Peter Eric Wakeland

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.

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.

A Laser Trigger System for ZR

Summary form only given. The Z pulsed power driver at Sandia National Laboratories is in the process of being refurbished as part of the ZR project to improve reliability and increase the energy delivered to the load. The new ZR gas switches currently close at a peak voltage of 6.2 MV to generate a projected 26 MA load current as compared to 4.6 MV and ~20 MA for the old switches on Z. The Laser Trigger System (LTS) has been redesigned to meet requirements that it trigger this higher voltage switch with comparable optic lifetime (>100 shots) and jitter (1sigma < 5ns) to the old Z switches. We will discuss critical design features of the LTS and show performance results from the Z20 test bed which was used to study the operation of a single ZR module.

Performance enhancements and results of testing a new 6.7 MV laser-triggered gas switch system on the refurbished Z accelerator

As the last command-triggered switch in the Refurbished Z accelerator at Sandia National Laboratories (SNL), the laser-triggered gas switch (LTGS) system is instrumental in the overall performance of Z and allows for flexibility in pulse shaping for various experimental campaigns. It is desirable to push the operating envelope of the switch to higher voltages and currents to allow for a higher peak power to be delivered to the load while at the same time reducing jitter and pre-fire rate for increased precision and reliability. We have accomplished this in a version of the LTGS that we call the C1.1 with the constraint of keeping the overall switch size consistent with physical space available. The C1.1 LTGS consists of laser-triggered and cascade portions which has been reported on previously.[1] However, the C1.1 eliminates the trigger plate and supports the cascade section in a cantilevered fashion. Improvements to this iteration of the LTGS were mainly mechanical in nature. Other minor electrical improvements were made to reduce regions of electric field enhancement and to reduce the likelihood of tracking by adding scalloping to the center support rod. Materials choice for the center support rod was important due to both the mechanical and electrical requirements placed on this component. Mechanical shock testing of the improved switch was performed on a shaker table available at SNL prior to installation on Z and showed that the improvements resulted in less displacement of the cantilevered end and less rotation of components in the cascade section. All electrical testing of the improved LTGS was performed on the Z machine. To date, we have accumulated 377 shots on C1.1 switches without a pre-fire. Runtime statistics are determined after each shot and show that the C1.1 switches are very tolerant to voltage and pressure variations exhibiting median runtimes of 43.9 ns with a jitter (1-σ) of 5.4 ns for all switch closures. The modifications made to and the performance results of the improved LTGS system are detailed in this manuscript.

Experimental validation of the first 1-MA water-insulated MYKONOS LTD voltage adder

The LTD technological approach can result in very compact devices that can deliver fast, high current and high voltage pulses straight out of the cavity without any complicated pulse forming and pulse-compression network. Through multistage inductively insulated voltage adders, the output pulse, increased in voltage amplitude, can be applied directly to the load. Because the output pulse rise time and width can be easily tailored (pulse shaped) to the specific application needs, the load may be a vacuum electron diode, a z-pinch wire array, a gas puff, a liner, an isentropic compression load (ICE) to study material behavior under very high magnetic fields, or a fusion energy (IFE) target. Ten 1-MA LTD cavities were originally designed and built to run in a vacuum or Magnetic Insulated Transmission Line (MITL) voltage adder configuration and, after successful operation in this mode, were modified and made capable to operate assembled in a de-ionized water insulated voltage adder. Special care has been taken to de-aerate the water and eliminate air bubbles. Our motivation is to test the advantages of water insulation compared to the MITL transmission approach. The desired effect is that the vacuum sheath electron current losses and pulse front erosion would be avoided without any new difficulties caused by the de-ionized water insulator. Presently, we have assembled and are testing a two-cavity, water insulated voltage adder with a liquid resistor load. Experimental results of up to 95kV capacitor charging are presented and compared with circuit code simulations.

