B.G. Anderson

Design, fabrication, and operation of a high-energy liner implosion experiment at 16 megamperes

We discuss the design, fabrication, and operation of a liner implosion system at peak currents of 16 MA. Liners of 1100 aluminum, with initial length, radius, and thickness of 4 cm, 5 cm, and 1 mm, respectively, implode under the action of an axial current, rising in 8 /spl mu/s. Fields on conductor surfaces exceed 0.6 MG. Design and fabrication issues that were successfully addressed include: Pulsed Power-especially current joints at high magnetic fields and the possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; Liner Physics-including the angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; Diagnostics-X-radiography through cassette insulator and outer conductor without shrapnel damage to film.

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.

PROCYON: 18-MJ, 2-/spl mu/s pulsed power system

The Procyon high explosive pulsed power (HEPP) system was designed to drive plasma Z-pinch experiments that produce Megajoule soft X-ray pulses when the plasma stagnates on axis. In the proceedings of the Ninth IEEE Pulsed Power Conference, the authors published results from system development tests. At this time, they have fielded seven tests in which the focus was on either vacuum switching or load physics. Four of the tests concentrated on the performance of a plasma flow switch (PFS) which employed a l/r mass distribution in the PFS barrel. Of the four tests, two had dummy loads and one had an implosion load. In addition, one of the tests broke down near the vacuum dielectric interface, and the result demonstrated what Procyon could deliver to an 18 nH load. The authors summarize PFS results and the 18 nH test which is pertinent to upcoming solid/liquid liner experiments. On their other three tests, they eliminated the PFS switching and powered the Z-pinch directly with the HEPP system. From the best of these direct drive tests, they obtained 1.5 MJ of radiation in a 250 ns pulse, their best radiation pulse to date. They also summarize direct drive test results. More details are given in other papers in this conference for both the PFS and direct drive experiments, and an updated analysis of their opening switch performance is also included. The remainder of this paper describes the parameters and capabilities of their system, and they use the data from several experiments to provide more precise information than previously available.

Radiographic results from the NTLX series of hydrodynamic experiments

The NTLX series of experiments are focused on measuring the shock induced hydrodynamic flow of a Sn-PMMA target. For these experiments multi-frame flash X-ray radiography is used to measure the position of the Sn-PMMA target interface and the location of shock in the PMMA as a function of time. Four radiographs are acquired at 700 ns intervals having a line-of-sight following the targets axis of symmetry. Because the X-ray spectrum from the sources has an end-point energy of /spl sim/300 keV with a strong component of /spl sim/60 keV tungsten K-line radiation, the Sn portion of the target is radiographically opaque. However, X-rays are transmitted through the PMMA portion of the target thereby allowing motion of the Sn-PMMA interface to be imaged. Also, the shock location is tracked as a function of time due to the density increase in the shocked PMMA. The resulting radiographs are analyzed to provide the trajectory and shape of both the shock and Sn-PMMA interface. In addition, the shock velocity in the Sn is determined for asymmetric target geometries.

Linear experiment on verification of Rayleigh-Taylor instability magnetic stabilization effect (joint LANL/VNIIEF experiment Pegasus-2)

A liner implosion experiment was conducted on facility Pegasus-2, in which two perturbation type growth was compared. On one half (through height) of the cylindrical liner sinusoidal azimuthally symmetric perturbations were produced. On the other liner half the perturbations were of the same wavelength and the same amplitude, but the angle between the wave vector and the cylinder axis was 45/spl deg/ (screw perturbations). The experimental radiographs show that there is essentially no screw perturbation growth, while the azimuthally symmetric perturbations grow many-fold. This result agrees with the theoretical predictions.

