M. A. Salazar

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.

METALLURGICAL CHARACTERIZATION OF ATLAS CYLINDRICALLY CONVERGENT SPALLATION EXPERIMENTS.

The microstructural distribution and nature of damage from three different cylindrically convergent spallation experiments performed on the pulsed power machine named Atlas are presented. Longitudinal momentum trapping was used to minimize the influence of release waves and thereby decrease the dimensionality of the experiments. Two of the experiments involved soft capture of the spalled piece. The material used is a proprietary directionally cast Al alloy with a mostly equiaxed grain morphology and essentially random texture in the region of spallation. The damage was most distributed in the lowest impact velocity shot and became progressively more narrow with increasing impact velocity. The effectiveness of the momentum trap design increased with increasing impact velocity.

A proposed Atlas liner design fabricated for hydrodynamic experiments on Shiva Star

An entirely new cylindrical liner system has been designed and fabricated for use on the Shiva Star capacitor bank. The design incorporates features expected to be applicable to a future power flow channel of the Atlas capacitor bank with the intention of keeping any required liner design modifications to a minimum when the power flow channel at Atlas is available. Four shots were successfully conducted at Shiva Star that continued a series of hydrodynamics physics experiments started on the Los Alamos Pegasus capacitor bank. Departures from the diagnostic suite that had previously been used at Pegasus required new techniques in the fabrication of the experiment insert package.

Dynamic Friction Experiments at the Atlas Pulsed Power Facility

A Series of dynamic friction experiments has been conducted at the Atlas Pulsed Power Facility. Pulsed currents in excess of 21 MAmps were delivered to a cylindrical liner in about 15 ¿s. The liner was accelerated to km/s velocities and symmetrically impacted a hollow Ta/Al/Ta target. Due to the shock speed difference in Ta and Al, sliding velocities of almost a km/s were achieved at the Ta/Al interfaces. Initial analysis indicates that the machine performed to within a few percent of the design specifications. The primary diagnostic for these experiments was three radiographic lines-of-sight to look at thin gold wires embedded within the Al piece of the target. The magnitude of the displacement and the amount of distortion of the wires near the material interface is used as a measure of the dynamic frictional forces occurring there. Other diagnostics included a single-point VISAR and line-ORVIS to measure the breakout time and velocity on the inside of the target. Also, the Faraday rotation of a laser beam through a circular loop of optical fiber located in the power-flow channel of the experiment is used to measure the total current delivered to the experimental load. Data are being compared to a theoretical dynamic friction model for high sliding velocities. The model is based on molecular dynamics simulations and predicts an inverse power law dependence of frictional forces at very high sliding velocities.

Machining damaged surface hydrodynamic (DSH) spall target assemblies used in high energy compression physics experiments

Summary form only given. Targets were designed and manufactured to simulate a damaged surface in a series of high energy density (HED) experiments. The simulated damaged surface was a zero strength epoxy powder of nominally 1 mum and 5 mum diameter tungsten particles. This design feature introduced a number of manufacturing issues such as two precision grooves in the target wall and glide planes (upper and lower), alignment slots in the upper glide plane, developing the zero strength tungsten powder surface, a cryogenic interference fit, and a precision surface finish. Custom tooling, fixturing, and unconventional machining methods were utilized to mitigate manufacturing issues and are described in the poster. The targets were fielded at the Nevada Test Site (NTS) on the Atlas pulse power machine.

Development of Atlas Liner Cassette Power Flow Channel

The original Atlas upper liner cassette joint was based on the Shiva Star design and were of the cryogenic interference type. The upper joint relied exclusively on the interference of the glide plane/liner interface for electrical conductivity and structural integrity. The second-generation cassette was designed to accommodate unpredictable changes in inner power flow channel z-axis geometry by fabricating the liner and liner current joint electrodes as a single piece of aluminum. Deforming a preformed section of the liner as the cassette is assembled makes a current joint between the liner and the return current conductor. Experiments have been conducted with variations of the deformable current joint designs to better understand how the materials fit together, joint configuration, tensile force associated with the joint and the reliability of vacuum integrity at the joint and the resulting shape of the liner. Laser interferometer measurements and finite element analysis are used to analyze the joint.

