One class of modern pulse power generators use deionized water as an energy storage, switching and transmission dielectric. Water is chosen for its high dielectric constant and relatively high resistivity, which allows reasonably sized and efficient low-impedance high-voltage pulse lines where pulse durations are less than 100 µs. Water/ethylene glycol mixtures are being researched, so that rotating machinery, rather than the usual Marx generator, can be used as the primary energy store. The high resistivity and high dielectric constant of these mixtures at low temperature permit low-loss operation on millisecond time scales. Simple design criteria linking load parameters and charging circuit characteristics to the liquid dielectric are developed which show that the dielectric constant, breakdown strength, and relaxation time are the primary properties of interest to the pulse power engineer. On time scales greater than 100 µs, injection of space charge, with density q and mobility µ, affects the charging and discharging circuit characteristics, introduces the time constant of the time of flight for injected charge to migrate between electrodes, and increases the effective ohmic conductivity σ to σ + qµ. A drift-dominated conduction model is used to describe measured space-charge effects. Kerr electrooptic field mapping measurements show strong space-charge effects with significant distortions in the electric field distribution a few hundred microseconds after high voltage is applied. The injected charge magnitude and sign depends on the electrode material. Thus by appropriate choice of electrode material combinations and voltage polarity, it is possible to have uncharged liquid, unipolar-charged negative or positive, or bipolar-charged liquid. An important case is that of bipolar injection, which has allowed up to a 40 percent higher applied voltage without breakdown than with no charge injection, and thus a doubling of stored energy due to the space-charge shielding which lowers the electric field strengths at both electrodes. Although injected space charge increases the stored electric energy over the capacitive space-charge-free energy, (1/2)CV2, more energy is required from a source during charging and the energy delivered to a resistive load is reduced because of internal dissipation in the capacitor as the charge is conducted to the electrodes. However, it appears that this extra dissipation due to injected charge can be made negligibly small and well worth the price if the space charge allows higher voltage operation for long charging time or repetitively operated machines.

Dielectric properties of water and water/ethylene glycol mixtures for use in pulsed power system design

A review of the state of the art in opening switches is presented. The general operating principles and present and potential future operating parameters for several switch categories are discussed. Among the switch categories described are: mechanical, solid state, vacuum arc, crossed field, fuse, explosive, plasma gun, superconducting, thermal, MHD instability, diffuse discharge, plasma erosion switches, and reflex triodes.

A review of opening switch technology for inductive energy storage

TITLE UV RADIATION TRIGGERED RAIL-GAP SWITCH INVENTIORSRoderick S. Taylor Kurt E. Leopold A. John Alcock ABSTRACT OF THE DISCLOSUREThe UV radiation triggered rail-gap switch applies a high volt-age from a low impedance source to a low impedance load. The switch includes first and second parallel elongated electrodes spaced to form a uniform gap along their length. The first electrode is adapted to be connected to the high voltage source, while the second electrode is adapted to be connected to the low impedance load. When a voltage from the source is applied to the electrodes, one electrode will be positive relative to the other electrode. In addition, the cross-section of each of the electrodes is sufficiently smooth to prevent points of high field concentration between the electrode. This cross-section is defined by the field enhancement factor of each electrode which is preferably less than 1.5. The switch also includes an enclosure in which the electrodes are located and which contains a gas mixture for maintaining the break-down threshold between the electrodes. A preferred gas mixture includes Ar, N2, and SF6 at a gas pressure selected to prevent breakdown in the switch until triggered by a UV radiation source. The W radiation source directs a beam of radiation substantially parallel to the pair of electrodes, preferably near the positive electrode, for initiating multi-channel, sub-nanosecond jitter, breakdown in the gap between the elec-trodes. The UV radiation may be obtained from either an incoherent radiation source or an UV laser source, but should be of short duration in the form of a narrow beam which is uniform along its cross-section.

Uv radiation triggered rail-gap switch

An oscillator-amplifier KrF discharge laser system capable of providing up to 400 mJ output with 0.1-cm-1linewidth in a 20-ns pulse is described. The output beam divergence is described and shown to be affected by the discharge current. The output beam is near the diffraction limit in the direction parallel to the discharge and about three times the diffraction limit transverse to the discharge. Operation with XeCl at up to 250 mJ with similar linewidth and spatial quality is also described.

