K.C. Hodge

Optical and pressure diagnostics of 4-MV water switches in the Z-20 test Facility

We are studying the behavior of self-breaking, high-voltage water switches for the Z refurbishment project. In Z-20, three or four water switches in parallel are charged to 4 MV in /spl sim/220 ns. The water gap between switch electrodes is 13-15 cm, and the enhancement of the positive and negative electrodes is varied to study time-evolution of the breakdown arcs, current sharing, and switch simultaneity. In addition to the standard electrical diagnostics (V,I), we are looking at one or more of the switches during the breakdown phase with two optical diagnostics: a streak camera and a fast framing camera. The streak camera has /spl sim/1-ns resolution, and the framing camera provides seven frames with >5 ns exposure times. For identical electric fields, the streamers originating on the positive electrode form earlier and move more rapidly than the streamers originating on the negative electrode. We observe four distinct phases in the closure of the water switches that depend on the macroscopic electric fields in the water: 1) No streamers propagate at E-fields below /spl sim/100 kV/cm from positive electrodes or voltages below /spl sim/140 kV/cm for negative electrodes; 2) streamers propagate with constant velocity between 100 and /spl sim/300 kV/cm; 3) above 300 kV/cm, the streamer velocities become linearly proportional to the electric field; 4) above 600 kV/cm, the velocity of streamers from the negative electrodes appears to saturate at /spl sim/100 cm//spl mu/s. The velocity of the streamers from the positive electrode continues to increase with E-field, reaching /spl sim/1% of the speed of light when the switch reaches closure.

Development/tests of 6-MV triggered gas switches at SNL

Gas switch development for application in Z refurbishment (ZR) has continued since PPPS Conference 2001. Several iterations have been tested both in oil and water dielectrics. The first switch tested was an evolved version of the Sandia designed HERMES III switch [G. J. Denison et al., 1985]. The 6 MV ZR baseline switch consists of a self-breakdown (cascade) section in which the discharge current flows in several parallel channels, and a trigger section where the current flows through a single spark channel. The second switch tested is a cantilevered switch. The entire cascade section is cantilevered on a plastic support rod allowing for a single continuous insulator housing but having the same characteristics as the baseline. The third switch tested is a Sandia hybrid switch. The laser triggered section of the hybrid switch includes 5 cascade like electrodes connected in parallel to a triggered gap via a /spl sim/2 /spl mu/H series isolation inductor. The discharge current in the hybrid switch trigger section flows in several parallel channels eliminating the single channel flow as in the baseline switch trigger section. The design iterations of these switches and results of these tests are presented.

Performance of the hermes-III laser-triggered gas switches

This paper reports the performance of the SF/sub 6/ insulated, multistage, laser-triggered gas switch used in the Hermes-III accelerator.sup 1/ In this accelerator, 20 of these switches are used to transfer energy from intermediate energy storage water dielectric capacitors to the pulse forming lines (PFLs). Approximately 8,000 laser-triggered switch shots have been taken with seven prefires. Nearly 70% of these shots have been at nominal operating parameters. The average first-to last spread in firing times for 20 switches is approximately 8 ns. Removal of systematic differences reduces this spread to /spl tilde/6 ns. This spread implies a one-sigma jitter for a single switch of <2 ns at 70-75% of self-breakdown voltage. Results show that the jitter does not change significantly over an operating range of 70-90% of self-breakdown. In addition, the jitter is insensitive to the factor of two variation in the laser energy delivered to the various switches by the optical system. A detailed summary on the performance, reliability, and maintenance of the switches and optical system is presented. Initial results of a study to investigate the performance of these switches under varying laser trigger conditions is also presented.

Genesis: A 5-MA Programmable Pulsed-Power Driver for Isentropic Compression Experiments

Enabling technologies are being developed at Sandia National Laboratories to improve the performance and flexibility of compact pulsed power drivers for magnetically driven dynamic materials properties research. We have designed a modular system capable of precision current pulse shaping through the selective triggering of pulse forming components into a disk transmission line feeding a strip line load. The system is comprised of two hundred and forty 200 kV, 60 kA modules in a low inductance configuration capable of producing 250–350 kbar of magnetic pressure in a 1.75 nH, 20 mm wide strip line load. The system, called Genesis, measures approximately 5 meters in diameter and is capable of producing shaped currents greater than 5 MA. This performance is enabled through the use of a serviceable solid dielectric insulator system which minimizes the system inductance and reduces the stored energy and operating voltage requirements. Genesis can be programmed by the user to generate precision pulse shapes with rise times of 220–500 ns, allowing characterization of a range of materials from tungsten to polypropylene. This paper provides an overview of the Genesis design including the use of genetic optimization to shape currents through selective module triggering.

