Showing papers presented at "International Power Modulator Symposium in 1996"

Biofouling prevention with pulsed electric fields

Abstract: The clogging of pipes due to biofouling organisms capable of rapid population growth is a major problem in cooling systems where untreated lake, river or sea water is used. In estuarine and marine ecosystems, it is blue mussels and barnacles which cause biofouling problems; in freshwater ecosystems the uncontrolled growth of zebra mussels has significant economic implications. In order to explore the preventive effect of pulsed electric fields on biofoulants, the authors have, following laboratory studies, performed field tests with a pulsed power system which was to designed to provide /spl mu/s pulses of up to 40 kV into a low impedance (<10 /spl Omega/) load. The system was used to treat water from the Elizabeth River in Norfolk, USA. Two tests, each performed over a period of three weeks with pulsed electric fields of 12 kV/cm, and 6.4 kV/cm, respectively, in the treatment cell have demonstrated the feasibility of the electric field method for biofouling prevention. Whereas strong growth of barnacles, blue mussels and hydrazoans was seen in the control system, the test system was completely free of these biofoulants. The highest efficiency reached so far was 1,400 gallons of treated tidal water per kWh of electrical energy. It is expected that the efficiency can be further increased by optimizing the effect with respect to pulse duration and electric field, and by concentrating on fresh water biofoulants, such as zebra mussels.

New superfast power closing switched-dynistors on delayed ionization

Abstract: The fastest from known semiconductor electrically triggered power closing switches have turn on times of dozens of nanoseconds at /spl sim/1 kV blocking voltage and /spl sim/100 A switching current. Any attempt to increase the device voltage leads to an increase of the device thickness and an increase of turn-on times. Spark gaps, although faster, have severely limited lifetimes. The effect of delayed ionization has allowed for the development of a new kind of power superfast switch-silicon avalanche shapers (SAS)-in which time of flight limitation on speed has bean overcome. This approach has been used to develop new power superfast devices-fast ionization dynistors (FID)-which behave like a thyristor in that they have two steady states: nonconducting; and high-conducting. The fast (less than 1 ns) transition from the nonconducting to high-conducting state is induced by delayed ionization by the application of a short (nanosecond) high-voltage (kilovolts) pulse to the blocking p-n junction of the multilayered n/sup +/pnp/sup +/ semiconductor structure. Due to regenerative feedback, as in usual thyristors, the structure remains in the high-conducting state infinitely. To switch on the device, it is necessary to break the current by using an external circuit.

Solid state switch application for the LHC extraction kicker pulse generator

Abstract: A semiconductor solid state switch has been constructed and tested in the prototype extraction kicker pulse generator of CERN's Large Hadron Collider (LHC). The switch is made of 10 modified 4.5 kV, 66 mm symmetric GTOs (also called FHCT-fast high current thyristor), connected in series. It holds off a DC voltage of 30 kV and conducts a 5 /spl mu/s half-sine wave current of 20 kA with an initial di/dt of 10 kA//spl mu/s. Major advantages of the switch are the extremely low self-firing hazard, no power consumption during the ready-to-go status, instantaneous availability, simple condition control, very low noise emission during soft turn-on switching and easy maintenance. However, the inherent soft, relatively slow turn-on time is a nonnegligible part of the required rise time and this involves adaptation of generator components. A dynamic current range of 16 is achieved with variations in rise time, which stay within acceptable limits. Important generator improvements have been made with the series diodes and freewheel diodes. A more efficient droop compensation circuit is being studied. It is directly connected in series with the freewheel diode stack and maintains an acceptable flattop variation of 5% of the magnet current during 90 /spl mu/s. This paper presents the complete generator, in particular the solid state switch, and discusses related electrical measurements.

High gain GaAs photoconductive semiconductor switches for ground penetrating radar

Abstract: The ability of high gain GaAs photoconductive semiconductor switches (PCSS) to deliver high peak power, fast risetime pulses when triggered with small laser diode arrays makes them suitable for their use in radars that rely on fast impulses. This type of direct time domain radar is uniquely suited for observation of large structures underground because it can operate at low frequencies and at high average power. This paper summarizes the state-of-the-art in high gain GaAs switches and discusses their use in a radar transmitter. The authors also present a summary of an analysis of the effectiveness of different pulser geometries that result in transmitted pulses with varying frequency content. To this end, they developed a simple model that includes transmit and receive antenna response, attenuation and dispersion of the electromagnetic impulses by the soil and target cross-sections.

