L.J. Tabaka

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.

RANCHERO explosive pulsed power experiments

The authors are developing the RANCHERO high explosive pulsed power (HEPP) system to power cylindrically imploding solid-density liners for hydrodynamics experiments. Their near-term goal is to conduct experiments in the regime pertinent to the Atlas capacitor bank. That is, they will attempt to implode liners of /spl sim/50 g mass at velocities approaching 15 km/sec. The basic building block of the HEPP system is a coaxial generator with a 304.8 mm diameter stator, and an initial armature diameter of 152 mm. The armature is expanded by a high explosive (HE) charge detonated simultaneously along its axis. The authors have reported a variety of experiments conducted with generator modules 43 cm long and have presented an initial design for hydrodynamic liner experiments. In this paper, they give a synopsis of their first system test, and a status report on the development of a generator module that is 1.4 m long.

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.

The Ranchero explosive pulsed power system

We are developing a high explosive pulsed power system concept that we call Ranchero. Ranchero systems consist of series-parallel combinations of simultaneously initiated coaxial magnetic flux compression generators, and are intended to operate in the range from 50 MA to a few hundred MA currents. One example of a Ranchero system is shown. The coaxial modules lend themselves to extracting the current output either from one end or along the generator midplane. In this paper we concentrate on the system that we will use for our first imploding liner tests, a single module with end output. The module is 1.4 m long and expands the armature by a factor of two to reach the 30 cm OD stator. Our first heavy liner implosion experiments will be conducted in the range of 40-50 MA currents. Electrical tests, to date, have employed high explosive (HE) charges 43 cm long. We have performed tests and related 1D MHD calculations at the 45-MA current level with small loads. From these results, we determine that we can deliver currents of approximately 50 MA to loads of 8 nH.

A new 40 MA Ranchero explosive pulsed power system

We are developing a new high explosive pulsed power (HEPP) system based on the 1.4 m long Ranchero generator which was developed in 1999 for driving solid density z-pinch loads. The new application requires approximately 40 MA to implode similar liners, but the liners cannot tolerate the 65µs, 3 MA current pulse associated with delivering the initial magnetic flux to the 200 nH generator. To circumvent this problem, we have designed a system with an internal start switch and four explosively formed fuse (EFF) opening switches. The integral start switch is installed between the output glide plane and the armature. It functions in the same manner as a standard input crowbar switch when armature motion begins, but initially isolates the load. The circuit is completed during the flux loading phase using post hole convolutes. Each convolute attaches the inner (coaxial output transmission line to the outside of the outer coax through a penetration of the outer coaxial line. The attachment is made with the conductor of an EFF at each location. The EFFs conduct 0.75 MA each, and are actuated just after the internal start switch connects to the load. EFFs operating at these parameters have been tested in the past. The post hole convolutes must withstand as much as 80 kV at peak dI/dt during the Ranchero load current pulse. We describe the design of this new HEPP system in detail, and give the experimental results available at conference time. In addition, we discuss the work we are doing to test the upper current limits of a single standard size Ranchero module. Calculations have suggested that the generator could function at up to ∼120 MA, the rule of thumb we follow (1 MA/cm) suggests 90 MA, and simple flux compression calculations, along with the ∼4 MA seed current available from our capacitor bank, suggests 118 MA is the currently available upper limit.

SOME ASPECTS OF DATA PROCESSING FOR AN OPTICAL ABSORPTION EXPERIMENT IN A PULSED 1000-TESLA MAGNET

A procedure is given for the analysis of optical absorption data acquired in the hostile environment of a pulsed 1000-Tesla magnet. The 1000-Tesla magnetic field is achieved with an explosive charge which generates a shock wave that travels toward the sample of octachlorodirhenate.

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.

High current performance of the 43 cm long Ranchero generator

A Ranchero flux compression generator (FCG) was recently tested at the 76 MA level. Ranchero generators were designed to be cost effective high current devices, and a variety of configurations have been tested. The Ranchero armature is a 152 mm diameter aluminum cylinder with a 6 mm thick wall. The high explosive (HE) is detonated simultaneously on axis, and the armature is expanded by a factor of two. At the final 300 mm diameter, the circumference is over 950 mm, which has been anticipated to accommodate currents approaching 100 MA. The test was performed with a 43 cm long module having an initial inductance of 56 nH, and a load inductance of 0.55 nH, consisting of a short load feed slot and probe grooves. A 12 mF capacitor bank at ~16.5 kV provided the initial 3.75 MA seed current. The performance of the generator with this load was calculated using a 2D MHD ALE code, and agreement was excellent. Generator design, test data, and details of the MHD calculations are given. The most important aspect of the results is verification of codes at the 76 MA level. The same codes do not show excessive losses up to the 100 MA level, and it is a much smaller extrapolation to that level at this time.