J.S. Shlachter

Instability growth in magnetically imploded high-conductivity cylindrical liners with material strength

Magnetically imploded cylindrical metal shells (z-pinch liners) are attractive drivers for experiments exploring hydrodynamics and properties of materials at extreme conditions. As in all z-pinches, the outer surface of a liner is unstable to magneto Rayleigh-Taylor (RT) modes during acceleration, and large-scale distortion arising from RT modes could make such liners unuseable. On the other hand, material strength in the liner should, from first principles, reduce the growth rate of RT modes, and material strength can render some combinations of wavelength and amplitude analytically stable. A series of experiments has been conducted in which high-conductivity, soft, cylindrical aluminum liners were accelerated with 6-MA, 7-/spl mu/s rise-time driving currents. Small perturbations were machined into the outer surface of the liner and perturbation growth monitored. Two-dimensional magneto-hydrodynamic (2-D-MHD) calculations of the growth of the initial perturbations were in good agreement with experimentally observed perturbation growth through the entire course of the implosions. In general, for high-conductivity and soft materials, theory and simulation adequately predicted the behavior of magneto-RT modes in liners where elastic-plastic behavior applies. This is the first direct verification of the growth of magneto-RT in solids with strength known to the authors.

Recent results on dense Z pinches

Abstract A detailed description of the ongoing high density Z-pinch experiments at Los Alamos is given. A review of past dense experiments is included. A model for a dense Z pinch as a high-Q, low yield, reactor system is presented.

Two‐dimensional direct simulation of deuterium‐fiber‐initiated Z pinches with detailed comparison to experiment

Deuterium‐fiber‐initiated Z‐pinch experiments have been simulated using a two‐dimensional resistive magnetohydrodynamic model, which includes many important experimental details, such as ‘‘cold‐start’’ initial conditions, thermal conduction, radiation, actual discharge current versus time, and grids of sufficient size and resolution to allow realistic development of the plasma. When the fiber becomes fully ionized (at a time depending on current ramp and fiber thickness), the simulations show rapidly developing m=0 instabilities, which originated in the corona surrounding the fiber, drive intense nonuniform heating and rapid expansion of the plasma column. Diagnostics generated from the simulation results, such as shadowgrams and interferograms, are in good agreement with experiment.

The Atlas project-a new pulsed power facility for high energy density physics experiments

Atlas is a facility being designed at Los Alamos National Laboratory (LANL) to perform high-energy-density experiments in support of weapon physics and basic research programs. It is designed to be an international user facility, providing experimental opportunities to researchers from national laboratories and academic institutions. For hydrodynamic experiments, it will be capable of achieving a pressure exceeding 30 Mbar in a several cubic centimeter volume. With the development of a suitable opening switch, it will be capable of producing more than 3 MJ of soft X-rays. The capacitor bank design consists of a 36 MJ array of 240 kV Marx modules. The system is designed to deliver a peak current of 45-50 MA with a 4-5-/spl mu/s rise time. The Marx modules are designed to be reconfigured to a 480-kV configuration for opening switch development. The capacitor bank is resistively damped to limit fault currents and capacitor voltage reversal. An experimental program for testing and certifying prototype components is currently under way. The capacitor bank design contains 300 closing switches. These switches are a modified version of a railgap switch originally designed for the DNA-ACE machines. Because of the large number of switches in the system, individual switch prefire rates must be less than 10/sup -4/ to protect the expensive target assemblies. Experiments are under way to determine if the switch-prefire probability can be reduced with rapid capacitor charging.

Instability heating of a solid fiber Z‐pinch

A dense Z‐pinch formed by the electrical breakdown of solid CD2 fibers in an 800 kA, 100 ns risetime pulse generator has been studied with optical and radiation diagnostics. It has been found that, contrary to calculations based on classical joule heating of the plasma that predict approximate dynamic equilibrium, the pinch always expands explosively while displaying intense m=0 hydromagnetic instability activity. Excellent agreement with the observed expansion rate as well as with measured electron temperatures and neutron yield has been obtained by including in a simulation code the direct heating of ions by turbulence arising from instability growth.

