Keith R. LeChien

Development of a terawatt test stand at the University of Missouri for fast, multichannel switching analysis

The University of Missouri Terawatt Test Stand (MUTTS) began assembly in January 2003. Construction of MUTTS is progressing rapidly with the design and development of its high energy Marx bank. The Marx bank consists of 32, 100 kV, 0.7 /spl mu/F capacitors switched by 16 Physics International T508 spark gaps. The Marx is switched into two parallel 7 nF, intermediate storage capacitors, which are fired into a dummy load through a fast multi-channeling output switch. The Marx stores 100 kJ and can deliver a voltage of 2 MV at 500 kA into a 4 /spl Omega/ load delivering 1 TW to the load. Initial testing will be of a multichanneling 2 MV output switch, which scales nicely to a 6 MV switch design for future very high energy machines at Sandia National Laboratories. The output switch is to reliably multichannel, or close with many parallel arc channels. The goal is to adapt an existing multichanneling switch to create a multichanneling output switch with significant operational advantages, including lower inductance, compared to existing multichannel switches. The target switch inductance is 100 nH or less. The facility and tank were assembled from January to June 2003, with testing to begin in July 2003. Simulations of the test stand and specifications of the output switch will be presented. Electrode configurations and switch augmentations that will facilitate a reliable multi-channeling switch will be introduced. Details describing the development of the MUTTS facility will be included.

Multichannel and Impedance Analysis of the Laser-Triggered Rimfire Gas Switch

Many pulsed power applications require multichannel switching because of stringent low erosion and low impedance requirements. Rimfire, which is a multigap multichanneling switch, has been implemented extensively for this purpose at Sandia National Laboratories for several decades, but with incomplete understanding. This paper presents a thorough experimental analysis of impedance and multichanneling characteristics for a multigap switch under the influence of a laser trigger and in an SF6 environment. The implications of these results on the future design of multichanneling multigap gas switches are also presented

Initial results from the University of Missouri terawatt test stand

The University of Missouri Terawatt Test Stand (MUTTS) is fitted with a 2.7 MV multichanneling laser triggered gas switch scaled from a 4 MV switch developed at Sandia National Laboratories. The long term goals of the research at MUTTS are to improve the multichanneling reliability and jitter of the switch when the electrode rings in the cascade section are either increased in number or in diameter. Multichanneling is important for two main reasons: 1) a reduction of electrode wear and 2) reduced inductance. The test facility provides a high voltage/high current test bed for a number of experiments that are easily scalable to large accelerators, such that, we expect the results to be directly applicable to the requirements of the Z and ZR accelerators. The first series of shots at MUTTS included dummy load and open load configurations to determine parasitic circuit elements. Diagnostics include monitoring the load current and Marx total current, voltage at the Marx output, load, and Marx trigger unit. Equivalent series resistance, series inductance, Marx capacitance, and shunt resistance were determined from these diagnostics. This is used to develop a first order circuit model of the energy storage section by comparison to experimental data. Characteristics of the scaled switch and initial pulsed power tests at the facility are presented

Electrical Effects of Multichanneling in the 2.5 MV Rimfire Gas Switch using a Laser Trigger

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.

Requirements for optimal performance and the consequences of using toroidal shaped electrodes in multichanneling switches

In many multi-channel switch designs, toroidal shaped electrodes are used to form the cascade breakdown section of a closing switch. Advantages of using toroidal electrodes include providing evenly graded electric fields from one gap to the next during charging and shaping the electric fields away from the mechanical connections that hold the electrodes in place. Often the inductance of arc channels is considered, but the inductance of electrodes must be considered when evaluating a multi-channeling switch. In typical geometries, electrode inductances are one to two orders of magnitude larger than of the inductance of the arcs themselves and may not be neglected. It is generally assumed that achieving more channels ad infinitum is desired to produce a low switching inductance. Calculations suggest this is not necessarily the case. Calculations also suggest that channel distribution, not simply the number of channels, strongly affect the operating inductance of the switch. For example, if one gap in the cascade section does not multi- channel, the bottleneck caused by this will dictate switch inductance, even if every other gap closes with many channels. Suggestions and supporting calculations are presented for minimizing arc inductance and electrode inductance of a toroidal arrangement. There must be the same number of channels from gap to gap to minimize the effects of arc inductance. The distribution of channels must be identical from gap to gap to minimize electrode inductance and azimuthal current flow on the electrodes with respect to the axial direction of the cascade section. Future work on optimizing fast, multi-channeling switch electrodes at the University of Missouri terawatt test stand (MUTTS) will be introduced.

Charge voltage, trigger voltage and gas dielectric effects on multi-channel closing of a Russian multi gap switch

The Z machine at Sandia National Laboratories is equipped with 30 spark gaps in each of its 36 Marx banks. It is crucial that each switch operate flawlessly to obtain the desired pulse and avoid pre fires. A Russian Multi Gap Switch (MGS) is being tested for several possible advantages over current technology. Air is the primary dielectric gas used in the MGS instead of SF/sub 6/, which is used in the existing spark gap system. It is triggered at low trigger voltages with a commercial trigger generator reducing wear on electrodes. Most interestingly, it is designed to produce multi-channels when fired, reducing the firing inductance and increasing pulse rise time. To analyze channel firing properties, the Russian MGS is fixed in single stage Marx, similar to the arrangement in the Z machine. Capacitor charge voltage and switch trigger voltage is varied over typical operating conditions to determine how these factors effect the closing of the MGS. Air, SF/sub 6/, SF/sub 6/-air, and SF/sub 6/-argon mixtures are used to study gas dielectric effects on MGS multi-channeling. The switchs opaque casing is replaced with clear casing so a very fast camera may document switch closing characteristics with an emphasis on multi-channel operation. The MGS is adapted so it may be opened with conventional tools. Results from all permutations of variables are obtained and correlated to multi-channeling of the switch. A complete description of the MGS is also presented.

Insulator Breakdown Tests Preceding a Study on Magnetic Flashover Inhibition

Improving the operating electrical stress level of insulator stacks is one goal for future pulsed power machines at Sandia National Laboratories [1]. This paper discusses experiments being conducted to understand the insulator vacuum flashover process under various conditions in order to improve insulator stress levels. Tests show flashover voltage to increase as risetime decreases. Initial shielded and unshielded cathode tests result in a lower flashover voltage with a shielded cathode. A flashover experiment was also designed with a flat electric to magnetic field ratio across the insulator surface for magnetic flashover inhibition tests.