Pseudospark switch development for the LHC extraction kicker pulse generator

Abstract: CERN, the European Laboratory for Particle Physics, has started construction of the Large Hadron Collider (LHC), a superconducting accelerator that will collide protons at a center of mass energy of 14 TeV from the year 2005 onwards. The kicker magnet pulse generators of the LHC beam extraction system require fast high power switches. One possible type is the pseudospark switch (PSS) which has several advantages for this application. A PSS fulfilling most of the requirements has been developed in the past years. Two outstanding problems, prefiring at high operating voltages and sudden current interruptions (quenching) at low voltage could have been solved recently. Prefiring can be avoided for this special application by conditioning the switch at two times the nominal voltage after each power pulse. Quenching can be suppressed by choosing an appropriate electrode geometry and by mixing Krypton to the D/sub 2/ gas atmosphere. One remaining problem, related to the required large dynamic voltage range (1.7 kV to 30 kV), is under active investigation; steps in forward voltage during conduction, occurring at low operation voltage at irregular time instants and causing a pulse to pulse jitter of the peak current. This paper presents results of electrical measurements concerning prefiring and quenching and explains how these problems have been solved. Furthermore the plans to cure the forward voltage step problem are discussed.

Power nanosecond semiconductor opening plasma switches

Abstract: Opening switches (OS) with inductive storage system are very promising in pulsed power applications. The density of energy stored in an inductor is higher than in a capacitor. The pulsed voltage generated during a short time at the load may be many times (dozens) higher than the voltage at which the energy has been stored. In the early 1980s a new effect of super fast voltage restoration in high voltage silicon p-n junctions, when the junction current is switched from forward to reverse direction, was discovered. This discovery gave rise to a new generation of solid state plasma opening switches, called drift step recovery diodes (DSRD). Being semiconductor devices, DSRD have unlimited life time, low jitter. A maximum repetition rate is limited mainly by heat and may be as high as megahertz. In this work the performance of DSRD and their limits are considered.

Effects of pulse risetime and pulse width on the destruction of toluene and NO/sub x/ in a coaxial pulsed corona reactor

Abstract: The efficient abatement of hazardous air pollutants has become a major concern for many private and military installations. The semiconductor processing industry is being scrutinized for emission of many perfluorinated compounds (PFCs) and volatile organic compounds (VOCs). A major concern in the military is the effluent from paint-spray applications. Most of the air emission from the military installations are VOCs from the paint-spray booths and associated drying booths. Also, there is interest in the destruction of oxides of nitrogen (NO/sub x/) from both stationary and mobile diesel engines. The pulsed corona reactor (PCR) is a nonthermal plasma discharge device which has been used to efficiently convert a variety of hazardous air pollutants into byproducts that can be filtered by standard technology. In this work, the authors show the effects of pulse parameters (pulsewidth and risetime) on the destruction of toluene and NO/sub x/ using a PCR.

Optically-activated GaAs switches for compact accelerators and short pulse sensors

Abstract: This paper describes research and development of high gain GaAs photoconductive semiconductor switches (PCSS) for two very different types of applications: compact, repetitive accelerators and short pulse, active optical sensors. The accelerator is being tested with a spark gap driven modulator. It is a short pulse, linear induction accelerator (LIA) with an electron diode. Its design goals are: 700 kV, 7 kA, 30 ns pulses at 50 Hz. After characterizing the accelerator with the spark gap modulator, it will be tested with a GaAs PCSS modulator, which is under construction. Forty-eight, 2 inch diameter PCSS will switch 70 kA in a 250 kV coaxial Blumlein to deliver 220 kV, 35 kA, 30 ns pulses to the LIA. One module (1/8/sup th/) of the PCSS modulator is being tested. Results from these tests and projections for the complete system are discussed. The short pulse sensors are for military and commercial applications in optical and electrical range sensing, 3D laser radar, and high speed photography. The highest optical power produced with PCSS-driven laser diode arrays is presently 50 kW in 75 ps wide pulses or 12 kW in 1 ns wide pulses. A variety of sizes of GaAs PCSS are being tested around voltage and current specifications of several applications. Voltages range from 2 to 100 kV, currents range from 10 to 500 A, and electrical pulse lengths range from 1 to 50 ns. This paper discusses developmental issues of GaAs PCSS, which are common to all: fundamental research in high gain GaAs, device longevity, optical triggering, circuit configuration, and switch performance.

