R.D. Speer

An ultra-compact Marx-type high-voltage generator

This paper discusses the design of an ultra-compact, Marx-type, high-voltage generator. This system incorporates high-performance components that are closely coupled and integrated into an extremely compact assembly. Low profile, custom ceramic capacitors with coplanar extended electrodes provide primary energy storage. Low-inductance, spark-gap switches incorporate miniature gas cavities imbedded within the central region of the annular shaped capacitors, with very thin dielectric sections separating the energy storage capacitors. Carefully shaped electrodes and insulator surfaces are used throughout to minimize field enhancements, reduce fields at triple-point regions, and enable operation at stress levels closer to the intrinsic breakdown limits of the dielectric materials. Specially shaped resistors and inductors are used for charging and isolation during operation. Forward-coupling ceramic capacitors are connected across successive switch-capacitor-switch stages to assist in switching. Pressurized SF/sub 6/ gas is used for electrical insulation in the spark-gap switches and throughout the unit. The pressure housing is constructed entirely of dielectric materials, with segments that interlock with the low-profile switch bodies to provide an integrated support structure for all of the components. This ultra-compact Marx generator employs a modular design that can be sized as needed for a particular application. Units have been assembled with 4, 10, and 30 stages and operated at levels up to 100 kV per stage.

A low-profile high-voltage compact gas switch

This paper discusses the development and testing of a low-profile, high-voltage, spark-gap switch designed to be closely coupled with other components into an integrated high-energy pulsed-power source. The switch is designed to operate at 100 kV using SF/sub 6/ gas pressurized to less than 0.7 MPa. The volume of the switch cavity region is less than 1.5 cm/sup 3/, and the field stress along the gas-dielectric interface is as high as 130 kV/cm. The dielectric switch body has a low profile that is only 1-cm tall at its greatest extent and nominally 2-mm thick over most of its area. Field modeling was done to determine the appropriate shape for the highly stressed insulator and electrodes, and special manufacturing techniques were employed to mitigate the usual mechanisms that induce breakdown and failure in solid dielectrics. Static breakdown tests verified that the switch operates satisfactorily at 100 kV levels. The unit has been characterized with different shaped electrodes. Capacitor discharge tests in a low inductance test fixture exhibited peak currents up to 25 kA with characteristic frequencies of the ringdown circuit ranging from 10 to 20 MHz. The ringdown waveforms and scaling of measured parameters agree well with circuit modeling of the switch and test fixture. Repetitive operation has been demonstrated at moderate rep-rates up to 15 Hz, limited by the power supply being used. Preliminary tests to evaluate lifetime of the compact switch assembly have been encouraging.

Analysis of a Small Loop Antenna With Inductive Coupling to Nearby Loops

This paper analyzes the inductive coupling that occurs when a loop antenna is near other conductive objects that form complete loops and are excited by incident low-frequency magnetic fields. The currents developed on the closed loops from the time changing magnetic fields generate their own magnetic fields that alter the voltage received by nearby open loop antennas. We will demonstrate how inductance theory can be used to model the system of loops. Using this theory, time domain circuit models are developed to find the open circuit voltage of a loop near one closed loop and for the open circuit voltage of one loop near two closed loops. We will show that the model is in good agreement with measurements that have been made in a TEM cell. One important application of this work is for electroexplosive device safety. It is necessary to ensure that if lightning strikes a facility that the electromagnetic fields generated inside do not have strong enough coupling to a detonator cable to cause initiation of explosives. We will show how the model can be used to analyze magnetic field coupling into a detonator cable attached to explosives in one typical type of work stand.

Modular High Current Test Facility at LLNL

This paper describes the 1 MA, 225 kJ test facility in operation at Lawrence Livermore National Laboratory (LLNL). The capacitor bank is constructed from three parallel 1.5 mF modules. The modules are capable of switching simultaneously or sequentially via solid dielectric puncture switches. The bank nominally operates up to 10 kV and reaches peak current with all three cabled modules in approximately 30degs. Parallel output plates from the bank allow for cable or busbar interfacing to the load. This versatile bank is currently in use for code validation experiments, railgun related activities, switch testing, and diagnostic development.

An elegant impulser developed for flat beam injection

The following report describes the design, construction, and checkout of a high-voltage (HV) impulser built for the heavy ion fusion (HIF) project. The purpose of this impulser is to provide an adjustable diode voltage source of sufficient quality and level to allow the optimization of beam transport and accelerator sections of HIF. An elegant, low-impedance, high-energy storage capacitor circuit has been selected for this application. Circuit parameters of the retrofit to the diode region have been included to provide the controlled rise time. The critical part of this circuit that is common to all candidates is the impedance matching component. The following report provides a description of the implemented circuit, the basic circuit variables for wave shaping, screening techniques revealing the weakest circuit component, and the resulting output of the injector.

