Brian Stoltzfus

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.

Analysis of a laser induced plasma in high pressure SF6 gas

The Laser Triggered Switch Program at Sandia National Laboratories is an intensive development study to understand and optimize the laser triggered gas switch (LTGS) for the Z-Refurbishment (ZR) project. The laser triggered gas switch is the final command-triggered switch in the machine. Reliability and performance of the switch is crucial.

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.

Comparison of the performance of the upgraded Z with circuit predictions

Since the completion of the ZR upgrade of the Z accelerator at the Sandia National Laboratories in the fall of 2007, many shots have been taken on the accelerator, and there has been much opportunity to compare circuit-code predictions of the performance of the machine with actual measurements. We therefore show comparisons of measurements, and describe a full-circuit, 36-line Bertha circuit model of the machine. The model has been used for both short-pulse and long-pulse (tailored pulse) modes of operation. We also present the as-built circuit parameters of the machine and indicate how these were derived. We discuss enhancements to the circuit model that include 2D effects in the water lines, but show that these have little effect on the fidelity of the simulations. Finally, we discuss how further improvements can be made to handle azimuthal coupling of the multiple lines at the vacuum insulator stack.

PPPS-2013: A robust, low-inductance, low-jitter switch for petawatt-class pulsed power accelerators

Spark gap switches are integral components in pulsed power applications ranging from pulsed microwaves and flash x-ray sources to, more recently, the linear transformer drivers (LTDs) powering next-generation petawatt-class pulsed power accelerators at Sandia National Laboratories (SNL) in Albuquerque, New Mexico. An LTD system uses a triggered switch to conduct current from two capacitors to form a magnetic field which accelerates particles down magnetically insulated transmission lines (MITLs). Existing spark gap switches suffer from limited lifetimes 1 and levels of jitter and inductance in excess of what will be required for these new accelerators2. Kinetech, under a contract from SNL, has designed, developed, and demonstrated a switch that exceeds all of the established requirements3 by firing for 57,200 shots with an inductance of 35 nH and a one-sigma switch jitter of 1.2 ns variation over a 1,000 shot sampling after 1k, 25k, and 50k shots. This paper will detail the design, development, and demonstration of this switch by Kinetech and SNL.

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.

Measurement of the effective length of laser-plasma channels in a laser triggered gas switch by guided microwave backscattering in SF 6 and N 2 /SF 6

Backscattering of guided microwaves has been utilized to measure the effective electrical length of laser produced plasma channels (LPPCs) in a laser triggered gas switch configuration. The LPPC was introduced into the waveguide by focusing the UV trigger laser beam through small holes in the broad wall. By translating the laser-focusing lens, the plasma channel could be scanned through the waveguide. A measure of the channel length was obtained by determination of the limits over which scattering is observed. Backscattering channel length measurements were compared with three other measurements (D-dot probe, visible imaging, Schlieren imaging). It was found that visible and Schlieren imaging significantly underestimate the plasma length. Additionally, the plasma length has been studied as a function of lens focal length, laser energy, and neutral gas fill pressure in both pure SF6 and an N2/SF6 mixture (90%/10%).

Performance of a radial vacuum insulator stack

Electrostatic breakdown (“flashover”) can limit the maximum delivered power in pulsed power systems. One problematic region is the solid-vacuum interface. The solid/vacuum interface surface is generally the component most likely to suffer undesired breakdown in a well-designed pulsed power system driving a load in vacuum. A relatively small (in total number) and localized amount of neutrals or plasma can be enough to allow a self-sustaining discharge. A discharge along the solid-vacuum interface can readily and quickly desorb ions and neutrals from the insulator material. The spurious discharge can carry almost unlimited current, generally constrained only by the current source or the inductance of the current path. Radial insulator assemblies, where power flows along the axis of the insulator (used in a coaxial power feed with concentric insulators of differing diameters, maintaining radial electric field direction) can be lower insertion inductance and smaller in total size than axial insulators. Radial insulators have additional electric field grading and mechanical stress issues compared to an axial insulator system. With an interest in quantifying and improving performance of a radial insulator system, we have built a prototype 60 cm diameter assembly with four series insulator rings and 0.23 m2 total stressed area. By driving the insulator with a ~2 MV, 15 ns (effective time) negative pulse from a low energy system, we are able to study insulator performance of a multi-level stack with a reasonable data rate in a relevant regime (~180 kV/cm). The prototype insulator assembly is designed to demonstrate key concepts important for a large system, including 1) improved electric field uniformity using equipotential redistribution, 2) mechanical assembly viability with reasonable machining tolerances, and 3) useful electric field hold-off. The system is scalable to larger size insulators with acceptable electric field uniformity and mechanical stress in the insulating plastic. Mated to a (generally optimal) 40Ω oil-insulated transmission line, the electric field uniformity on the plastic-vacuum interface tested here is better than ±1%. Without equipotential redistribution, the electric field non-uniformity would be of order ±46%. To facilitate future fabrication of larger insulator assemblies, the Rexolite® 1422 cross-linked polystyrene plastic rings are cut from sheet or individually cast, then machined; a large monolithic slab of insulator material is not needed. We will discuss basic properties of vacuum interfaces, discuss scaling formulae used to relate performance of different systems, and show results of testing.

Millimeter-gap magnetically insulated transmission line power flow experiments

An experiment platform has been designed to study vacuum power flow in magnetically insulated transmission lines (MITLs). The platform is driven by the Mykonos-V LTD accelerator to drive a coaxial MITL with a millimeter-scale anode-cathode gap. The experiments conducted quantify the current loss in the MITL with respect to vacuum pumpdown time and vacuum pressure. MITL gaps between 1.0 mm and 1.3 mm were tested. The experiment results revealed large differences in performance for the 1.0 and 1.3 mm gaps. The 1.0 mm gap resulted in current losses of 40%-60% of the peak current. The 1.3 mm gap resulted in current losses of less than 5% of peak current. Classical MITL models that neglect plasma expansion predict that there should be zero current loss, after magnetic insulation is established, for both of these gaps.