A compact lightweight 125 mm thyristor for pulse power applications

Abstract: A compact, lightweight, symmetric 125 mm diameter silicon thyristor is under development by SPCO for high current millisecond pulse power applications, i.e. electric launchers. This thyristor has a drastically reduced size and weight. This is due to a new design which eliminates the need for large, heavy refractory metal substrates and endpieces, employs an alloyed anode interface for high surge currents, and a high integrity plastic disk package. The package is less than one-fifth the volume and weight of a package utilizing standard construction technology, and provides for double sided cooling and a lower thermal resistance from junction to sink of 0.0033 K/W. Exploratory testing has been performed at ARL using a 10 milliohm, 0.5 ms (FWHM) PFN. With a matched load the pulser at 4 kV can provide peak currents of 200 kA with a di/dt in excess of 1 kA//spl mu/s. Initial results on the temporal behaviour as a function of operating levels under matched load conditions are presented. A computer simulation of the behaviour of the device using the actual current pulse shape was consistent with the experimental results. The present six star gate design does not provide the rate of plasma spreading required for 200 kA operation. A computer model using an involute gate structure indicates that the peak current objectives are achievable. The development of this switch will permit the construction of compact, lightweight high voltage series switch stacks for electric guns.

A fault control system using solid state circuit breakers and high temperature superconducting fault current limiters

Abstract: High voltage power systems and loads can suffer from extreme peak currents during load fault conditions. Because conventional circuit breakers require several cycles and many milliseconds to open, the large amount of energy deposited into the load during this fault can be destructive to the load as well as disruptive to the power system. A new method of combining a high temperature superconducting fault current limiter in conjunction with a solid-state circuit breaker has been developed to reduce the potential damage of a fault by limiting fault power and energy.

Stacked Blumlein pulse generators

Abstract: Extensive characterization of stacked Blumlein pulsers has been performed over the past few years. Results indicate that these devices are capable of producing high-power waveforms with risetimes and repetition rates in the range of 0.2-50 ns and 1-300 Hz, respectively, using a conventional thyratron, spark gap or photoconductive switch. This paper reviews the progress made to date in the development of these novel pulsers. It is shown that, with slight design modifications, they can produce pulsed power waveforms with fast risetimes and a wide range of pulse durations and peak values. An analysis of the behavior of the stacked Blumlein pulsers is given and the results of performance tests are discussed.

Investigation of power MOSFET switching at cryogenic temperatures

Abstract: Operation of power MOSFETs (metal oxide semiconductor field effect transistor) at cryogenic temperatures significantly reduces conduction losses and increases power handling capability. High voltage (1000 V+) devices exhibit the largest reduction of conduction losses. The breakdown voltage of the devices is reduced by about 20% when cooled from 75/spl deg/F down to -319/spl deg/F.

Space power technology

Abstract: The trends today are toward smaller satellites with increased capability. These two trends place enormous demands on the technology used to power such space systems. Typical power sources are nuclear, photovoltaic, and chemical. As the power available on a typical satellite decreases, the use of long pulsed power sources becomes more attractive. Technologies such as electrochemical capacitors, pulsed batteries and flywheels offer the possibility of employing high power subsystems for limited times without undue burden on the spacecraft power train. The state of the art in each of the pulsed storage devices is discussed and preliminary experiments described which use an electrochemical power source to power a spacecraft propulsion unit such as an arc-jet or Hall effect thruster.