Analysis of conductor impedances accounting for skin effect and nonlinear permeability

It is often necessary to protect sensitive electrical equipment from pulsed electric and magnetic fields. To accomplish this electromagnetic shielding structures similar to Faraday Cages are often implemented. If the equipment is inside a facility that has been reinforced with rebar, the rebar can be used as part of a lighting protection system. Unfortunately, such shields are not perfect and allow electromagnetic fields to be created inside due to discontinuities in the structure, penetrations, and finite conductivity of the shield. In order to perform an analysis of such a structure it is important to first determine the effect of the finite impedance of the conductors used in the shield. In this paper we will discuss the impedances of different cylindrical conductors in the time domain. For a time varying pulse the currents created in the conductor will have different spectral components, which will affect the current density due to skin effects. Many construction materials use iron and different types of steels that have a nonlinear permeability. The nonlinear material can have an effect on the impedance of the conductor depending on the B-H curve. Although closed form solutions exist for the impedances of cylindrical conductors made of linear materials, computational techniques are needed for nonlinear materials. Simulations of such impedances are often technically challenging due to the need for a computational mesh to be able to resolve the skin depths for the different spectral components in the pulse. The results of such simulations in the time domain will be shown and used to determine the impedances of cylindrical conductors for lightning current pulses that have low frequency content.

Faraday rotation data analysis with least-squares elliptical fitting

A method of analyzing Faraday rotation data from pulsed magnetic field measurements is described. The method uses direct least-squares elliptical fitting to measured data. The least-squares fit conic parameters are used to rotate, translate, and rescale the measured data. Interpretation of the transformed data provides improved accuracy and time-resolution characteristics compared with many existing methods of analyzing Faraday rotation data. The method is especially useful when linear birefringence is present at the input or output of the sensing medium, or when the relative angle of the polarizers used in analysis is not aligned with precision; under these circumstances the method is shown to return the analytically correct input signal. The method may be pertinent to other applications where analysis of Lissajous figures is required, such as the velocity interferometer system for any reflector (VISAR) diagnostics. The entire algorithm is fully automated and requires no user interaction. An example of algorithm execution is shown, using data from a fiber-based Faraday rotation sensor on a capacitive discharge experiment.

Understanding high voltage vacuum insulators for microsecond pulses

High voltage insulation is one of the main areas of pulsed power research and development since the surface of an insulator exposed to vacuum can fail electrically at an applied field more than an order or magnitude below the bulk dielectric strength of the insulator. This is troublesome for applications where high voltage conditioning of the insulator and electrodes is not practical and where relatively long pulses, on the order of several microseconds, are required. Here we give a summary of our approach to modeling and simulation efforts and experimental investigations for understanding flashover mechanism. The computational work is comprised of both filed and particle-in-cell modeling with state-of-the-art commercial codes. Experiments were performed in using an available 100-kV, 10-μs pulse generator and vacuum chamber. The initial experiments were done with polyethylene insulator material in the shape of a truncated cone cut at +45° angle between flat electrodes with a gap of 1.0 cm. The insulator was sized so there were no flashovers or breakdowns under nominal operating conditions. Insulator flashover or gap closure was induced by introducing a plasma source, a tuft of velvet, in proximity to the insulator or electrode.

Measuring helical FCG voltage with an electric field antenna

A method of measuring the voltage produced by a helical explosive flux compression generator using a remote electric field antenna is described in detail. The diagnostic has been successfully implemented on several experiments. Measured data from the diagnostic compare favorably with voltages predicted using the code CAGEN [1], validating our predictive modeling tools. The measured data is important to understanding generator performance, and is measured with a low-risk, minimally intrusive approach.

Bonded penetration analysis for a severe lightning strike to a facility

Lightning strikes pose a serious threat to facilities and their subsystems. If a facility takes a direct strike, large amounts of pulsed electromagnetic (EM) energy can radiate into the interior of the facility. This energy can couple into electronic systems causing failures. Often, proper shielding of the facility can reduce the radiated energy by an order of magnitude. In an attempt to reduce pulsed EM energy, facilities are built to resemble a Faraday cage. However, most facilities have several imperfections which limit the effectiveness of their shielding capabilities. Penetrations into the facility are a type of imperfection that allows EM fields to be produced in the interior of the facility. Therefore, penetrations must be connected to the Faraday cage through bond wires to maintain the shields integrity and protect sensitive components. Finite element computer simulations have been performed to determine the effects of bonded penetrations, using 6 AWG bond wires. In an attempt to offer guidelines, which optimize the facilitys shielding effectiveness; several bond wire configurations have been investigated. Bond wire lengths, bond wire orientation, single and multiple bond wire configurations and varying the angle between bond wires have been investigated. Simulation results have shown that multiple bond wires result in greater than 40dB reduction of pulsed EM fields in the interior of the facility and a spacing of greater than 45° is optimum for bond wire spacing, for the simulated facility. In addition, the penetration current diverted by the bond wire was monitored. For severe direct lightning strikes, i.e. Ipeak=200 kA and dI/dt=400 kA/μs, the simulation suggest greater than 90% of the lightning current is diverted through the bond wire into the Faraday cage for the configurations examined. The high current nature of the severe lightning pulse produces large Lorentz forces on the bond wire. Laboratory experiments are being developed at the LLNL pulsed power lab to ensure that bond wires maintain proper connection when exposed to high currents, ensuring desired shielding throughout a direct strike.