G. Kirkman

High-power pseudospark and BLT switches

A review is presented of recent developments in a new group of high-power hollow-electrode switches, including the pseudospark and the backlighted thyratron (BLT). Experiments demonstrate that for several key high-power switching performance factors, the pseudospark and BLT switches are superior to either high-pressure spark gap switches or thyratrons or, in some cases, both. High performance has been demonstrated in peak current (>100 kA), current rate of rise (>10/sup 12/ A/s), switching precision, trigger efficiency, current reversal (100%), and recovery time. Several electrical and optical trigger methods have been demonstrated and are described. >

Low pressure, light initiated, glow discharge switch for high power applications

A low pressure glow discharge switch that has a number of features that are desirable for high power applications is described. The switch has achieved high stand‐off voltage and peak current, has potential for very fast current rate of rise, and operates near the glow‐to‐arc transition in hydrogen or helium. Closure is initiated by light incident on the back of the cathode, and it is observed that surface damage within the area of illumination is less than surrounding areas. Several aspects of the switch are similar to thyratrons and pseudosparks.

An analysis of the anomalous high-current cathode emission in pseudospark and back-of-the-cathode lighted thyratron switches

An analysis of the anomalously large cathode emission recently observed in the superdense glow of pseudospark and back‐lighted thyratrons is presented. These switches are low‐pressure (27 Pa H2) glow‐discharge pulsed‐power devices. After operating at peak discharge currents of 6–8 kA and pulse durations of 0.5–1 μs, the surface surrounding the cathode hole was found to have been homogeneously melted within a radius of ≊4 mm indicating that the discharge is a superdense glow discharge, not an arc, with a cross‐sectional area on the order of 1 cm2. This conclusion is also supported by streak camera measurements. The current density at the cathode surface under these conditions is 5–10 kA/cm2, several orders of magnitude larger than that of thermionic cathodes in common thyratrons. This high‐current density is explained by intense cathode heating from a high‐current density ion ‘‘beam’’ produced in the cathode fall during the initial stage of current buildup. The surface heating resulting from this ‘‘beam’’ ...

Flash‐lamp‐triggered high‐power thyratron‐type switch

Extremely high cold cathode emission and a plasma current density that is much higher than commercial glow discharge hot cathode switches have been obtained in a flash‐triggered hollow cathode thyratron‐type switch that operates in a glow mode. The cold cathode emission is higher by several orders of magnitude than emission from thermionic cathodes in high‐power hydrogen thyratrons, without arc formation. Peak current >17 kA, dI/dt>3×1011 A/s, and power gain >1400 are observed. The switch has been successfully operated as a thyratron substitute in a commercial XeCl excimer laser. The work demonstrates a new approach to thyratron‐type switches, and a wide range of technologically useful physical processes that are poorly understood.

Evidence for large-area superemission into a high-current glow discharge

This letter presents evidence for large‐area (≊1 cm2) cathode superemission (∼10 000 A/cm2) into a high‐current glow discharge in a pseudospark or back lighted thyratron switch. Cathodes studied with a scannning electron microscope following operation at 6–8 kA, ≊1 μs pulse length, and 105 pulses in a low‐pressure H2 discharge show evidence of melting of a thin surface layer within a radius of ∼4 mm, indicating that the discharge is a superdense glow with a cross‐sectional area of the order of 1 cm2, rather than an arc. Further supporting evidence is provided by streak camera data. An ion beam present during the avalanche phase of the discharge is responsible for heating the cathode surface resulting in a significant field‐enhanced thermionic emission.

Fiber-optic-triggered high-power low-pressure glow discharge switches

The development of a novel version of an optically triggered low-pressure high-power glow discharge switch is reported. Characteristic of these type of switches are high-voltage hold-off, high peak current capability, excellent current rise rate, low delay and jitter, and a simple device geometry. This method of triggering guides a UV light pulse through a fiber-optic cable to the aperture in the cathode electrode. Photoelectrons are produced and quickly avalanche to initiate the discharge, and a glow discharge is formed. Jitter and delay of 0.4 ns half-width half-maximum (HWHM) and 78 ns, respectively, have been obtained and reliable triggering has been demonstrated with energies as low as 10 mu J of UV light. This device is considered suitable for scaling switches to very high current and voltages by simultaneously triggering multiple gaps in parallel or series. >

Current quenching in the pseudospark

A study of current quenching in the pseudospark is presented. Current quenching during the conductive phase limits peak conduction current in certain thyratron switches. The quenching phenomenon also occurs in the pseudospark. However, a remarkable feature is that current quenching is observed only below currents of ≊ 2–3 kA, near the onset of superemissive behavior, and thus in the pseudospark is not an upper limit to high current operation. A mechanism involving an instability caused by ion depletion at the plasma boundary is discussed.

A super-emissive self-heated cathode for high-power applications

A superemissive cathode for high-power glow-discharge applications, such as high-current thyratrons, based on a self-heated thermionic emission mechanism is reported, and the mechanism is discussed. The results suggest that high-brightness cathode design for glow-discharge switches can be significantly improved. By tailoring the area of ion-beam heating, it should be possible to optimize operating conditions according to pulse length, current risetime, voltage falltime, peak current and electrode material. The superemission characteristics which are approximately=2 orders of magnitude larger than typical thermionic emitters, suggest that there are a number of important applications. >

An optically triggered, glow switch Marx bank

Operation of high-powers lasers, electron and other accelerators, and other devices is often limited by power modulator performance. Results of an investigation of a concept for modulator switching that has the potential to significantly extend these limitations presented. The switching device is a three-stage Marx bank based on back-lighted thyratrons that are triggered using optical fibers. Potential advantages include optical isolation of triggering, operation of the switches in a glow mode, no external cathode heater, reduced volume and supporting systems, and high-precision (subnanosecond) triggering. A three-stage proof-of-principle Marx bank has been operated at 105 kV with a rise time of >

High Power Evaluation of Hermetically Sealed BLT Switches

This paper reports the evaluation of hermetically sealed BLT switches operating at high current and high repetition rates in continuous operation. A single gap BLT-250-T has been operated at 25kV, 12kA, 120Hz with a 160nsec pulse length for 8x107 shots and 160,OOOC total charge transfer. This result is more than an order of magnitude longer life than could be expected from a hermetically sealed spark gap operating under the same conditions. A two gap BLT-250-2T has been operated at 104kV, 26kA and 1OHz. Performance of these switches in various test circuits and an analysis of electrode erosion is presented.