Progress towards a 20 kV, 2 kA plasma source, ion implantation modulator for automotive production of diamond film on aluminum

Abstract: This paper provides the process requirements and the electrical design topology being developed to facilitate large scale production of amorphous diamond films on aluminum. The patented recipe, that includes other surface modification processes, requires various operational voltages, duty cycles and current load regimes to ensure a high quality film. It is desirable to utilize a common modulator design for this relatively "low voltage" recipe. Processing may include target part cleaning, ion implantation, plasma deposition and vacuum chamber cleaning. Modulator performance will have a direct impact on plant size and system economics. Unfortunately, process requirements are in a regime that is not easily achievable by solid state or very efficiently by vacuum tube devices.

A novel high current rate SCR for pulse power applications

Abstract: Novel high-power thyristors with a blocking capability of up to 6 kV and excellent turn-on behaviour are presented. These devices have been investigated as closing switches for very short current pulses as they occur, e.g., with laser applications for very short pulse durations, or with electric launchers for longer pulses at very high di/dt. The maximum current capability of the devices is of the order of a few hundred kiloamps, and the current rise rate can go up to several ten kA//spl mu/s. The triggering circuit is arranged directly around the thyristor in order to minimize the parasitic inductance in the gate-cathode loop and to optimize the compactness of the whole switch. In this way a very high gate current can instantaneously be applied, and the device turns on very fast. This is essential for a series connection of devices as well as for a minimization of the turn-on losses. The power needed for the triggering circuit can be supplied by external sources or can be extracted from the main pulse power circuit. The combination of the above high-power thyristor with the low-inductance triggering circuit forms an ideal switch that can be used in a wide range of pulse power applications.

A two-stage opening switch for inductive energy storage systems

Abstract: A two-stage opening switch comprises of a vacuum circuit breaker (VS) as a first stage and a high voltage fuse (HVF) in series with an SCR as a second stage. The switch offers low resistance of 20 /spl mu//spl Omega/ during charge intervals of several hundred milliseconds, controlled time to opening, minimal fuse size and a relatively fast opening of 0.25-0.7 ms. It serves as a closing switch as well. In a series of experiments, the current of 30-40 kA was commutated routinely from a 130 /spl mu/H inductor into the resistive load at a voltage of 3 kV. Various quenching media were examined. The liquids were found to yield maximum inductively generated voltage and transfer efficiency. A strong correlation exists between the fuse performance and the pressure generated in the fuse. It was found that a two-three millisecond HVF conduction time suffices for the recovery of the VS. A PSpice circuit simulation using an action-dependent fuse model gave good agreement with the experimental results.

Silicon carbide power devices for high temperature, high power density switching applications

Abstract: Silicon carbide (SiC) has been attracting much attention because of its potential application to high performance power devices with the capability of operating at high temperature. Here, the authors briefly describe SiC's advantages for high pulsed power devices, review the state-of the-art in prototype SiC power device performance and present simulation results and thermal calculations which predict the capability of SiC thyristors in high-power pulse applications.

A 100 kV, 1 kHz triggered pulse generator

Abstract: The performance of a repetitive pulse generator system is described. The generator is capable of operation under triggered, untriggered and single shot conditions. When operated in a triggered mode, the generator produces output voltages of up to 100 kV, at a pulse repetition frequency (PRF) which is fully controllable over the range 1 Hz to 1.2 kHz. In an untriggered mode, the PRF can be increased to 5 kHz.

Capacitors-past, present, and future: a trans-national perspective

Abstract: Over the last decade, significant increases in capacitor reliability have been achieved through a combination of advanced manufacturing techniques, new materials, and diagnostic methodologies to provide requisite life-cycle reliability for high energy pulse applications. Innovations in analysis of aging, including dimensional analysis, are introduced for predicting component performance and fault tolerance.

All-solid-state exciter for high-power, high-repetition-rate excimer laser

Abstract: An all-solid-state exciter has been developed for a high-power, high-repetition-rate XeCl excimer laser. It is composed of a semiconductor switch using MAGTs (MOS assisted gate-triggered thyristors) and a single-stage magnetic pulse compression circuit (MPC). The semiconductor switch, comprising thirty-two series and two parallel MAGTs, generated a 25 kV, 2 kA, 0.8 /spl mu/s pulse, which was compressed to a 200 ns pulse by the MPC. Continuous operation with an average laser power of 260 W at a repetition rate of 2 kHz was verified by applying the exciter to a XeCl excimer laser. A switch efficiency of 77% was achieved at 2 kHz operation for a switching energy of 11.3 J. Furthermore, 440 mJ, 220 W at 500 Hz was obtained by increasing the switching energy (=33.8 J) and discharge volume. Experimental results verified that this all-solid-state exciter can replace thyratrons.