An injection-locked unstable resonator rare-gas halide discharge laser of narrow linewidth and high spatial quality

By using a kinetic model developed for a self‐sustained discharge pumped XeCl laser (B→X, 308 nm) with the Ne/Xe/HCl mixture, we determined operational conditions of the XeCl laser with commonly used pulse width of about 100 ns for attaining efficient lasing. According to the analysis of electron kinetics in the laser discharge, the XeCl* formation, the XeCl* relaxation, and the absorption of the B→X laser radiation, it is found that the discharge and kinetic characteristics are affected predominantly by the electron number density which is determined mainly by the ratio of Xe to HCl concentrations. Although the XeCl* formation, the XeCl* collisional quenching, and the absorption of the laser radiation increase with increasing total gas pressure, the optimum Xe and HCl concentrations which are almost independent of the total gas pressure can be determined. Furthermore, since the laser output energy does not increase linearly with the excitation rate, the optimum excitation rate can be determined for a given total gas pressure as it shifts to a higher value with increasing total gas pressure. As a result of the analysis by using the most reliable rate constants and cross sections for the reactions published so far involved in the XeCl laser kinetics, the maximum intrinsic efficiency of 12.5% is found to be obtainable with a 3‐atm laser mixture of 0.2% HCl and 1% Xe in the Ne diluent pumped with an excitation rate of 3.5 MW/cm3.

Theoretical analysis of efficiency scaling laws for a self‐sustained discharge pumped XeCl laser

An optical filter is employed to transport a 15-ns light pulse from a high power ruby laser for precise triggering of a gas filed high voltage spark gap. The maximum power density that can be transmitted by the fiber is limited to 6 × 1012 W/m2 above which laser induced damage occurs on the fiber entrance face. The overall throughput efficiency of the optical system was measured as 62 percent. Results are presented for the switching delay time and associated jitter for various mixtures of A and N2 gas, and as a function of the voltage across a pulse-charged Blumlein generator gap. Pulse charging of the Blumlein generator was accomplished by a three-stage Marx generator, resulting in output voltages up to 250 kV. It was conclusively demonstrated that an optical fiber will transport a sufficiently intense laser pulse to evince subnanosecond jitter in the triggering of a pressurized gas switch under the conditions studied.

Laser Triggering through Fiber Optics of a Low Jitter Spark Gap

The performance of externally controlled, high‐pressure, diffuse discharges as switching elements in pulse power systems is strongly determined by the recombination and attachment processes in the fill gas. To obtain high control efficiency and fast response of the diffuse discharge switch the discharge must be attachment dominated with the attachment rate coefficient increasing with field strength. An electron‐beam controlled diffuse discharge system was constructed to study the behavior of pulsed discharges in the submicrosecond range in gas mixtures containing N2 as a buffer gas and small additives of electronegative gases. The results of experiments in N2 plus N2O were compared with values obtained with a Monte Carlo code and a rate equation calculation.

An electron‐beam controlled diffuse discharge switch

The dual channel triggering of a spark gap switch by fiberoptic transported ruby laser radiation is discussed. The spark gap is the output switch of a 20-ns water dielectric Blumlein generator. The Blumlein generator is pulse charged in approximately 250 ns by a three-stage Marx bank to 150 kV. The spark gap is operated at a pressure of 2540 torr with a mixture of Ar and N2 gas and an electrode separation of 2 cm. Two 1-mm diameter quartz optical fibers are used to transport 2 2-MW laser beams into the spark gap onto points 6 cm apart on the target electrode. The two beams are obtained by optical splitting of the output of a single laser. Under appropriate conditions, two arc channels are initiated by the laser beams along their paths. A small improvement in current rise time for dual channel events over single channel events is observed. Moreover, the number of successful dual channel events is observed to depend on the time of laser entry with reference to the beginning of the charging pulse, and not the gap polarity. The correlation of this behavior with the space charge build up in the slightly over-volted gap is discussed.

Space-Charge Effects in a Laser Fiber-Optics Triggered Multichannel Spark Gap

: Externally controlled diffuse discharge switches offer the potential of submicrosecond opening time and high repetition rate. For the investigation of an electron-beam controlled diffuse discharge, an e-beam gun was constructed which allows submicrosecond, multiple pulse operation. The 200 kV, 100 cm2, e-beam with thermionic cathode is designed as a triode. The control grid is pulsed to generate several successive e-beam pulses of 100 ns duration and variable pulse-to-pulse interval. Electrical and optical diagnostics are used to monitor the triode performance. Preliminary switch experiments in N2-02 gas mixtures are discussed.

H. C. Harjes

Papers

Laser Triggering through Fiber Optics of a Low Jitter Spark Gap

An electron‐beam controlled diffuse discharge switch

Space-Charge Effects in a Laser Fiber-Optics Triggered Multichannel Spark Gap

E-Beam Triode for Multiple Submicrosecond Pulse Operation