Publications

A fully three-dimensional electromagnetic model of the major pulsed power components of the 26-MA ZR accelerator is presented. This large-scale simulation model tracks the evolution of electromagnetic waves through the intermediate storage capacitors, laser-triggered gas switches, pulse-forming lines, water switches, tri-plate transmission lines, and water convolute to the vacuum insulator stack. The plates at the insulator stack are coupled to a transmission line circuit model of the four-level magnetically-insulated transmission line section and post-hole convolutes. The vacuum section circuit model is terminated by either a short-circuit load or dynamic models of imploding z-pinch loads. The simulations results are compared with electrical measurements made throughout the ZR accelerator and good agreement is found, especially for times before and up to peak load power. This modeling effort represents new opportunities for modeling existing and future large-scale pulsed power systems used in a variety of high energy density physics and radiographic applications.

Abstract not provided.

Abstract not provided.

Experiment demonstrates the first direct transformation of a tungsten wire core to the plasma state by Joule heating during nanosecond electrical explosion in vacuum. Energy of ∼130 eV/atom was deposited into the 12 μm W wire coated by 2 μm polyimide during the first ∼10 ns. All the metal rapidly transformed to highly ionized plasma, while the surrounding polyimide coating remained primarily in a gaseous state. This coating totally suppressed corona formation. The expansion velocity of the wire was ∼12-18 km/s, the average wire ionization at 50 ns reached ∼67% with corresponding LTE temperature of ∼1.2 eV. Explosion of bare W wire demonstrated earlier termination of the wire core heating due to shunting corona generation. Magnetohydrodynamic (MHD) simulation reproduces the main features of coated and uncoated W wire explosion. © 2008 The American Physical Society.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

During the last few years Sandia is actively pursuing the development of new accelerators based on the novel technology of Linear Transformer Driver (LTD). This effort is done in close collaboration with the High Current Electronic Institute (HCEI) in Tomsk, Russia, where the LTD idea was first conceived and developed. LTD based drivers are currently considered for many applications including future very high current Z-pinch drivers like ZX and IFE (Inertial Fusion Energy), medium current drivers with adjustable pulse length for ICE (Isentropic Compression Experiments), and finally relatively lower current accelerators for radiography and x-pinch. Currently we have in operation the following devices: One 500-kA, 100-kV LTD cavity, a 1-MVvoltage adder composed of seven smaller LTD cavities for radiography, and one 1-MA, 100-kV cavity. The first two are in Sandia while the latter one is still in Tomsk. In addition a number of stackable 1-MA cavities are under construction to be utilized as building blocks for a 1-MA, 1-MV voltage adder module. This module will serve as a prototype for longer, higher voltage modules, a number of which, connected in parallel, could become the driver of an IFE fusion reactor or a high current Z-pinch driver (ZX). The IFE requirements are more demanding since the driver must operate in rep-rated mode with a frequency of 0.1 Hz. In this paper we mainly concentrate on the higher current LTDs: We briefly outline the principles of operation and architecture and present a first cut design of an IFE, LTD z-pinch driver. © 2005 IEEE.

The University of Missouri Terawatt Test Stand (MUTTS) has conducted many untriggered experiments on a Rimfire gas switch scaled to 2.5 MV. The focus of these experiments was to evaluate what methods may be used to control the distribution of cascade arcs. The untriggered data indicates that the rise time of switch current does not statistically improve, as expected, as the number of cascade arcs per gap increased beyond two channels. For the same data, the number of arcs in the cascade section more dramatically affects the output current period. This indicates that in late time increased multichanneling has a more pronounced effect than in early time. The switch is triggered with a frequency quadrupled Nd:YAG laser at 30 mJ with a 3-5 ns pulse width. Since the focused laser does not ionize the full length of the trigger section, there is little effect on current rise time when compared to untriggered data, but more channels form in the cascade section for an air filled switch. The cascade section was shorted and data are presented describing the contribution of the single channeling trigger section to overall switch impedance. The electrical effects of multichanneling using a laser trigger, the formation of arc channels in the cascade section, and the implications the results have on the future design of fast gas switches are discussed. © 2005 IEEE.

