A nonequilibrium atmospheric streamer discharge was investigated as a means to seed a large-gap arc breakdown. The dynamics of the streamer were analyzed with high speed imaging, photodiode light intensity, and current-voltage measurements. The temporal evolution of the discharge included a localized surface corona and a positive surface streamer. With the addition of an impurity gas (methane), the ionization was suppressed, which inhibited surface streamer propagation. The electron temperature was determined from time and spatially averaged spectra, coupled with a collisional-radiative model. The electron temperature in argon was measured at 1.25 eV for an electron density range of 1019 -10(20)m(-3). Partial local thermodynamic equilibrium calculations showed that the Ar II 4p states followed a Boltzmann distribution with an excitation temperature of 0.7 eV. The gas temperature was estimated at 815 K from a black-body distribution. The velocity of the surface streamer in argon was estimated at similar to 100 km/s with a diameter of similar to 500 mu m.

This paper proposes that in some instances, the magnetic saw effect is purely a melt phenomenon and describes a series of experiments where the magnetic saw effect was studied by varying the submillimeter radius of the enhancement point on a set of highly controlled samples. The magnetic saw effect is an instability that can occur when high currents are required to make sharp bends, thereby greatly enhancing the magnetic field and current densities. These sharp bends generally occur at joints, cuts, or other flaws and can cause failure of high-current conductors. The magnetic saw effect has also been proposed as a possible metal-cutting mechanism for manufacturing purposes. Due to the very small length scales of the features associated with magnetic sawing, it is difficult to make samples that are controlled enough to get reproducible experimental results for systematic studies. In these experiments, printed circuit board technology was used to accurately make samples with very fine enhancement features. The conditions for the magnetic saw effect were modeled using an electromagnetic code to establish the important parameters that predict the onset of damage.

A non-equilibrium atmospheric streamer discharge has been investigated as a means to seed a large gap arc breakdown. The dynamics of a surface streamer discharge has been analyzed with high speed imaging, voltage and current characterization as well as light intensity measurements. The temporal evolution of the discharge has been examined and a mechanism for the formation and propagation of the discharge presented. Thermodynamic properties of the atmospheric streamer discharge were determined from a time and spatially averaged spectra and a collisional-radiative model. The electron temperature and density were measured to be 1.25 eV and 10 m respectively. The velocity of the surface streamer was estimated to be ~110 km/s with a diameter of ~400 μm.

A gas-fed capillary plasma source has been developed to study plasma-surface interactions under pulsed high pressure arc conditions, without the use of an exploding fuse wire or ablative liner. A nonintrusive preionization source has been developed to break down relatively large interelectrode gaps at low charge voltages of 2-6 kV. The preionization source comprises a nonequilibrium surface streamer discharge that forms a conducting channel through which the main thermal arc discharge is initiated. The arc electron temperature and number density are estimated to be Te ~ 1-2 eV and ne ~ 1023 m-3. Silicon and sapphire samples were exposed to the arc plasma and revealed deposition of electrode and wall materials. Substitution of Elkonite 50W3 for brass electrodes reduced plasma contamination to acceptable levels. The plasma-material interactions were examined and quantified using scanning electron microscopy and energy dispersive X-ray spectroscopy.

This paper describes an exploding wire experiment to measure the electrical specific action to melt of structural alloys of copper and aluminum, including C10100, C11000, C18000, C18200, Al6061, and Al7075. These alloys, which are commonly used in railguns and other pulsed power devices, are not produced in fine wire form. Instead of wires, we developed a technique to test macroscopic samples (0.25 mm × 0.5 mm cross section) manufactured with wire electrical discharge machining. This paper includes a description of the design considerations for such macroscopic exploding wire experiments.

This paper describes a novel technique for making fine cuts in metal plates and foils. The process is based on a variant of the magnetic saw effect, a magnetic instability associated with high pulsed magnetic fields. When not controlled, the magnetic saw effect can lead to failure in high-field pulsed conductors. By limiting energy and pulse duration, the authors have produced controlled cuts with kerfs as fine as 0.01 mm. These promising results motivated a systematic study of the parameters that govern the quality and reproducibility of magnetic saw cuts. This paper provides an overview of the technique, of some key experiments, and of the practical issues associated with using pulsed magnetic fields for manufacturing.

Non-Contact Magnetic Cutting is a potential manufacturing process that uses repeated, low-energy magnetic pulses to produce and direct fine cuts in metal plates or foils. The process is based on the magnetic saw effect which, when not controlled, can lead to failure in high-field pulsed conductors. By limiting energy and pulse duration, the authors have produced controlled cuts with kerfs as fine as 0.013 mm. These promising results motivated a systematic study of the parameters that govern the quality and reproducibility of magnetic saw cuts.This paper reports on a series of simulations and experiments to identify the best conditions for applying the NCMC process to cutting 1100 aluminum plate. In the simulations, three parameters were varied: the location of the drive coil relative to the plate cut, the duration of the magnetic pulse, and the thickness of the sample. Joule heating and magnetic forces were computed in the simulation. The simulated conditions were used in the experiments, scaled in magnitude so that the pulses were at the minimum level that produced magnetic saw cuts. The results provided some insights into the relative importance of melting and fracture in the cutting process.

