I. Petzenhauser

Excimer emission from microhollow cathode argon discharges

Microhollow cathode discharges (MHCDs) operated in rare gases are sources of intense excimer emission. Of particular interest is argon, because of its relatively low cost and the short wavelength (128 nm) of its excimer emission. The measured internal efficiency, obtained in static argon at atmospheric pressure, was found to be on the order of 1%. Flowing argon through a direct current (DC) MHCD at atmospheric pressure caused the argon excimer internal efficiency to increase to 6%, indicating that the low efficiency in static argon is mainly due to impurities. Applying 10 ns pulses to the DC plasma resulted in an increase in excimer power from 30 mW DC to 180 mW peak power, at an efficiency of 5–6%. The increase in excimer power correlates with an increase in the electron density. For DC operation, electron densities of 1015 cm−3 were measured in atmospheric pressure argon micro-plasmas, which increased to values beyond 1016 cm−3 for nanosecond pulsed operation. This increase in electron density and excimer power is due to pulsed electron heating, an effect that has allowed us to raise the mean electron energy from 1 eV, for DC operation, to 2.25 eV in the pulsed mode.

Switching characteristics of microplasmas in a planar electrode gap

Microplasmas at high pressure have been the authors’ special interest for its practical relevance to the development of a switch. They concentrated on repetitive switching with a possibility to exceed the up to now known values for plasma closing switches and simultaneously maintaining a subnanosecond rise time of the switched pulses at a load. They examined several parameters for this purpose such as the electrode gap spacing, the electrode geometry, the gas type, the gas pressure, and including the applied voltage and current rating to operate these plasmas.

Xenon excimer emission from pulsed high-pressure capillary microdischarges

Intense xenon vacuum ultraviolet (VUV) emission is observed from a high-pressure capillary cathode microdischarge in direct current operation, by superimposing a high-voltage pulse of 50 ns duration. Under stagnant gas conditions, the total VUV light intensity increases linearly with pressure from 400 to 1013 mbar for a fixed voltage pulse. At fixed pressure, however, the VUV light intensity increases superlinearly with voltage pulse height ranging from 0.8 to 2.8 kV. Gains in emission intensity are obtained by inducing gas flow through the capillary cathode, presumably because of excimer dimer survival due to gas cooling.

Multi-gap Pseudospark Switches for High Voltage Applications

The long-term goal of the described experiments is to construct multi-gap pseudospark switches, which can handle reliably the specifications of the pulse forming network (PFN) for the injection/extraction kicker magnet system of the future heavy ion synchrotron accelerator complex SIS100/300 at the Gesellschaft fuer Schwerionenforschung (GSI). The PFN requires an high-voltage switch, that can handle an hold-off voltage up to 70 kV, peak currents up to 6 kA and pulse durations up to 7 mus. In this paper most of the reported experimental data have been obtained by using a two-gap prototype pseudospark switch with total hold-off voltage up to 30 kV. There are two major problems correlated with the development of such multi-gap systems. The first one concerns efficient and reliable triggering of switch for the overall switch lifetime. Two trigger systems have been studied, the so-called high-dielectric trigger (HDT), which already has proven its long term capability, and second, a novel system, based upon the electron emission from carbon nanotubes (CNT). The CNT- trigger still suffers from an insufficient small lifetime of 104 discharges. The second critical part concerns the instant breakdown initiation in the individual gaps, separated by a drift space. In the experiments with this prototype an active pre-ionization of the drift space was nor foreseen. For both trigger systems the measured delay and jitter in the first gap vary from 50 to 60 ns and 15 to 30 ns, respectively; corresponding the second gap has a delay and a jitter of up to 200 and 45 ns, respectively.

Comparison between the ultraviolet emission from pulsed microhollow cathode discharges in xenon and argon

We measured the dynamic I–V characteristics and vacuum ultraviolet (VUV) emission lines of the second continuum in xenon (170 nm) and argon (130.5 nm) from pulsed microhollow cathode discharges (MHCD). For pulse lengths between 1 and 100 μs the dynamic I–V characteristics are similar in both inert gases. Only the time variation of the VUV emission line at 170 nm for xenon can be related to the dimer excited states. In argon the energy transfer between the Ar2* dimers and the oxygen impurity atoms is responsible for a qualitatively different time behavior of the resonance line at 130.5 nm. Consequently, the relative VUV efficiency reveals an inverse dependence on the electrical pulse lengths for the MHCD in xenon and argon, respectively.