The ZR gas switch, located between the Intermediate Store capacitor (I-Store) and the Pulsed Forming Line (PFL), requires a laser pulse for its triggering. There are several routes for the beam to reach the gas switch but all of them cross over the high voltage regions. The Z laser tube crosses over the outer to inner PFL electrodes with a voltage difference no larger than 3.5 MV. The ZR gas switch was designed to be in oil, given the higher operational voltages, as a consequence the laser tube is in the oil side of the PFL interface. The ZR laser tube is required to hold in excess of 5 MV across it using high pressure SF6 gas, the ID is 2.5″ to accommodate the laser beam, mechanically should tolerate the non-axial shock loading during the water switches firing. After a couple of iterations it was decided to use Polyurethane, it provided most of the desired mechanical properties, except that it outgases ether and ether based compounds. The effect of just a few ppm of ether on SF6 is a significant reduction on the HV hold off especially surface tracking or flashover. As a consequence the final design is such that the electric field distribution on the tube is as conservative as it was possible due to space constrains. We present the basic design, the field distribution, its relationship with available SF6 breakdown data and the present performance. © 2005 IEEE.

We have developed a system of differential-output monitors that diagnose current and voltage in the vacuum section of a 20-MA 3-MV pulsed-power accelerator. The system includes 62 gauges: 3 current and 6 voltage monitors that are fielded on each of the accelerator's 4 vacuum-insulator stacks, 6 current monitors on each of the accelerator's 4 outer magnetically insulated transmission lines (MITLs), and 2 current monitors on the accelerator's inner MITL. The inner-MITL monitors are located 6 cm from the axis of the load. Each of the stack and outer-MITL current monitors comprises two separate B-dot sensors, each of which consists of four 3-mm-diameter wire loops wound in series. The two sensors are separately located within adjacent cavities machined out of a single piece of copper. The high electrical conductivity of copper minimizes penetration of magnetic flux into the cavity walls, which minimizes changes in the sensitivity of the sensors on the 100-ns time scale of the accelerator's power pulse. A model of flux penetration has been developed and is used to correct (to first order) the B-dot signals for the penetration that does occur. The two sensors are designed to produce signals with opposite polarities; hence, each current monitor may be regarded as a single detector with differential outputs. Common-mode-noise rejection is achieved by combining these signals in a 50-{Omega} balun. The signal cables that connect the B-dot monitors to the balun are chosen to provide reasonable bandwidth and acceptable levels of Compton drive in the bremsstrahlung field of the accelerator. A single 50-{omega} cable transmits the output signal of each balun to a double-wall screen room, where the signals are attenuated, digitized (0.5-ns/sample), numerically compensated for cable losses, and numerically integrated. By contrast, each inner-MITL current monitor contains only a single B-dot sensor. These monitors are fielded in opposite-polarity pairs. The two signals from a pair are not combined in a balun; they are instead numerically processed for common-mode-noise rejection after digitization. All the current monitors are calibrated on a 76-cm-diameter axisymmetric radial transmission line that is driven by a 10-kA current pulse. The reference current is measured by a current-viewing resistor (CVR). The stack voltage monitors are also differential-output gauges, consisting of one 1.8-cm-diameter D-dot sensor and one null sensor. Hence, each voltage monitor is also a differential detector with two output signals, processed as described above. The voltage monitors are calibrated in situ at 1.5 MV on dedicated accelerator shots with a short-circuit load. Faraday's law of induction is used to generate the reference voltage: currents are obtained from calibrated outer-MITL B-dot monitors, and inductances from the system geometry. In this way, both current and voltage measurements are traceable to a single CVR. Dependable and consistent measurements are thus obtained with this system of calibrated diagnostics. On accelerator shots that deliver 22 MA to a low-impedance z-pinch load, the peak lineal current densities at the stack, outer-MITL, and inner-MITL monitor locations are 0.5, 1, and 58 MA/m, respectively. On such shots the peak currents measured at these three locations agree to within 1%.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.