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Nevada Shocker

Pulse Power Mission Statement

The Nevada Shocker with Surface Flashover Application

The Nevada Shocker is a moderate size pulsed power device that has the potential of generating a 540 kV, 7 W, 50 ns pulse based on Marx Bank and Blumlein technology. This 7 W machine tools the Pulsed Power Laboratory in the Engineering College at the University of Nevada Las Vegas with the ability to study the properties of materials in a harsh electromagnetic environment.

The Marx Bank consists of nine low inductance 60 kV, 0.28 mF capacitors charged in a parallel configuration using a 90 kV Gamma High Voltage DC power supply. Suitably rated Ross relays engage the power supply to the capacitor bank. For safety purposes, another set of Ross relays engage the Marx Bank to an electrical dump when required. Typically, the bank charges in thirty seconds. Four 3.6 nF capacitors, charged by a secondary source and activated manually, drive a trigatron in a gas manifold chamber supported just above the capacitor assembly in the oil bath. A spark generated in the gas manifold containing the capacitor electrodes immersed in 21 psi of 99.999% dry air environment initiates the erection of the capacitor bank into a series configuration. A direct line of sight exists between the capacitor bank and the trigatron. Others suggest that the ultraviolet light generated by the trigatron spark enhances the gas breakdown process resulting in closure or shorting of adjoining electrodes. At this pressure and dry air purity, the electrodes spaced about 3/8” hold off voltage levels as high as 63 kV. The embedded graphs show the self-breakdown properties of dry air at various pressures and electrode spacings. A Shell Diala AX oil prevents breakdown among capacitor electrodes in the bank. Typical ratings are: relative permittivity 2.25 and dielectric breakdown strength 28 kV in a 1.02 mm parallel plate gap. 330 W and 2500 W copper sulfate resistors interconnect capacitors for charging and discharging purposes. A resistive tee sensor monitors the Marx bank discharge. Embedded graphs textbook discharge results when the bank erects properly. A charging transmission line guides the energy discharged by the Marx bank to the Blumlein transmission line. The charging transmission line utilizes a coaxail cable, stripped of its outer jacket and shielding, as the center electrode of the line. The bank forces the inner conductor at a lower potential relative to the earth-ground, outer conductor. The outer conductor of the line contains two view ports useful for adjusting the self-breaking water switch in the Blumlein and allowing for the positioning of optical sensors near the water switch. The Blumlein composed of three cylindrical electrodes (inner conductor, intermediate conductor, and an outer conductor) temporarily stores and compresses the energy released by the Marx bank. For optimal operation, the capacitance of the erected Marx Bank well excedes the Blumlein capacitance allowing the voltage of the Marx Bank to be transferred to the Blumlein as illustrated in a simple circuit model. A self-breaking water switch, at the charging end of the line, releases the stored energy to a discharge transmission line. The characteristics of the line dictate the time interval for discharge. Typically, the discharge time of the Marx bank well exceeds that of the Blumlein. Consequently, a loosly wound, low inductance, inductor at the discharge end of the line connects the inner conductor to the outer conductor during the slow charging process and, in principle, open circuits this connection during the fast discharge process. A detailed three-line transmission line theory of the Blumlein illustrates the dynamics of the discharge process. The distance of separation of the water switch electrodes dictates the Blumlein discharge voltage. The optimal electrode spacing occurs when the water switch closes as the Blumlein voltage approaches the half voltage of the Marx bank. If the charging process exceeds the time interval for polarizing the water between the water switch electrodes, premature closing of the water switch results in limiting the discharge voltage. A small resistive sensor in series with the loosely wound inductor indirectly monitors the charging of the Blumlein and directly monitors the precursor voltage across the diode electrodes. The discharge transmission line guides the time-compressed energy of the Blumlein directly to the diode vacuum chamber through a special nylon water-vacuum barrier. The charging transmission line electrodes, the Blumlein electrodes and the charging transmission line electrodes supports a deionized water dielectric medium with conductivity on the order of 5 mS/cm. The water medium circulates through Barnstend deionizing filters for purification. Decreasing the conductivity of the deionized water hinders the operation of the self-breaking water switch resulting in delay switching due to the relatively slow polarization processes of the water. The vacuum chamber consists of eight ports supporting a number of diagnostics such as the residual gas analyzer (RGA), optical view ports for lasers, cameras, and optical sensors, feed throughs for electrical and optical sensors such as B-dots and optcal fiber sensors mounted on the diode electrode. The diode electrodes consist of two, parallel, flat, 7 3/8” diameter, circular, brass discs sharing a common axis. The negative electrode connects directly to the center electrode of the discharge transmission line. The grounded electrode attaches to an air cylinder piston and, via copper braids, to the grounded outer chamber wall. The air cylinder delivers up to 1,200 lbs (@250 psi of maximum input air pressure) of compression force to the element under test between the diode electrodes. A practical limit of 500 lbs at 100 psi exists in the laboratory using house air. A cryopump maintains vacuum pressures between 1.5 to 5.0 x 10 -6 Torr suitable for a number of material studies in a harsh electromagnetic environment. State of the art real-time 6 GHz digital scopes record signals intercepted by electrical and optical sensors. It is anticipated based on power flow arguments that the circular electrodes exhibit radial transmission line qualities during initial transit times. Consequently, the energy absorbed by the mismatched radial transmission line is voltage up-converted as the signal approaches the lines axis.

One use for the Nevada Shocker applies to the study of surface flashover (breakdown) on plastic insulators. The high vltage delivered by the Blumlein energizes a one inch diameter, one inch long plastic cylinders sandwiched in between the diode electrodes on diode axis. The ground electrode supports nine B-dots and eight fiber sensors. One set of four equally spaced B-dots and one set of four equally spaced fiber sensors exist each on a 2.5” radius and on a 1.25” radius perimeter circle. A single B-dot also exists at the center of the electrode under the test piece. Modeling studies and experimental studies investigate the initiation of flashover on plastics with the aim to understand the mechanism of surface breakdown. Initial modeling studies with Mission Research Corporation’s large scale plasma (LSP) code predicts field emission at the electrode edge and at the plastic, electrode, vacuum interface. The LSP particle simulation shows the extent of field emission over time. Click references for a list of authors investigating surface flashover.

 
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Last updated: 8/26/05 10:16 AM