P.T. Sheehey

Particle and heat transport in a dense wall-confined MTF plasma (theory and simulations)

Plasma beta in magnetized target fusion systems is sometimes much greater than 1, and the plasma may be in direct contact with the imploding liner. Plasma processes are strongly dominated by inter-particle collisions. Under such conditions, the plasma microturbulence, behaviour of α-particles, and plasma equilibria are very different from conventional fusion systems. This paper contains the most comprehensive analysis of the corresponding phenomena to date. Two-dimensional numerical simulations of plasma convection in the targets of a diffuse pinch type demonstrate an onset of convection in this configuration.

Hall and two-temperature magnetohydrodynamic simulation of deuterium-fiber-initiated Z pinches

Two-dimensional “cold-start” resistive magnetohydrodynamic computations of formation and evolution of deuterium-fiber-initiated Z pinches have been extended to include separate ion and electron energy equations and some finite-Larmor-radius ordered terms. In the Ohm’s law (magnetic field evolution) equation, Hall and diamagnetic pressure terms have been added, and corresponding terms have been added to the energy equation. None of the extended model computations show stabilizing effects for fiber-initiated Z pinches; in fact, further slight destabilization is noted. This continues the good agreement shown between previous computational results and experiment.

HEL-1: a DEMG based demonstration of solid liner implosions at 100 MA

In August 1997, the Los Alamos National Laboratory (LANL) and the All-Russian Scientific Research Institute of Experimental Physics (VNIIEF) conducted a joint experiment in Sarov, Russia to demonstrate the feasibility of applying explosive pulsed power technology to implode large scale, high velocity cylindrical liners. Kilogram mass metal liners imploding at velocities of 5-25 km/sec are useful scientific tools for producing high energy density environments, ultra-high pressure shocks and for the rapid compression of plasmas. To explore the issues associated with the design, operation and diagnosis of such implosions, VNIIEF and LANL designed and executed a practical demonstration experiment in which a liner of approximately 1 kg mass was accelerated to 5-10 km/sec while undergoing a convergence of about 4:1. The scientific objectives of the experiment were three-fold: first to explore the limits of very large, explosive, pulse power systems delivering about 100 MA as drivers for accelerating solid density imploding liners to kinetic energies of 25 MJ or greater; second to evaluate the behavior of single material (aluminum) liners imploding at 5-10 km/s velocities by comparing experimental data with 1-D and 2 D numerical simulations; and third, to evaluate the condition of the selected liner at radial convergence of 4 and a final radius of 6 cm. A liner of such parameters could be used as a driver for the equation of state measurements at megabar pressures or as a driver for a future experiment in which a magnetized fusion plasma would be compressed to approach ignition conditions.

Results of a 100-megaampere liner implosion experiment

A very high-current liner implosion experiment was conducted, using an explosive magnetic-compression generator (EMG) to deliver a peak current of 102 /spl plusmn/ 3 MA, to implode a 4.0-mm-thick aluminum liner. Analysis of experimental data showed that the inner surface of the liner had attained a velocity of between 6.8-8.4 km/s, consistent with detailed numerical calculations. Both calculations and data were consistent with a final liner state that was still substantially solid at target impact time and had a total kinetic energy of over 20 MJ.

Modeling and analysis of the high energy liner experiment, HEL-1

A high energy, massive liner experiment, driven by an explosive flux compressor generator, was conducted at VNIIEF firing point, Sarov, on August 22, 1996. We report results of numerical modeling and analysis we have performed on the solid liner dynamics of this 4.0 millimeter thick aluminum liner as it was imploded from an initial inner radius of 236 mm onto a central measuring unit (CMU), radius 55 mm. Both one- and two-dimensional MHD calculations have been performed, with emphasis on studies of Rayleigh-Taylor instability in the presence of strength and on liner/glide plane interactions. One-dimensional MHD calculations using the experimental current profile confirm that a peak generator current of 100-105 MA yields radial liner dynamics which are consistent with both glide plane and CMU impact diagnostics. These calculations indicate that the liner reached velocities of 6.9-7.5 km/s before CMU impact. Kinetic energy of the liner, integrated across its radial cross-section, is between 18-22 MJ. Since the initial goal was to accelerate the liner to at least 20 MJ, these calculations are consistent with overall success. Two-dimensional MHD calculations were employed for more detailed comparisons with the measured data set. The complete data set consisted of over 250 separate probe traces. From these data and from their correlation with the MHD calculations, we can conclude that the liner deviated from simple cylindrical shape during its implosion. Two-dimensional calculations have clarified our understanding of the mechanisms responsible for these deformations.

Material science experiments at the Atlas facility

Three material properties experiments that are to be performed on the Atlas pulsed power facility are described; friction at sliding metal interfaces, spallation and damage in convergent geometry, and plastic flow at high strain and high strain rate. Construction of this facility has been completed and experiments in high energy density hydrodynamics and material dynamics will begin in 2001.