Early-time turn-on characteristics of a high current thyristor

Abstract: Studies and experiments aimed at increasing the turn-on speed of high current thyristors have been carried out to meet an expanding need in the pulsed power field for switches with high peak power handling capacity and high pulse repetition rate capability. The present tests were conducted to measure the early-time turn-on characteristics of ABB 2003-45A02 thyristors and to determine whether special triggering techniques could raise the safe turn-on rate to the 100000 A//spl mu/S range which would qualify it for applications such as direct drive accelerator circuits. Results were encouraging. With these devices and special triggering techniques a maximum turn-on rate of 65373 A//spl mu/s and a peak current of 19040 A were recorded at a pulse repetition rate of 5 Hz.

Demonstration of a modular, repetitive-fired 1/4-MJ PFN for ETC guns

Abstract: A compact pulse forming network (PFN) storing 1/4 MJ at 16 kV has been assembled and tested to validate its performance as a driver for an electrothermal-chemical (ETC) gun. The PFN features a triggered vacuum gap output switch; shielded inductors at energy density near 10 MJ/m/sup 3/, with low external magnetic fields; 2.5 MJ/m/sup 3/ metallized electrode, PVDF dielectric capacitors; and a computer-based controller. The PFN was repetitively fired at 1/3 Hz in 5-shot bursts. Variability of the output energy over the burst was less than 2%. Energy density for the overall PFN module was 1.2 MJ/m/sup 3/ and the energy transfer efficiency predicted for a 12-module PFN system to an ETC gun-like load was 85%.

A transportable 500 kV, high average power modulator with pulse length adjustable from 100 ns to 500 ns

Abstract: Physics International has developed and demonstrated a repetitive pulsed power system capable of producing up to 500 kV into a 50 /spl Omega/ load at low repetition rates and to 400 kV at 100 Hz. This transportable system has demonstrated 100 Hz operation in 10-second bursts. Prime power is supplied by a diesel generator. The pulsed power system consists of a two-stage thyratron-switched modulator, which pulse-charges an eleven-section ladder-network PFN through a step-up transformer (produced by Stangenes Industries). The number of active PFN sections can be selected to allow adjustment of the output pulse duration from 100 ns to 500 ns in 50 ns increments. Section capacitors are water-dielectric coaxial capacitors. Section inductors are oil-insulated coils. The output switch is an air-insulated 1 MV spark gap (produced by PI) with UV illumination to stabilize the self-break voltage. A blow-down air system is used to cool the spark gap and assist in recovery for 100 Hz operation.

Current issues in high power microwaves

Abstract: Technology developments in the field of high power microwaves is focused principally in three areas: narrowband sources; ultra-wideband sources; and antennas. This paper reviews the status of activity in each area and discusses challenges for power modulators and pulsers that derive from the work presently in progress. Basic research efforts are presently attempting to overcome barriers to further progress in the use of both narrowband and ultra-wideband technologies. Several classes of narrowband tubes are discussed and the course of present basic research efforts that bear upon them is summarized. Success in advancing the art in the areas discussed has the potential for introducing new commercial and military products for applications as diverse as advanced radars, welding, and waste treatment.

Synchronization of multiple magnetically switched modules to power linear induction adder accelerators

Abstract: In applications where multiple magnetic modulators are used to drive a single linear induction voltage adder (LIVA) or linear accelerator (LINAC), it is essential that the outputs of the modulators be synchronized. Output rise times are typically in the 10 ns to 20 ns range, often making it necessary to synchronize to within less than 1 ns. Microprocessor and electronic feedback schemes have been developed and demonstrated that achieve the required level of synchronization, however, they are sophisticated and potentially complex. In a quest for simplicity, this work seeks to determine the achievable level of modulator to modulator timing jitter that can be obtained with simple design practices and passive techniques. Sources of output pulse time jitter in magnetic modulators are reviewed and some basic modulator design principles that can be used to minimize the intrinsic time jitter between modulators are discussed. A novel technique for passive synchronization is presented.

