Foil Load Research Articles

The Electrothermal Instability on Pulsed Power Ablations of Thin Foils

Presented are results from the optical imaging of atmospheric ablations of thin aluminum foils. These experiments were performed to evaluate the growth of temperature perturbations attributed to the electrothermal instability (ETI). ETI has been shown to seed magnetohydrodynamic instabilities on pulsed power-driven ablations of initially solid metallic targets, a topic of interest to various programs in pulsed power-driven plasma physics that depend on stable liner implosions. Experimental observations presented herein demonstrate exponentially growing temperature perturbations perpendicular to the direction of current with growth rates consistent with the linear ETI theory. High-temperature regions were observed to enter the vapor phase before sufficient energy had been deposited in the bulk foil to overcome the latent heat of vaporization, indicating a significant spatial heterogeneity in energy deposition rates. The growth rates of these perturbations scale as the square of current density, the predicted behavior for long-wavelength ETI structures. The development of these structures was unchanged by physical deformation of the foil surface, but dramatically influenced by incorporating areas of local high resistance in the foil loads. Extending the observation window in time showed a transition from perpendicular to parallel filaments, which is significant because ETI is predicted to switch orientations when the bulk foil material transitions into the plasma state. Collectively, these results provide an experimental validation of many theoretical predictions regarding ETI.

Time-Resolved Thomson Scattering on Laboratory Plasma Jets

Thomson scattering measurements were performed on plasma jets created from a 15- $\mu \text{m}$ -thick radial Al foil load on a 1-MA pulsed power machine. The laser used for these measurements has a maximum energy of 10 J at 526.5 nm. Using the full energy, however, significantly heats the $5\times 10^{18}$ cm−3 jet by inverse bremsstrahlung, creating a density bubble in the jet. To measure the evolving plasma parameters of this laser-heated jet, a streak camera was used to record the scattered spectrum, resulting in the sub-ns time-resolved Thomson scattering. Analysis of the streak camera image showed that the electron temperature of the jet was about 25 eV prior to the laser pulse. The laser then heated the plasma to 80–100 eV within about 2 ns. The electron temperature then stabilized for about 0.5 ns prior to falling at the end of the laser pulse. Jets made from a radial Ti foil showed more heating by the laser than the Al jets, going from 50 to over 150 eV, and heating was detected even when only 1 J of laser energy was used. Also, the ion-acoustic peaks in the scattered spectrum from the Ti jets were significantly narrower than those from Al jets, a result of several possible differences in the plasma created from these two materials.

Diagnostic and Power Feed Upgrades to the MAIZE Facility

The Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) is a single-stage, 1-MA linear transformer driver at the University of Michigan. The MAIZE facility has been the site of various experimental studies, including wire array Z-pinches for X-ray source development and cylindrical foil loads for implosion stability analysis. In order to better understand and investigate these and other experiments, a 4-frame extreme ultraviolet camera was implemented, the optical stability of a 12-frame laser shadowgraphy system was improved, B-dot probes were upgraded, and a Rogowski coil was constructed. New conical load hardware was also developed to reduce the inductance of the transmission line and thus improve the current delivered to the loads.

Multi-angle multi-pulse time-resolved Thomson scattering on laboratory plasma jets.

A single channel sub-nanosecond time-resolved Thomson scattering system used for pulsed power-driven high energy density plasma measurements has been upgraded to give electron temperatures at two different times and from two different angles simultaneously. This system was used to study plasma jets created from a 15 μm thick radial Al foil load on a 1 MA pulsed power machine. Two laser pulses were generated by splitting the initial 2.3 ns duration, 10 J, 526.5 nm laser beam into two pulses, each with 2.5 J, and delaying one relative to the other by between 3 and 14 ns. Time resolution within each pulse was obtained using a streak camera to record the scattered spectra from the two beams from two scattering angles. Analysis of the scattering profile showed that the electron temperature of the Al jet increased from 20 eV up to as much as 45 eV within about 2 ns by inverse bremsstrahlung for both laser pulses. The Thomson scattering results from jets formed with opposite current polarities showed different laser heating of the electrons, as well as possibly different ion temperatures. The two-angle scattering determined that the electron density of the plasma jet was at least 2 × 1018 cm-3.

Plasma Pinch Research on University Pulsed-Power Generators in the United States

This paper covers ongoing research on pulsed-power-produced plasmas using wire arrays and foil loads at four universities in the U.S. It is directed toward prospective and current students, as well as program managers at government agencies, and others who have a general interest in applications of pulsed power. This paper presents several pinch configurations (Z-pinches, X-pinches, planar wire arrays, radial, and suspended foils), specific physics issues (e.g., wire ablation), applications (plasma jets, X-ray hohlraums), diagnostics (e.g., ultraviolet laser probing), and modeling. The areas covered are not meant to be comprehensive, but rather, the discussion demonstrates the breadth and vibrancy of the activities at these institutions and collates extensive references on this work over primarily the past six years.

