Z.R. Wallace

ZR laser triggered gas switch requirements and performance

The Z machine at Sandia National Laboratories is presently undergoing an upgrade, called Z-Refurbishment (ZR) [1], that is aimed at improving capacity, precision, and capability to essentially all of its pulsed power components, including its thirty six laser-triggered gas switches (LTGS). Voltage and current requirements for the ZR LTGS have increased 25% from the onset of the ZR program, with no allowable increase to the physical footprint (or inductance) for the device. Initial design studies indicated that a total machine peak current of 26 MA could be achieved with the each LTGS operating at 5 MV and 600 kA. Increases in the final design inductance in the transition from vertical water transmission lines to horizontal magnetically insulated transmission lines, higher inductance in vacuum from changes in the load position for improved diagnostic access, and conservatism in the vacuum power flow requirements caused the LTGS operational goal to become 5.4 MV and 750 kA for a total machine peak current of 23 MA in 100 ns to a 10 mm radius, 10 mm long wire array. A comprehensive research program was initiated in August 2005 to improve the performance of the ZR gas switch at the 5.4 MV level, and results of that effort to date are presented herein.

Emergence of the Positive Column in an Atmospheric Air Discharge

The initial stages of electrical breakdown of atmospheric air have been captured with a fast-framing camera. The peak voltage is 100 kV with a duration of 60 ns and is applied to a 4.6-cm gap. The camera captures the start of the positive column, the initial cathode spot, and the development by ionization waves into a hot channel.

Surface interactions involved in flashover with high density electronegative gases.

This report examines the interactions involved with flashover along a surface in high density electronegative gases. The focus is on fast ionization processes rather than the later time ionic drift or thermalization of the discharge. A kinetic simulation of the gas and surface is used to examine electron multiplication and includes gas collision, excitation and ionization, and attachment processes, gas photoionization and surface photoemission processes, as well as surface attachment. These rates are then used in a 1.5D fluid ionization wave (streamer) model to study streamer propagation with and without the surface in air and in SF6. The 1.5D model therefore includes rates for all these processes. To get a better estimate for the behavior of the radius we have studied radial expansion of the streamer in air and in SF6. The focus of the modeling is on voltage and field level changes (with and without a surface) rather than secondary effects, such as, velocities or changes in discharge path. An experiment has been set up to carry out measurements of threshold voltages, streamer velocities, and other discharge characteristics. This setup includes both electrical and photographic diagnostics (streak and framing cameras). We have observed little change in critical field levels (where avalanche multiplication sets in) in the gas alone versus with the surface. Comparisons between model calculations and experimental measurements are in agreement with this. We have examined streamer sustaining fields (field which maintains ionization wave propagation) in the gas and on the surface. Agreement of the gas levels with available literature is good and agreement between experiment and calculation is good also. Model calculations do not indicate much difference between the gas alone versus the surface levels. Experiments have identified differences in velocity between streamers on the surface and in the gas alone (the surface values being larger).

Towards the Autonomous Operation of Z20: A TW Pulsed Power Module

The Refurbished Z machine, ZR, has 3 distinct missions. The first is to increase the capability for the user community by providing higher peak currents. The second is to increase the precision by increasing pulse repeatability and pulse shape flexibility. The third is to increase the capacity by providing operational turnaround time consistent with conducting a shot per shift. The pursuit of the third mission relies heavily on the reliability of the components and well defined maintenance cycles. To test the performance of the ZR design, a system assessment module has been tested and found to meet the capability mission. The system assessment module will be used to test the reliability of the components comprising the ZR pulsed power modules. To assess the program goal of 1 failure in 50 ZR shots we plan to perform 7200 shots of the system assessment module, Z20. At a typical shot rate, this task would take approximately three years. To minimize the impact on the facility while obtaining the required reliability data, the system will be configured to fire and reset autonomously with the ultimate goal of unmanned operation. A systems approach was developed using National Instruments Lab VIEWreg software and Field Point I/O hardware. All communications between subsystems are provided via ethernet using fiber optic media converters. The major subsystems for operating the module which will be described are the gas pressure and purge, high voltage power supplies, oil diverter operation, triggering, precision monitoring of Marx system, personnel access control and remote operation of the laser trigger system.

Optical and electrical diagnostics of 100 micron diameter wires exploded in air

Current induced wire explosions have been studied for many years for applications in exploding bridge wires and as radiation sources. Typical media are air, water and vacuum. In this effort, experiments are conducted in ambient air for the purpose of validating numerical simulations performed with the magneto-hydrodynamics (MHD) code ALEGRA. In addition to the electrical quantities current, voltage, and the action integral, derivative parameters, such as pressure and shock wave velocity, are also measured and compared.

Investigations of surface flashover in high pressure environments

It is generally acknowledged that once a highly conductive channel is established between two charged and conducting materials, electrical breakdown is well established and difficult to interrupt. Thus, an understanding of the initiation mechanism for electrical breakdown is crucial for devising mitigating methods to avoid catastrophic failures. Both volumetric and surface discharges are of interest. An effort is underway where experiments and theory are being simultaneously developed. The experiment consists of an impedance matched discharge chamber capable of investigating various gases and pressures to ten atmospheres. In addition to current and voltage measurements, a high dynamic range streak camera records streamer velocities. The streamer velocities are particularly valuable for comparison with theory. A streamer model is being developed which includes photoionization and particle interactions with an insulating surface. The combined theoretical and experimental effort is aimed at detailed comparisons of streamer development as well as a quantitative understanding of how streamers interact with dielectric surfaces and the resulting effects on breakdown voltage.

Streamer Initiation in Volume and Surface Discharges in Atmospheric Gases

It is generally acknowledged that once a highly conductive channel is established between two charged and conducting materials, electrical breakdown is well established and difficult to interrupt. An understanding of the initiation mechanism for electrical breakdown is crucial for devising mitigating methods to avoid catastrophic failures. Both volumetric and surface discharges are of interest. An effort is underway where experiments and theory are being simultaneously developed. The experiment consists of an impedance matched discharge chamber capable of investigating various gases and pressures to ten atmospheres. In addition to current and voltage measurements, a high dynamic range streak camera records streamer velocities. The streamer velocities are particularly valuable for comparison with theory. A streamer model is being developed which includes photo-ionization and particle interactions with an insulating surface. The combined theoretical and experimental effort is aimed at detailed comparisons of streamer development as well as a quantitative understanding of how streamers interact with dielectric surfaces and the resulting effects on breakdown voltage.

Laser Triggered Gas Switches Utilizing Beam Transport Through 1 MΩ-cm Deionized Water

We report on the successful attempts to trigger high voltage pressurized gas switches by utilizing beam transport through 1 MΩ-cm deionized water. The wavelength of the laser radiation was 532 nm. We have investigated Nd:YAG laser triggering of a 6 MV, SF6 insulated gas switch for a range of laser and switch parameters. Laser wavelength of 532 nm with nominal pulse lengths of 10 ns full width half maximum (FWHM) were used to trigger the switch. The laser beam was transported through 67 cm-long cell of 1 MOmega-cm deionized water constructed with anti reflection UV grade fused silica windows. The laser beam was then focused to form a breakdown arc in the gas between switch electrodes. Less than 10 ns jitter in the operation of the switch was obtained for laser pulse energies of between 80-110 mJ. Breakdown arcs more than 35 mm-long were produced by using a 70 cm focusing optic.