRLs) is by adaptive spatial filtering of XRL apertures using geometric shaping in the form of bowtie or wedge XRLs. However, we must maintain the desired geometric shapes in exploding foil or slab configurations during the lasing period. As a first step toward understanding lasing in such geometries, we study the behavior of simple stripe XRLs. Past experience with stripe XRLs deposited on thick plastic substrates resulted in significantly weaker laser intensities as compared to line-focused slab XRLs. Possible reasons for this intensity reduction of stripe XRLs could include mixing at the laser boundary, and changes in plasma, kinetics, and hydrodynamic properties that affect laser gains and propagation. We present experimental and theoretical characterizations of germanium line-focused slab and stripe XRLs. Key experimental parameters we are studying include images of emission profiles of the laser blowoff, angular divergences, XRL output intensities, and ionization balances as we vary XRL designs. We compare the experimental results with 2-D laser deposition and hydrodynamics simulations using LASNEX, and study the changes in ionization balances and level populations from postprocessing LASNEX results.