Passive Mitigation of Load Debris in a Magnetically Insulated Transmission Line

The Z driver at Sandia National Laboratories delivers one to two megajoules of electromagnetic energy inside its ~10 cm radius final feed in 100 ns. The high current (~20 MA) at small diameter produces magnetic pressures well above yield strengths for metals. The metal conductors stay in place due to inertia long enough to deliver current to the load. Within milliseconds however, fragments of metal escape the load region at high velocity. Much of the hardware and diagnostics inside the vacuum chamber is protected from this debris by blast shields with small view ports, and fast-closing valves. The water-vacuum insulator requires different protection because the transmission line debris shield should not significantly raise the inductance or perturb the self- magnetically insulated electron flow. This report shows calculations and results from a design intended to protect the insulator assembly.

Hydrodynamic loading of structural components due to electrical discharge in fluids

Electrical discharge in a fluid produces a transient pressure wave, which should be taken into account when designing structural components of pulsed power machines. By combining theoretical approaches for underwater explosive theory with a self-consistent approach for modeling water as a compressible non-flow material, finite element models can be used to investigate mechanical response to arc driven pressure waves. Water switch testing includes impulse measurements of both multiple and single arc gaps compared with theory. Pressure wave interactions seen in computer models were in good agreement with peak pressures measured from a three-electrode water switch. Pressure measurements from a single electrode water switch were in close approximation to predictions based on explosive theory. Structural damage tests were also conducted in which damage to machine parts were related to arc energies.

1-MA Linear Transformer Driver (LTD) cavities as building blocks for ICE, ICF, and IFE drivers

Sandia in collaborations with a number of other institutions, is developing a new paradigm in pulsed power technology: the Linear Transformer Driver (LTD) technology. This approach can provide very compact devices that can deliver very fast high current and high voltage pulses. The output pulse rise time and width can be easily tailored to the specific application needs. We have extensively tested several induction LTD flat doughnut-shaped cavities which are 3m in diameter and have a 1.8 m bore and only ∼20 cm thickness, but they can deliver 0.1 TW, 70 ns rise time pulses to a matched ∼ 0.1 Ohm load. Experiments with a 1 TW Inductive Voltage Adder (IVA), which includes 10, 1-MA LTD cavities connected in series, are in preparation at MYKONOS LTD Laboratory in Sandia National Laboratories, Albuquerque, NM. This MYKONOS voltage adder will be the first ever IVA built with a transmission line insulated with de-ionized water. We present the design and experimental results of the first two-cavity water isolated voltage adder with a liquid salt solution resistive load. This is the first step towards the assembly of a ten cavity voltage adder.

Performance improvements of the 6.1-MV laser triggered gas-switch on the refurbished Z

A 6.1-MV, 790-kA laser triggered gas switch (LTGS) is utilized to synchronize the 36-modules of the Z-machine at Sandia National Laboratories. The switch is ∼81-cm in length, 45-cm in diameter, and is immersed in transformer oil. The switch is pulse-charged from a 780-kJ, 6-MV Marx generator in 1.4-µs. Closure of the switch allows energy stored in a 24-nF intermediate-store water capacitor to flow into subsequent pulse-forming stage. The entire system (36-modules) generates a ∼70-TW at the present system operating level, but after near term improvements are complete, this level will increase to ∼100-TW.

A 6.25 MV Laser Triggered Gas Switch for the Refurbished Z Accelerator

The Z machine at Sandia National Laboratories was recently refurbished to improve capacity, precision, and capability of nearly all of its pulsed power components in each of its thirty-six modules. The goal of this upgrade was to increase the total machine current by 30%, from ~20 MA to 26 MA in 120 ns into a wire-array z-pinch load. Initial design calculations indicated the operating point for an individual gas switch would be 5 MV and 600 kA to meet these design goals. This manuscript discusses design improvements that were required to achieve reliable performance at this level, the highest requirement for an LTGS in history. Significant factors include the electric potential grading along the switch length, the materials used in the switch, and the insulator housing design. It also discusses design criteria for robust SF6 gas switch design. Several design iterations are discussed that address performance improvements for each failure mode.