Genesis: A 5 MA programmable pulsed power driver for Isentropic Compression Experiments

Enabling technologies are being developed at Sandia National Laboratories to improve the performance and flexibility of compact pulsed-power drivers for magnetically driven dynamic materials properties research. We have designed a modular system that is capable of precision current pulse shaping through the selective triggering of pulse-forming components into a disk transmission line feeding a strip line load. The system is composed of 240 200-kV 60-kA modules in a low-inductance configuration that is capable of producing 250-350 kbar of magnetic pressure in a 1.75-nH 20-mm-wide strip line load. The system, called Genesis , measures approximately 5 m in diameter and is capable of producing shaped currents that are greater than 5 MA. This performance is enabled through the use of a serviceable solid-dielectric insulator system which minimizes the system inductance and reduces the stored energy and operating voltage requirements. Genesis can be programmed by the user to generate precision pulse shapes with rise times of 220-500 ns, allowing characterization of a range of materials from tungsten to polypropylene. This paper provides an overview of the Genesis design, including the use of genetic optimization to shape currents through selective module triggering.

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.

Performance of the hermes-III pulse forming lines

Hermes III is a new 20-MV, 730-kA, 40-ns pulsed power accelerator. In Hermes III, eighty pulse forming lines (PFLs) generate 1.1-MV, 220-kA pulses, which are then added in a series/parallel configuration to produce the desired output pulse. Under normal Hermes-III operation, the PFLs produce a /spl tilde/40-ns FWHM pulse with a /spl tilde/15-ns, 10-90% rise time, and a one-sigma jitter of /spl tilde/4 ns. The Hermes-III water-dielectric PFLs have undergone 18 months of testing. During this period, over 59,000 PFL-shots have been accumulated, most of these at or near peak power. The PFLs have met design specifications, and they have proven to be highly reliable. This paper presents a brief overview of the PFL design, together with a detailed discussion of the system performance and limitations during this extensive testing period. A modification to the PFLs to produce a ramped output pulse was designed and tested. Results indicate that a ramped output pulse can be produced using a stepped impedance PFL.

A 5-Megavolt, 600-Kiloampere Laser-Triggered Gas Switch for use on Z-R: Comparison of Experiments and Simulations

The Z refurbishment project is designed to increase the peak current to the load on Z to ~26 MA in a 100-ns wide power pulse. This current is achieved by summing the current from 36 independent pulse-power modules. To meet these requirements, we have designed and constructed an SF6-insulated gas switch that can hold off 5.5 MV and conduct a peak current of 600 kA for over a hundred shots. The gas switch is charged by a Marx generator in ~1 microsecond and transfers about 200- kilojoules of energy and 0.25 Coulombs of charge to a pulse-forming line in a ~150-ns-wide power pulse peaking at 2.5 TW. The gas switch consists of a laser- triggered section holding off 15% of the voltage followed by 25 self-breakdown gaps. The self-breaking gaps are designed to provide multiple breakdown arcs in order to lower the overall inductance of the switch. The gas switch is submerged in transformer oil during operation. In this work, we show how simulation and experiment have worked together, first to verify proper operation of the switch, and then to solve problems with the switch design that arose during testing.

Performance of Self-Closing Diverter Switches for ZR/Z20 Marx and Intermediate Store Protection

The ZR refurbishment project [1] at Sandia National Laboratories (SNL) required a set of diverter switches to protect the Marx generators and intermediate storage (IS) capacitors from Marx pre-fire and/or laser triggered output switch (LTS) no-fire. Thirty-six such diverters, one for each Marx-IS set, will need to operate reliably over the full range of Marx charge voltages and LTS anticipated closure times. Operating voltage is up to 6 MV. A self-closing oil switch diverter was selected and design work began in late 2002. The first diverter (Phase I or just PI) was delivered in the summer of 2003 and tested on SNLs Z20 test-bed. Based on test results, operational experience and overall project budgetary concerns, it was decided to re-design the diverter, resulting a simpler, less costly switch. This new self- closing oil switch (Phase II or P2) was fielded at SNL on the Z20 test-bed in late 2004. Both designs include adjustable electrodes to control the closure time. Also incorporated is a mechanical clamp that minimizes or shorts the oil gap until Marx charge is complete. Both diverters feature liquid resistors sized to safely absorb the energy stored in the Marx or IS. This paper describes the design and test results from these diverters.

SaTPro: the system assessment test program for ZR

In the mid-90s, breakthroughs were achieved at Sandia with z-pinches for high energy density physics on the Saturn machine. These initial tests led to the modification of the PBFA II machine to provide high currents rather than the high voltage it was initially designed for. The success of z-pinch for high energy density physics experiments insured a new mission for the converted accelerator, known as Z since 1997. Z now provides a unique capability to a number of basic science communities and has expanded its mission to include radiation effects research, inertial confinement fusion and material properties research. To achieve continued success, the physics community has requested higher peak current, better precision and pulse shaping versatility be incorporated into the refurbishment of the Z machine, known as ZR. In addition to the performance specification for ZR of a peak current of 26 MA with an implosion time of 100 ns, the machine also has a reliability specification to achieve 400 shots per year. While changes to the basic architecture of the Z machine are minor, the vast majority of its components have been redesigned. Moreover the increase in peak current from its present 18 MA to ZRs peak current of 26 MA at nominal operating parameters requires significantly higher voltages. These higher voltages, along with the reliability requirement, mandate a system assessment be performed to insure the requirements have been met. This paper will describe the System Assessment Test Program (SATPro) for the ZR project and report on the results