This paper describes a novel method for determining the specific action to melt the metals, and reports the values for action to melt that measure for several elements and three alloys: aluminum 2024, aluminum 6061, and C27400 brass. We electrically heat small diameter wires (127 $\mu{\rm m}$ ) to the point of vaporization using a slow regime exploding wire experiment. Using high-resolution voltage and current data, we compute the derivative of electrical resistivity with respect to specific electrical action. Features in the plot of this derivative clearly show the onset of melting for many of the materials we tested. We compare our results for copper, silver, aluminum, molybdenum, and titanium to those published by Tucker and Toth in the 1970s. Our data agree with their published values for silver and molybdenum, but not with those for copper, aluminum, and titanium. This paper presents our results and discusses possible reasons for the discrepancies between some of our measurements and those of Tucker and Toth.

Summary form only given. The term “magnetic saw effect” was introduced in 1957 by Firth et. al. in [1] to describe an instability observed in high-field pulsed magnets. The instability produces fine, saw-like cuts in solid conductors, often followed by larger openings, which have since been termed “blow holes.” The magnetic saw effect is also been observed in high-current pulsed buswork and more recently in railguns. As instabilities go, magnetic sawing has not been the subject of much research, in part because for most applications it is possible to engineer ones way out of the problem by using larger conductors. It is only within the last 10 years that researchers have sought to better understand the causes of magnetic sawing and the conditions for which magnetic sawing occurs. This paper surveys the literature on magnetic sawing and discusses some of the outstanding research questions, such as the relative importance of melting versus fracture as a driver for magnetic sawing.

This paper describes temperature measurements made on the high-energy medium-caliber launcher at the Institute for Advanced Technology. Simulations performed in Maxwell 3-D and E-Physics showed that Joule heating from current diffusing into the rails accounts for most of the temperature rise in the conductors. Temporal skin effects increase thermal dissipation significantly over what would be expected by the ohmic losses under fully diffused conditions. Based on this analysis, Joule heating is the overwhelmingly dominant source of heating in low-speed tests. As the velocity of the armature increases, Joule heating remains the dominant source of heat; however, additional mechanisms-which may include frictional heating, arcing energy, aluminum deposition, and temperature-dependent properties-are required to more satisfactorily explain the temperature profile obtained.

The Institute for Advanced Technology has been conducting experiments on plasma-armature railguns for the past several years. To control the damage mechanisms associated with this class of railgun, an electrothermal (ET) launcher preinjects the projectile into the plasma railgun bore milliseconds before the arc is energized. During the initial testing of the ET launcher, it was found that injection velocities were significantly lower than expected for a given input energy. Subsequent experiments using photonic Doppler velocimetry to resolve velocity as a function of time confirmed this finding. Using an ablative capillary discharge code written by Powell and Zielinski in 1991, the velocity discrepancy is analyzed, and the properties of the propellant gas-such as ionization fractions, the mass of the propellant gas, and the magnitude and decay of the pressure pulse seen by the projectile-are determined.

This paper provides a brief overview of railgun armature development undertaken at the Institute for Advanced Technology (IAT) and elsewhere over the last decade. The fundamental physics issues that govern the armature requirements are described. These include the operating requirements, minimum parasitic mass, material action limits, contact interface pressures, electromagnetic skin effects and current nonuniformities, magnetic sawing, launch package interactions, material surface treatments, melt lubrication, gouging, and transition to arcing contact. Different bore geometries-square, rectangular, round, augmented, -turn-are also described and require matching armature and launch package designs. Novel designs are discussed, including forward tabs, magnetic obturators, splined armatures, fiber contacts, pseudoliquid armatures, plasma, and hybrid armatures.

The Institute for Advanced Technology (IAT) at The University of Texas at Austin has been conducting research aimed at achieving muzzle velocities in excess of 7 km/s using a plasma-driven electromagnetic launcher. Plasma-driven railguns are susceptible to bore effects that limit muzzle speeds to about 6 km/s. The velocity ceiling is believed to be the result of the viscous drag of material ablated from the bore of the launcher. Ablation of bore materials, especially the insulators, is caused by the intense heat radiated by plasma armatures, especially at low speeds. The experiments reported in this paper are focused on overcoming the velocity ceiling of 6 km/s. A proof-of-principle experiment has been designed to launch 5-10 g polycarbonate projectiles to muzzle velocities in excess of 7 km/s with modest acceleration loads of similar to 500 kG. Our approach to controlling bore ablation involves the following elements: 1) using magnetic augmentation to reduce power dissipation in the plasma; 2) using high-purity alumina insulators to raise the ablation resistance of the bore; 3) using pre-acceleration to prevent ablation of the bore materials at low velocity; and 4) using a synchronously driven distributed power supply to electrically isolate stages. This paper describes the consequences of excessive bore ablation, the rationale for the IAT experiment, and the results obtained during testing in 2009.

This paper describes how small permanent magnets embedded in a launch package can be used to locate armature position during launch with greater accuracy than can be achieved by detecting the dipole field of the armature itself. Using this technique, we determined armature position in a medium-caliber launcher to within a millimeter of the value measured with photonic Doppler velocimetry, a precise optical technique. This paper describes the technique and some experimental results.