K. R. LeChien

Status of the Z pulsed power driver

The Z pulsed power driver at Sandia National Laboratories is used for a variety of high energy physics experiments. The Z system stores 20 megajoules at the present nominal ±85 kV Marx charge voltage. The Z system consists of 36 basically identical modules, each with a DC-charged Marx generator, water insulated intermediate store capacitor, laser triggered gas switch, and water insulated pulse-forming section. High current drivers such as Z are able to create energy densities of megajoules per cubic centimeter in ∼1 cm3 volumes, used for creating extremes of temperature and pressure on a nanosecond time scale. The Z driver has delivered currents up to 27 MA with 85 nanosecond rise time (10%–90%) into fixed inductance and imploding plasma loads. Z has generated over 300 TW peak X-ray power, with more than 2 MJ total energy radiated from imploding tungsten plasma shells. Z is also used for studying dynamic compression of solid materials at megabar levels, using magnetic pressure from current densities of tens of megamperes per cm. Z is a confluence of pulsed energy storage and switching technologies. DC charged Marx generators are triggered with ten-nanosecond precision, transferring energy to water-insulated coaxial capacitors. Six megavolt laser triggered gas switches perform the final nanosecond time synchronization, and self-closing water switches further compress the pulse. Issues with the system include improving the reliability and performance of the components, and improving our conceptual understanding of the entire machine. Many of the Z components are unique: Z requires laser triggered gas switches operating at six million volts, water switches operating at three million volts, and large-area solid-vacuum interfaces operating reliably at 140 kV/cm. Synchronization with fast load diagnostics requires nanosecond predictability of the load current timing, while the pre-fire probability of any switch must be extremely small. We will describe the state of the machine, and specific measurements of subsystem performance.

A 1-MV, 1-MA, 0.1-Hz linear transformer driver utilizing an internal water transmission line

Sandia National Laboratories is investigating linear transformer driver (LTD) architecture as a potential replacement of conventional Marx generator based pulsed power systems. Such systems have traditionally been utilized as the primary driver for z-pinch wire-array experiments, radiography, inertial fusion energy (IFE) concepts, and dynamic materials experiments (i.e. ICE).

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.

Temporally shaped current pulses on a two-cavity linear transformer driver system

An important application for low impedance pulsed power drivers is creating high pressures for shock compression of solids. These experiments are useful for studying material properties under kilobar to megabar pressures. The Z driver at Sandia National Laboratories has been used for such studies on a variety of materials, including heavy water, diamond, and tantalum, to name a few. In such experiments, it is important to prevent shock formation in the material samples. Shocks can form as the sound speed increases with loading; at some depth in the sample a pressure significantly higher than the surface pressure can result. The optimum pressure pulse shape to prevent such shocks depends on the test material and the sample thickness, and is generally not a simple sinusoidal-shaped current as a function of time. A system that can create a variety of pulse shapes would be desirable for testing various materials and sample thicknesses. A large number of relatively fast pulses, combined, could create the widest variety of pulse shapes. Linear transformer driver systems, whose cavities consist of many parallel capacitor-switch circuits, could have considerable agility in pulse shape.

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.

Linear Transformer Drivers (LTD) for high voltage, high current rep-rated systems

The Linear Transformer Driver (LTD) technology can provide very compact devices that 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. Trains of large number of high current pulses can be produced with variable inter-pulse separation from nanoseconds to microseconds. Most importantly these devices can be rep-rated to frequencies only limited by the capacitor specifications that usually is 10Hz. To date we have completed the experimental evaluation of two (LTD I, LTD II) 500-kA cavities in both single and rep-rated modes. A larger 1-MA cavity was tested in a single shot mode and in a voltage adder configuration. The first built inductively isolated 1-MA LTD Voltage Adder (IVA) was composed of five cavity connected in series with a vacuum insulated transmission line. It was tested with both resistive and electron diode loads. The experimental results are in excellent agreement with numerical simulations. Experiments with a 1 TW Inductive Voltage Adder (IVA), which includes ten, 1-MA LTD cavities connected in series, are in preparation at Mykonos LTD Laboratory in Sandia National Laboratories, Albuquerque, NM. This time around the Mykonos voltage adder will be built with a transmission line insulated with deionized water. In this experimental work we aim to test the advantages of water insulation as compared to self Magnetic Insulated Transmission Line transport (MITL). It is hoped that the vacuum sheath electron current losses will be avoided without any new difficulties caused by the deionized water. This voltage adder will be rep-rated at 6 shots per minute. In the present paper we briefly present the LTD cavity architecture, describe the two LTD cavity types built and experimentally tested in single and rep-rated modes and present the design of the first two-cavity water insulated voltage adder with a liquid salt solution resistive load. This is the first step towards the assembly of a ten-cavity voltage adder of which most of the components are already built and/or fully designed.

High powers from large diameter wire arrays on the refurbished Z generator

We present results from initial large diameter wire array experiments at the refurbished Z generator. 65mm on 32.5mm nested stainless steel wire arrays were fielded at two different total masses (2.5mg & 5mg). Each array was 20mm tall and had an inner array half the mass of the outer. The experiments were designed to evaluate trends in Fe K-shell power and yield and re-establish previous output, and large diameter tungsten wire arrays are further planned to study total x-ray output.

Large diameter copper wire array implosions for K-shell x-ray generation on the refurbished Z machine

Copper wire arrays have previously been fielded in large diameter (up to 70 mm) nested configurations on the 20 MA Z machine, with average Cu K-shell (8.4 keV) yields of 20 kJ typical. More recently, Cu K-shell source development studies have been performed on the refurbished Z machine, which in initial commissioning shots has produced just over 20 MA current in these loads. At full capability, the refurbished Z is anticipated to drive 25 MA z-pinch implosions with double the stored energy as previously available at the facility.

Current losses in wire array Z-pinches on the Z generator

Compact tungsten wire array z-pinches imploded on the Z generator at Sandia National Laboratories have proven to be a powerful, reproducible, x-ray source. Wire arrays have also been used in dynamic hohlraum radiation flow experiments, and as an intense K-shell source, while the generator has been used extensively for isentropic compression experiments. A problem shared by all these applications is current loss preventing the ∼20MA drive current from being reliably coupled to the load. This potentially degrades performance, while uncertainties in how this loss is described limits our predictive capability.