Assembly of Atlas Power Flow Channel

Assembly of the Atlas power flow channel (PFC) containing the target cassette was a difficult operation prior to moving the machine to the Nevada test site (NTS). The assembly operation consisted of the target cassette manufactured at the Target Fabrication Facility of Los Alamos National Laboratory (TFF), vacuum containment hardware, and the PFC. The combination was made up of approximately 317 separate components that were required to fit accurately and become vacuum tight. The assembly process also included wired diagnostic feed through and x-ray diagnostic components inside the vacuum containment. All this activity occurred for each experiment on the deck of the Atlas machine. Each experiment altered the position of the conductor plates due to the dynamic forces of the electrical and magnetic pulse causing each subsequent installation to be unique with its own unique problems. An evolutionary design for the Atlas vacuum envelope (VE) eliminates the fit problems on the machine and reduces components assembled on the Atlas deck to one vacuum envelope assembled in a laboratory consisting of approximately 223 parts, and one part for mating to the PFC. The original Atlas PFC had a pumped volume of approximately 37.7 liters. The VE has a pumped volume of approximately 0.8 liters.

New residual stress mapping tool applied to Atlas current joint design

A redesigned cylindrical liner system has been implemented for use on the Atlas capacitor bank. This new design dramatically changes how the liner, glide planes and current joints of the system are formed. The previous design relied on interference of the liner with the glide plane by thermal shrink fit using liquid nitrogen coolant to form current joints. The new design achieves the required fit by mechanically distorting soft metals with a swaged joint. In this paper, we present the results of the first application of a new residual stress mapping technique, the contour method, to the design and fabrication process of the Atlas upper current joint. One of the strengths of the contour method is that it provides a full cross-sectional map of the residual-stress component normal to the cross section. The results showed significant stresses in the stainless steel glide plane with expected maximum compression near the joint and stresses in the aluminum part liner and return current conductor that corresponds well with measured form distortions.

Summary of the Rayleigh-Taylor instability studies at the Pegasus facility

Summary form only given, as follows. The goal of the RTMIX series on Pegasus was to study Rayleigh-Taylor instability growth and mixing, at a high/low density interface in a convergent geometry, as a function of material strength and initial perturbation amplitude. In this series of experiments a solid multi-layered core was compressed in a Z-pinch configuration using an approximately 5.5 MA sinusoidal current driven in the outer layer of aluminum. The 800-micron thick aluminum liner drives a 200-micron thick layer of dense metal (Cu or In/Sn) against a lower density solid core initially made of foam. The results of the four experiments completed are summarized. Preliminary results of the first three experiments were reported in the two previous Pulse Power Conferences. For the first experiment the high/low density interface was smooth and no instability growth or mixing was observed within the resolution limits of the diagnostics. In the second and third experiments, axially symmetric sine-wave perturbations were machined on to the inner surface of the dense material. The wavelength of the perturbations was 1 mm. On half of the interface the perturbation amplitude was 12.5 microns, on the other half the amplitude was 50 microns. Initial predictions indicated that with copper the large amplitude perturbations would grow and the small perturbations would remain stable. Since the In/Sn alloy used in the third experiment was predicted to melt, both perturbations should have experienced strong growth. In all cases the actual experiments showed no growth in either case. This enigma was resolved in the fourth experiment by replacing the foam inner core with liquid butane. The radiographs for the fourth experiment clearly show growth of the perturbations and recovery of the expected fluid-on-fluid behavior. This result indicated that the complex nature of the foam material used in the central core suppressed growth of the Rayleigh-Taylor instability.

Design and operation of high energy liner implosions at 16 MA for studies of converging shocks

Electromagnetically-driven implosion of solid-density, cylindrical liners can launch shocks with excellent precision at impact speeds exceeding 5 km/s. We discuss the design and operation of liner implosions driven at peak currents of 16MA, using the Shiva Star capacitor bank at the Air Force Research Laboratory. 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. The inner surface of the liner achieves a speed of 6.25 km/s when it impacts a concentric target cylinder of tin at a radius of 2 cm. Magnetic probes and radially-aligned X-radiography follow the motion of the liner and its impact on the tin cylinder. This cylinder holds a solid cylinder of acrylic of 1.5 cm radius in which the motion of a converging shock is followed by optical shadowgraphy and axially-aligned, X-radiography. Design issues that were successfully addressed include: Pulsed Power - current joints at high magnetic fields in the vicinity of the liner and glide-plane/electrodes, where magnetic pressures quickly exceed values for mechanical pre-stress, requiring dynamic solutions; surface temperature enhancements at changes in current direction; possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; need for magnetic inhibition of breakdown (MIB) between liner surface and insulator; Liner Physics - angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; nonlinear magnetic diffusion into liner and associated melting; Diagnostics X-radiography through cassette insulator and outer conductor without shrapnel damage to film.