T. A. Oliphant

Two‐dimensional modeling of magnetically driven Rayleigh–Taylor instabilities in cylindrical Z pinches

A two‐dimensional computational methodology has been developed that uses a phenomenological representation of initial perturbations to model the evolution of magnetically driven Rayleigh–Taylor instabilities in a hollow Z pinch. The perturbed drive current waveform and x‐ray output obtained from the two‐dimensional models differ qualitatively from the results of unperturbed (one‐dimensional) models. Furthermore, the perturbed results reproduce the principle features measured in a series of capacitor bank‐driven pulsed power experiments. In this paper we discuss the computational approach and the computational sensitivity to initial conditions (including the initial perturbations). Representative examples are also presented of instability evolution during implosions, and the results are compared with experimentally measured current waveforms and visible framing camera images of perturbed implosions. Standard magnetohydrodynamic modeling, which includes instability growth in two dimensions, is found to repr...

A computational model of exploding metallic fuses for multimegajoule switching

A new model for determining the time‐dependent behavior of exploding metallic fuses is formulated. The model draws on an atomic data base and gives insight into the temporal behavior of the material density and temperature of the fuse as well as the nonlinear electrical circuit interaction. The model includes an embedding insulating tamper and leads to a plausible explanation of fuse ‘‘restrike.’’ The model predicts time‐scale compression of 500 for inductive store systems powered by explosive driven magnetic flux compression generators. A scenario for achieving multimegajoule foil implosions is predicted.

Calculational evaluation of plasma flow switches for the los alamos foil implosion project

The next system design under consideration for the Los Alamos Foil Implosion Project (Trailmaster) is projected to deliver in excess of 15 MA of electrical current produced by high-explosive driven flux compression generators to a foil load. A plasma flow switch is being investigated as the final pulse shaping step in this system. The performance of these switches is being evaluated using a wide variety of computational tools including zero-, one- and two-dimensional MHD codes and a 3-D view-factor radiation transport code. We are concerned with the effects of radiation from the switch plasma prior to switching current to the load, and the inter action of the switch plasma on existing perturbations and as a source of perturbation on the imploding load.

The laser‐initiated, gas‐embedded z‐pinch: Two‐dimensional computations

A high‐density, laser‐initiated, gas‐embedded Z‐pinch is modeled computationally in two dimensions. All aspects of an actual experiment are modeled: the computer model includes a laser optics package, a Marx‐bank/transmission line package, an electron avalanche prescription, and two‐dimensional magnetohydrodynamics. The experimental observations are reproduced to a very satisfactory degree. Predictions of performance in nonstandard operating modes are given.

Solution of the Fokker-Planck transport equation by matrix factorization

A matrix factorization method is used to solve the Fokker-Pianck (Landau) charged particle transport equation. By treating all phase space variables as discrete in analogy to S, neutronics, the collision term takes on a five-point difference form which is readily treatable by this method. In order to illustrate this technique, the energy deposited by fast ions in a geometrically spherical, Maxwellian background plasma is calculated. Although this technique can be generalized to other geometries, its essential elements are best illustrated in this simple context. In the study of charged particle transport in plasmas, numerical techniques for solving the Fokker-Planck equation have been developed which closely parallel those used in neutron transport. This was a natural step in the development of solution methods in charged particle transport (CPT) in view of the fact that the theory and methods of neutron transport have been well developed [ 1,2]. Moreover, since ~~~~ of the pioneering work in CPT was carried out in conjunction with the ongoing effort to build controlled fusion devices, the early methodologies developed to solve the transport equation were made more applicable to those machines. In the well--known analysis of transport in mirror machines by Killeen and Marx [3], for example, the calculations of spatial changes along the magnetic field are based on

ENTROPY PRODUCTION AND HEAT FLOW IN LIQUID He II

An analysis of the effect of mutual friction between normal fluid and superfluid in the flow of liquid He II through narrow channels under large temperature gradients was recently given by Craig, Keller, and Hammel. This analysis is based on the Gorter-Mellink thermodynamical equations. An alternative treatment of the problem is presented in which the conservation of entropy is introduced naturally into the theory at an early stage. The role played by entropy production in the processes involved in the thermohydrodynamics is thereby considerably clarified and a simple relation is given for an estimate of the relative importance of normal fluid dissipation and mutual friction dissipation. (auth)

Computational simulations of the Laguna foil implosion experiments

The Los Alamos foil implosion project is intended to produce a source of intense laboratory x‐radiation for physics and fusion studies. Following the Pioneer shot series, the project is now embarking on the Laguna foil implosion experiments. In this series a Mark‐IX helical generator will be coupled to an explosively‐formed‐fuse opening switch, a surface discharge closing switch, and a vacuum power flow and load chamber. The system design will be discussed and an overview of zero‐, one‐, and two‐dimensional MHD preshot simulations will be presented. The generator should provide more than 11 MA of which ∼5.5 MA will be switched to the 5‐cm‐radius, 2‐cm‐high, 250‐nm‐thick aluminum foil load. This should give rise to a 1.1 μs implosion with tens of kilojoules of kinetic energy.Zero‐dimensional calculations serve to optimize the pulse‐power system. One‐dimensional, Lagrangian, MHD calculations are made to estimate temperature, densities and radiation output. The temperature and density profiles predicted by t...

Transient neutral gas interactions with plasmas and walls

The use of neutral gas layers for confinement and/ or heat removal in controlled fusion reactors has been the subject of many investigations in recent years. Alfven and Smars [l] appear to be the first to propose the use of a “gas blanket” when they suggested that heat losses from a hot plasma can be reduced by a strong magnetic field in such a way as to allow a layer of high pressure neutral gas near the wall to surround the plasma and balance its pressure. Although it was later shown by Lehnert [2] that the plasma pressure cannot be balanced by the neutral gas pressure in steady state, the question of such balance in the case of fully ionized plasma cores bounded by a partially ionized layer, as well as the question of the penetration process of neutral gas into a plasma, were addressed at an early date also by Lehnert [3]. A mathematical model for the application of a neutral gas layer to the heat removal problem in a ThetaPinch reactor has been presented by Oliphant [4] in which estimates of the rate of heat flux from the plasma using a short-mean-free-path limit for thermal conduction, and a quasi-steady state assumption for the plasma and neutral gas profiles, were obtained. This paper represents an extension of the work cited in ref. 4. A time-dependent one-dimensional computer code is utilized to study the transient development of a neutral gas blanket by examining the effects of diatomic neutral gas streaming into a magnetized plasma. The interactions of the incoming diatomic gas and the resulting monatomic gas with the plasma ions, electrons and alphas are examined in