Instabilities in foil implosions and the effect on radiation output

One of the aims of the Athena program at the Los Alamos National Laboratory is the generation of a high fluence of soft X-rays from the thermalization of a radially imploding foil. In experiments in the Athena program, a large axial current is passed through a cylindrical aluminum foil. Under the action of the Lorentz force, the resulting plasma accelerates toward the axis, thermalizes, and produces a fast soft X-ray pulse with a blackbody temperature up to several hundred electron volts. We present visible light images and X-ray data designed to study the effects of foil mass, current, and initial perturbations on the instability growth during foil implosion. Representative data is presented from several experiments using the Pegasus capacitor bank system and the explosively driven Procyon system. These experiments are labeled Peg 25 and Peg 33 for the Pegasus experiments and PDD1, PDD2 and PRFO for the Procyon experiments. In these experiments, all foils had radii of 5 cm but varied in mass and initial conditions. Experimental data from several shots were compared with each other and to a radiation magnetohydrodynamic (RMHD) computation. The data obtained from these experiments and the analysis has given us understanding of the physical mechanisms involved and insight for future experiments and has lead us to propose methods for minimizing the instability growth and maximizing the radiation output. In particular, we observed that wrinkles and other physical anomalies in the initial shape of foil do not appear to contribute to the growth of the instabilities.

Study of high-energy liner compression in HEL-1 experiment

The paper describes arrangement and the results of the first joint experiment between VNIIEF and LANL with explosive magnetic generators (EMG) of 1 m diameter and a nonevaporating liner. The experiment took place in August 22, 1996. The goal of the experiment was to accelerate magnetically cylindrical relatively thin aluminum liner and to get kinetic energy of 20 MJ or more. As the energy source for the experimental device we chose 5-module DEMG of 1000 mm diameter, tested many times in the experiments for rigid and liner loads. This EMG can store more energy than any other EMG created at VNIIEF. The physical scene of the liner unit was chosen so that the growth of disturbances would have less influence on the liner shape during flight, especially on the liners inner surface. The shape of glade plane on which the liner slides during flight and the way of contact liner with walls were chosen on the grounds of 2-D calculations, proceeding from the necessity to ensure electrical contact during the liner flight.

Explosively formed fuse opening switches for multi-megajoule applications

High explosive pulsed power (HEPP) systems are capable of generating very high energies in magnetic fields. Such stored energy is usually developed on time scales of a few tens or hundreds of microseconds. Many applications require shorter pulses and opening switches provide one way to use the large energy available for faster applications. With current flowing in an inductive circuit, introducing resistance produces voltage that can be used to drive current into a load. For an opening switch with a fast rising resistance, the load current rise time is determined by the R/L time constant of the circuit. A significant fraction of the circuit energy must be dissipated in the process, and in applications where very large energies must be dealt with only a few types of switches can be used. Experiments with high explosive driven opening switches have produced a few switches that can carry tens of MA current, and open on the time scale of one or a few /spl mu/s. We have specialized in a type of switch that we call an explosively formed fuse (EFF), and the use of this switch in the is MJ Procyon system is the subject of this paper. Operation of the EFF switch at levels of /spl sim/3 TW for 2 /spl mu/s has become routine, and we describe its characteristics and give data from a number of tests.

High energy imploding liner experiment HEL-1: experimental results

The imploding liner is a cylinder of conducting material through which a current is passed in the longitudinal direction. Interaction of the current with its own magnetic field causes the liner to implode. In August, 1996, a high energy liner experiment (HEL-1) was conducted at the All-Russia Scientific Research Institute (VNIIEF) in Sarov, Russia. A 5 tier 1 meter diameter explosive disk generator provided electrical energy to drive a 48 cm outside diameter, 4 mm thick, aluminum alloy liner having a mass of about 1 kg onto an 11 cm diameter diagnostic package. The purpose of the experiment was to measure performance of the explosive pulse power generator and the heavy imploding liner. Electrical performance diagnostics included inductive (B-dot) probes, Faraday rotation current measurement, Rogowski total current measurement, and voltage probes, flux loss and conductor motion diagnostics included current-joint voltage measurements and motion sensing contact pins. Optical and electrical impact pins, inductive (B-dot) probes, manganin pressure probes, and continuously recording resistance probes in the central measuring unit (CMU) and piezo and manganin pressure probes, optical beam breakers, and inductive probes located in the glide planes were used as liner symmetry and velocity diagnostics. Preliminary analysis of the data indicate that a peak current of more than 100 MA was attained and the liner velocity was between 6.7 km/sec and 7.5 km/sec. Liner kinetic energy was between 22 MJ and 35 MJ.