The performance of a simple PFN Marx generator

Abstract: The performance of a 5-stage pulse forming network (PFN) Marx generator is described. Each stage of the generator was constructed from HV co-axial cable instead of discrete inductors and kV capacitors. The output voltage pulse from the generator is rectangular in profile, with a risetime of /spl sim/30 ns and a duration of /spl sim/100 ns. The maximum output voltage is 200 kV and the output impedance is 125 /spl Omega/. Due to the relatively small amount of energy stored in the device (/spl sim/10J), it may be operated at pulse repetition frequencies of /spl sim/100 Hz.

Corona processing of insulating oil

Abstract: It is well known that sustained corona discharge in insulating oil lowers its dielectric strength and simultaneously reduces its corona resistance. Therefore, for operating stresses in the corona regime, activity typically increases with time and, if allowed to continue, eventually leads to breakdown of the oil and failure of the component or system. It is, therefore, common practice to periodically replace oil in devices such as large power transformers and switchgear before breakdown occurs. Sealed components such as capacitors are typically replaced. Experiments have demonstrated that the dielectric properties of corona weakened oil can not only be restored, but actually improved by a simple regeneration process. These experiments were carried out on high voltage pulse transformer windings which were operated at high repetition rates until partial discharges formed. Reprocessing the oil after each operating cycle resulted in successively longer operational periods before partial discharges appeared. In a separate experiment, a process was developed to precondition transformer oil to raise its corona inception voltage before using it to insulate a high voltage component, thus giving it a longer initial service life for a given operating stress or permitting higher stress operation for limited operating times.

Power fast modulator thyristors

Abstract: Fast power silicon thyristors are very promising for power modulators, but they have some drawbacks preventing their wide use. The main drawbacks are: instability of uniform current distribution which could lead to current localization (filamentation of current) and to burnout of the device under high currents; turn on time depends on the thickness of base layers and as well on maximum voltage. High voltage thyristors have longer turn on times. So to turn on high voltage during short time it is necessary to connect many low voltage fast devices in series. In this case the main question, which should be answered is on how to synchronize all thyristors. The objective of the paper is to show how to overcome these drawbacks.

Characteristics of pseudospark switches for pulsed power modulators

Abstract: Pseudospark switches (PSS) are considered to be an alternative not only to thyratrons or medium-power spark gaps but also to ignitrons. A lifetime of more than 10/sup 8/ discharges with a corresponding total charge transfer of 600 kC was achieved in laser circuits. Electrode erosion is still the main limiting factor to make high power PSS a serious competitor to commercially available switches. One approach to solve this problem is to distribute the total current on several discharge channels with a common hollow cathode. The other one is to look for suitable electrode materials or compounds of them. In order to distribute the discharge homogeneously over more than one single channel synchronous triggering of all channels is indispensable. The trigger method used is a pulsed glow discharge. The characteristics of the trigger discharge as a function of delay and jitter was investigated in combination with actual triggering of all channels. After successful triggering of all channels the corresponding current filaments try to pinch by self-magnetic fields. For long current pulses (i.e. more than 1 or 2 /spl mu/s for a 80 kA peak current) a single plasma column develops in the symmetry axis of the device. Streak photographs allow determination of pinch time as a function of the peak current, the number of channels, the gap distance and so on. A better understanding of electrode phenomena is essential in order to improve PSSs to carry high peak currents and simultaneously allow high charge transfer. The critical current density at which electrode erosion dramatically rises is still an unknown parameter.

Prototype 2-stage magnetic pulse compression modulator for pulse power applications

Abstract: A magnetic pulse compression (MPC) scheme using a saturable reactor together with pulse capacitor is a useful technology for shaping input pulses into narrow output pulses of a much higher power level. Modulators with MPC circuits can meet the long life and high reliability requirements which are very important in industrial applications. A prototype modulator with a two-stage MPC switch (assembled with amorphous cores) is constructed to generate repetitive pulses of maximum 500 MW peak power with a pulse width (FWHM) of 500 nsec at a maximum pulse repetition rate of 300 Hz into a 24 /spl Omega/ dummy load. In this paper, the detailed system design and operational characteristics of the prototype MPC modulator are presented.