Self-organization observed in numerical simulations of a hard-core diffuse Z pinch

A hard-core Z-pinch plasma (metal conductor on axis) with an unstable pressure profile can rearrange itself through m=0 interchange motions to produce a stable pressure profile. In this paper the self-organization process is demonstrated in numerical simulations of an experimental plasma formation process, using a two-dimensional compressible two-fluid magnetohydrodynamic code. The stabilization process results in m=0 turbulence, which has a level of kinetic energy that is saturated typically at a few percent of the plasma thermal energy. Using idealized initial conditions for simulations with an axial sinusoidal density perturbation, it is possible to observe in detail the development of instability and then turbulence. At first a coherent Rayleigh–Taylor type motion grows exponentially, with localized isentropic heating and cooling associated with the motion. Then the bubble and spike structure breaks up and incoherent m=0 turbulence develops.

Progress with developing a target for magnetized target fusion

Summary form only given, as follows. Magnetized target fusion (MTF) is an approach to fusion where a preheated and magnetized plasma is adiabatically compressed to fusion conditions. Successful MTF requires a suitable initial target plasma with an embedded magnetic field of at least 5 T in a closed-field-line topology, a density of roughly 10/sup 18/ cm/sup -3/, a temperature of at least 50 eV, and must be free of impurities which would raise radiation losses. Target plasma generation experiments are underway at Los Alamos National Laboratory using the Colt facility; a 0.25 MJ, 2-3 /spl mu/s rise-time capacitor bank. In the first experiments, a Z-pinch is produced in a 2 cm radius by 2 cm high conducting wall using a static gas-fill of hydrogen or deuterium gas in the range of 0.5 to 2 torr. Follow-on experiments will use a frozen deuterium fiber along the axis (without a gas-fill). The diagnostics include B-dot probes, framing camera, gated OMA visible spectrometer, time-resolved monochrometer, silicon photodiodes, neutron yield, and plasma-density interferometer. Operation to date has been with drive currents ranging from 0.8 MA to 1.9 MA. Optical diagnostics show that the plasma produced in the containment region lasts roughly 20 to 30 /spl mu/s, and the B-dot probes show a broad current-profile in the containment region. The experimental design and data will be presented.Magnetized target fusion (MTF) is an approach to fusion where a preheated and magnetized plasma is adiabatically compressed to fusion conditions. Successful MTF requires a suitable initial target plasma with an embedded magnetic field of at least 5 T in a closed-field-line topology, a density of roughly 10/sup 18/ cm/sup -3/, a temperature of at least 50 eV, and must be free of impurities which would raise radiation losses. Target plasma generation experiments are underway at Los Alamos National Laboratory using the Colt facility; a 0.25 MJ, 2-3 /spl mu/s rise-time capacitor bank. The goal of these experiments is to demonstrate plasma conditions meeting the minimum requirements for a MTF initial target plasma. In the first experiments, a Z-pinch is produced inside a 2 cm radius by 2 cm high conducting cylindrical metal container using a static gas-fill of hydrogen or deuterium gas in the range of 0.5 to 2 ton. Thus far, the diagnostics include an array of 12 B-dot probes, a framing camera, a gated OMA visible spectrometer, a time-resolved monochrometer, filtered silicon photodiodes, neutron yield, and plasma-density interferometers. These diagnostics show that a plasma is produced in the containment region that lasts roughly 10 to 20 /spl mu/s with a maximum plasma density exceeding 10/sup 18/ cm/sup -3/. The experimental design and data are presented.

2D MHD computer modeling of dense plasma focus accelerators

Dense Plasma Focus (DPF) discharge is a very promising mechanism to achieve high neutron production yield (about 10/sup 11/-10/sup 12/ per pulse) from the D-T reactions. Focus is conjectured to be a finite 2D Z-pinch formation near the end of the coaxial plasma accelerator, typically having the density above the level of 10/sup 19//cm/sup 3/ and temperature of a few keV during 100 to 150 ns. Interest to DPF appeared since the pioneer experiments of Filippov and the first theoretical reviews of Dyachenko and Imshennik and Mather. During the last decades theoretical investigation of DPF lagged behind the experiments, giving scant explanation of the experimental results. At the same time modeling and diagnostic capabilities have been dramatically improved. This presentation shows that the computer simulation by the code MHRDR (Magneto Hydro Radiative Dynamic Research) can help meet the goals and challenges of the LANL-Bechtel Nevada Dense Plasma Focus Accelerator project. Theoretical estimations, made in this report, represent the bounds between the parameters of source generator, geometry of the electrodes and feeding circuit and the initial density of the background gas in the form of the simple scaling laws.

The inverse Z-pinch as a physics test bed, and, possibly, a target plasma, for magnetized target fusion (MTF)

From an overall fusion system perspective, there remains an untested and interesting possibility of compressing a magnetized target plasma with beta greater than unity by a magnetically driven imploding liner, or other target pusher driver. This approach, known as Magnetized Target Fusion (MTF), operates in an intermediate density regime and time scale between magnetic and inertial fusion, which are separated by twelve orders of magnitude. Even if magnetized plasma transport is Bohm-like, fusion gain in the MTF parameter space appears accessible with existing drivers, which means MTF does not require a major financial investment in driver technology.