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Experiments were conducted that simulate the time-varying mechanical and thermal conditions typically found in electromagnetic launchers. While materials that are slowly heated isothermally have adequate time for fundamental microstructural changes that weaken them to occur, the mechanical strength of metals subjected to rapidly varying high-temperature pulses is dependent on heating rate and duration. Previous work has shown that tabulated properties of high-temperature materials inadequately describe such materials under pulsed heating conditions. In this paper, electrolytic tough pitch copper ring samples have been heated to a peak temperature in the range of 450[degrees]C over about 25 ms. The samples are first inductively preheated with low current before using a short-duration high-current pulse to expand the ring. A new photonic Doppler velocimetry system was used to accurately measure the velocity profile during the dynamic thermal and mechanical conditions. Preheating the specimens to 200[degrees]C resulted in a 30% drop in strength when compared to samples without preheating. Index Terms--Expanding ring, photonic doppler velocimetry (PDV), pulsed adiabatic conditions.

This paper describes a series of experiments to study the efficiency of an electrothermal (ET) launcher. The launcher serves as a preaccelerator for a plasma-driven hypervelocity railgun experiment at the Institute for Advanced Technology. The ET launcher is designed to accelerate small polycarbonate projectiles to about 1 km/s. The objective of this paper was to understand how pulse duration affects the efficiency with which the pressure of the ET discharge is coupled to the projectile. This paper consisted of multiple tests for which the total energy of the discharge was kept constant, but the pulselength was varied. The dependent variable in the tests was the muzzle velocity of the projectile. The experimental results are presented and compared with gas-dynamic considerations. Index Terms--Electrothermal (ET) launcher, gas dynamics, plasma-driven railgun.

The Institute for Advanced Technology (IAT) is developing a plasma-driven railgun to launch low-mass projectiles of 5-10 g to a velocity in excess of 7 km/s. Accomplishing this goal requires overcoming the problem of bore ablation, which has been linked to an observed velocity ceiling of about 6 km/s in plasma-armature launchers. Bore ablation is a direct consequence of the intense heat radiated by plasma armatures. Controlling bore ablation requires a coordinated approach that includes magnetic augmentation to reduce power dissipation in the plasma, high-purity alumina insulators to raise the ablation resistance of the bore, and pre-acceleration to prevent ablation of the bore materials at low velocity. This paper describes the consequences of excessive bore ablation, the rationale for the IAT experiment, and results obtained from the hardware that has been designed and tested to date. Index Terms--Electromagnetic launch, hypervelocity, plasma, railgun.

Experimental results comparing a breech-fed scheme and two distributed energy schemes for a free-running arc are presented. Analysis and observations of the issues associated with distributed energy switching of a plasma arc in the railgun are explored. The use of a free-running arc allows experiments to emulate the ablation and restrike phenomenon of a plasma armature railgun at high speeds (> 5 km/s) without the requirement of a large amount of stored energy. Numerous experimental tests were conducted to investigate the dynamics of plasma arcs within a distributed energy source railgun. Variations of switch timing, bore pressure, bore material, current amplitude, and current pulse length within each stage have been tested. These data reveal important design parameters for distributed energy railguns. The arc length, stage length, and stage trigger timing play a crucial role in distributed energy railgun performance. Failure to take these parameters into consideration will result in velocity reduction through plasma arc restrike and/or splitting. Index Terms--Distributed energy railgun, plasma armature railgun, railgun.

A unique theoretical model of the breakdown mechanism in water has been developed and further tested in both simulation software and experimentation. The conducted experiments test the degree to which electrode material, surface roughness, and surface area impact the dielectric strength of water. Voltage pulses with respective rise times of roughly 200 and 20 ns were applied to a water test gap producing electric fields in excess of 1.5 MV/cm. In experiments testing various electrode materials, thin film coatings of various metallic alloys and oxides were applied to Bruce-profiled stainless steel electrodes, with an effective area of 5 [cm.sup.2], through ion beam deposition. Similar Bruce-profiled stainless steel electrodes with surface roughness ranging from 0.26 to 1.96 [micro]m and effective areas ranging from 0.5 to 75 [cm.sup.2] were used in the study of surface roughness and area. Additionally, shadowgraph images of a point plane geometry were taken to further understand the breakdown processes that occur. Index Terms--Compact pulsed power, pulsed breakdown, water breakdown.

Experimental results are presented on the degree to which electrode surface conditioning and surface area impact the dielectric strength of water. The applied pulse to the test gap reached electric field levels greater than 1.5 MV/cm with risetimes around 200 and 50 ns, respectively. The test gap is composed of 304 stainless steel electrodes machined with a Bruce profile. Electrode surface roughness ranged from 0.34 to 1.41 [micro]m and effective areas ranged from 0.5 to 75 [cm.sup.2]. Additional results are presented on the pulsed breakdown strength of Rexolite with various surface finishes ranging from .025 to 5.715 [micro]m. Conclusions are made as to the effect electrode surface area and surface roughness has on the holdoff voltage of water dielectric systems. Conclusions are also made as to the impact of the surface condition of Rexolite has on its bulk breakdown strength. Index Terms--Compact pulsed power, pulsed breakdown, water breakdown.