Loads for Pulsed Power Cylindrical Implosion Experiments

Pulse power can be used to generate high energy density conditions in convergent hollow cylindrical geometry through the use of appropriate electrode configuration and cylindrical loads. Cylindrically symmetric experiments are conducted with the Pegasus-II inductive store, capacitor energized pulse power facility at Los Alamos using both precision machined cylindrical liner loads and low mass vapor deposited cylindrical foil loads. The liner experiments investigate solid density hydrodynamic topics. Foil loads vaporize from Joule heating to generate an imploding cylindrical plasma which can be used to simulate some fluxes associated with fusion energy processes. Similar experiments are conducted with “Procyon” inductive store pulse power assemblies energized by explosively driven magnetic flux compression.

Study on Characteristics of Foil Loads of High-Speed Ships with Hydrofoils

The present paper studies the characteristics of the foil load and the vertical motion of the high-speed ships with hydrofoils quantitatively. First, the hydrofoil load was measured directly using dynamometers. Time-domain non-linear simulation was carried out using TSLAM-FHF1.2.3.4) and was compared with the experimental data. It was shown that the simulation can predict not only the vertical ship motion but also the foil load quantitatively in case no hydrofoil emergence occurs. The experimental result, however, shows the necessity of further investigation on the characteristics of histeresis of the hydrofoil lift to simulate ship motion in case foil emergence occurs. The eigenvalue analysis for the vertical ship motion of the high-speed ships with hydrofoils was performed based on the linearized equation of motion. The effect of the ship speed, the area and locations of the foils on the ship motion in wave was clarified.

Enhancement of the radiation yield, in plasma flow switch experiments

The Shiva Star fast capacitor bank, an inductive store, and a plasma flow switch were used to deliver multimegaampere currents with submicrosecond rise times to cylindrical foil loads. A series of numerical simulations of the plasma flow switch/imploding load system were performed with the goal of discovering a way to boost the total power radiated by the imploding plasma as it stagnates on the axis of symmetry. The changes to the experimental design that were investigated include variations of the shape of the electrodes, size and mass of the load foil, structure of the axial view vanes, shape and mass of the switching plasma, material from which the load is constructed, the degree to which the load is bowed, and the energy of the capacitor bank. Radiation yields in the range 6-9 TW are predicted for future experiments on Shiva Star.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of intraoral muscle forces during functional exercises

Measurements of intraoral muscle force with foil strain gauges, load cells, and pressure transducers bonded to a Tru-Tain stent and to a lip bumper appliance were tested by means of seven functional exercises in five adult subjects over a 5-day interval. The measuring devices and the functional exercises were tested for replicability and validity. Results showed that the pressure transducer was the superior measuring device with respect to size, sensitivity, thermal compensation, factory uniformity, replicability, and validity. The device most susceptible to error, on the basis of these factors, was the foil strain gauge. Of the seven functional exercises used, the pronunciation of the words "phone," "mom," and "church" and the exercise of swallowing were replicable over time. The other three exercises--chewing gum, sucking, and blowing on a straw--were determined to be unreliable in terms of replicability over time. Overall pressure values recorded were significantly higher than in previous reports. Pressure values were higher for the Tru-Tain stent than for the lip bumper.

Enhanced load current delivery from the SHIVA star vacuum inductive store/plasma flow switch

The SHIVA Star device is a 1313- mu F, 120-kV capacitative storage device capable of storing 9.4-MJ electrical energy. The experimental operating voltage is 90 kV. An increased current delivery from the SHIVA Star capacitor band to a plasma-flow-switch-driven cylindrical foil load is reported. Modification of a plasma-flow-switch electrode produces a current delivery that is a factor of two better than the best previous comparable efforts. Peak driving current is 12.3 MA; the peak load current is over 9.4 MA. At peak load current, driving current is 11.2 MA for a current delivery of 85% or more. Current delivery in the experimental system is reported. The modification specified entails partially closing the outflow electrode of a plasma-flow switch. >

Simulations of a Plasma Flow Switch

In a portion of the experimental program using the SHIVA Star capacitor bank at the Air Force Weapons Laboratory (AFWL), a cylindrical foil load is imploded using an inductive store and a plasma flow switch. We have performed a number of two-dimensional simulations of the switch and load using the MHD code MACH2. In addition to explaining the data from the first series of experiments, the simulations led to design modifications of the basic plasma flow switch that resulted in improved current delivery and in enhanced radiation yield. The experimental results are reported in a companion paper by Degnan et al. The key modification was closing portions of the vane structure. The switch must be sealed shut or else substantial current will flow in the diffuse gas that is ablated from the walls of the switch barrel.