Production of solid D2 threads for dense Z‐pinch plasmas

A liquid‐He/liquid‐N2 cryostat system has been developed to freeze D2 gas and extrude fibers of solid deuterium into a vacuum chamber. Fibers that are 40 μm in diameter and 10 cm long have been successfully and routinely produced. The fibers are observed to remain constant in physical dimensions for several minutes. A fiber will be used as the electrical load for a pulsed power machine in a program investigating the properties of dense, ohmically heated plasmas for controlled thermonuclear fusion.

The Atlas High-Energy Density Physics Project

Atlas is a pulsed-power facility under development at Los Alamos National Laboratory to drive high-energy density experiments. Atlas will be operational in the summer of 2000 and is optimized for the study of dynamic material properties, hydrodynamics, and dense plasmas under extreme conditions. Atlas is designed to implode heavy-liner loads in a z-pinch configuration. The peak current of 30 MA is delivered in 4 µs. A typical Atlas liner is a 47-gram-aluminum cylinder with ∼ 4-cm radius and 4-cm length. Three to five MJ of kinetic energy will be delivered to the load. Using composite layers and a variety of interior target designs, a wide variety of experiments in ∼ cm3 volumes will be performed. Atlas applications, machine design, and the status of the project are reviewed.

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.

Pegasus II experiments and plans for the Atlas pulsed power facility

Atlas will be a high-energy (36 MJ stored), high-power (/spl sim/10 TW) pulsed power driver for high energy-density experiments, with an emphasis on hydrodynamics. Scheduled for completion in late 1999, Atlas is designed to produce currents in the 40-50 MA range with a quarter-cycle time of 4-5 /spl mu/s. It will drive implosions of heavy liners (typically 50 g) with implosion velocities exceeding 20 mm//spl mu/s. Under these conditions, very high pressures and magnetic fields are produced. Shock pressures in the 50 Mbar range and pressures exceeding 10 Mbar in an adiabatic compression will be possible. By performing flux compression of a seed field, axial magnetic fields in the 2000 T range may be achieved. A variety of concepts have been identified for the first experimental campaigns on Atlas. Experimental configurations, associated physics issues, and diagnostic strategies are all under investigation as the design of the Atlas facility proceeds. Near-term proof-of-principle experiments employing the smaller Pegasus II capacitor bank have been identified, and several of these experiments have now been performed. This paper discusses a number of recent Pegasus II experiments and identifies several areas of research presently planned on Atlas.

Atlas-a facility for high energy density physics research at Los Alamos National Laboratory

Atlas is a facility designed to perform high energy-density experiments in support of weapon-physics and basic-research programs at Los Alamos. The capacitor bank design consists of a 36-MJ array of 600-kV Marx modules. The system is designed to deliver a peak current of 20-25 MA with a 2-3 /spl mu/s rise time. The capacitor bank is resistively damped to limit fault currents and capacitor voltage reversal. Both oil- and air-insulated Marx module designs are being evaluated. An experimental program for testing both prototype components and the air-insulated concept is currently underway. The capacitor bank design contains 300 closing switches. The primary candidate is a modified version of a Maxwell railgap switch originally designed for the DNA-ACE machines. An alternative candidate is a low-inductance surface-discharge switch. Because of the large number of switches in the system, individual switch prefire rates are required to be less than 10/sup -4/ to protect the high-value loads and targets. Experiments are underway to determine if switch-prefire probability can be reduced by increased capacitor charging rates. A pulse-charging system is described which is capable of charging the 36-MJ capacitor bank to full voltage in 40 milliseconds. This system would use the LANL 1430-MVA generator and a 50-MJ set of intermediate energy-storage inductors. Charging the capacitor bank with a large rectifier connected directly to the generator is another option, and would produce charging times in the 1-6 s range. Conventional rectifiers and grid power would be used for charging times >6 seconds.