Temporal structure of the fast electron beam generated in the pseudospark discharge with external triggering

This paper deals with the investigation of the fast electron beams that form in the high-current pseudospark discharge at different stages of its development. Three short-duration peak of the beam current at the discharge axis have been revealed. The physical reasons for the appearance of these peaks of the beam are discussed. The main idea of the proposed physical mechanism is that the electrons are accelerated in a double electric layer, which forms between the hollow cathode plasma and the near-anode plasma.

Fundamental investigation in two flashover-based trigger methods for low-pressure gas discharge switches

Modern switches for pulse-power technology have special requirements such as long lifetime, reliability in a wide pressure and voltage range, as well as small delay time. In order to meet these requirements, two trigger methods were developed and examined. These two different trigger methods based on a flashover were tested for the emission behavior by variation of different parameters. The first configuration is a semiconductor surface flashover trigger, where electron emission is based on a surface flashover between the contact area of a copper spring and a carbide cylinder. The second trigger concept is the high-dielectric trigger, where electrons are released by the field emission effect at the transition between metal-vacuum and dielectric. For this system, high dielectric materials with dielectric constants in the order of 2000 are available. The electrical and optical measurements of both trigger systems were done in a modular structured vacuum chamber. For lower pressure, the high-dielectric trigger shows better performances and higher emitted charge of the electron emission within all adjusted parameters like gas pressure, applied voltage, and different wirings. In addition to the higher emitted charge, the emitted electrons from the high-dielectric material have higher energies. For the lifetime characteristic, the high-dielectric trigger shows lifetimes much higher than 100 million discharges.

Critical assessment of multistage pseudospark switches

In repetitive pulsed power there is a strong increasing demand for the development of high voltage pulse forming networks (PFN), which can be operated with high repetition rates and simultaneously with extremely long lifetime. Typical applications for such PFNs are modulators for the next generation of accelerators, pulse generators for flue gas cleaning with electrostatic precipitators, high power gas lasers, accelerators for medical radiography and drivers of high power microwaves. For instance, in the next generation linear collider the traditional thyratron/PFN modulators will no longer meet the new requirements. Multistage thyratrons are available for hold-off voltages up to 240 kV, but their handling is complicated, the reliability poor and the costs are high. To equip the 1600 modulators, which are planned for SLACs next linear collider, with multistage thyratrons is unaffordable. Solid-state devices in combination with step-up transformers are under discussion, but their performance is not yet satisfactory. Especially the size of the switches will hamper their application in the near future. The pseudospark switch belongs, with the thyratron, to the class of high-voltage, high-current low-pressure gas switches. In contrast to the thyratron, the pseudospark switch is much simpler, consumes less stand-by power, has a compact size and better overall switching parameters. First this paper presents an overview about the most critical design criteria for a multistage pseudospark switch in comparison to conventional multigap thyratrons, mainly focussed on the coupling of the conduction path between the single stages. In the second part, several different approaches to achieve fast transient coupling between the gaps will be discussed with respect to minimizing the overall inductance of the switch. In former experiments, with different multistage switch designs, a two-stage device using a floating intermediate electrode on the one hand and a three-stage configuration, developed for the beam-dump system of LHC at CERN, on the other hand, were tested. Based on the experience with these switches, a 100 kV-switch, which is planned to be integrated into the 100 kV-PFN of the injection/extraction unit of the future GSI (Gesellschaft fuer Schwerionenforschung at Darmstadt) accelerator complex, will be built.

Multigap pseudospark switch for fair

At the GSI Helmholtzzentrum fuer Schwerionenforschung GmbH a new accelerator complex, called Facility for Antiproton and Ion Research (FAIR), is under construction. Its main components are the SIS100 and SIS300 heavy ion synchrotrons. To operate their injection/extraction kicker magnet systems, modulators with pulse-forming networks (PFNs) are necessary. The PFNs will be charged to a high voltage up to 70 kV and discharged via a high-voltage switch. The switch has to handle currents up to 6 kA, pulse durations up to 7 microseconds with an overall lifetime exceeding 108 shots. The repetition rate is about 4 Hz and a current rise rate of at least 4*1010 A/s is required. The only commercially available switch in this parameter range is actually a multi-gap thyratron. As an alternative, a three-gap pseudospark switch is under development at GSI. It combines the major advantages of the thyratron with its low stand-by power as a cold-cathode device, as well as its insensitivity to large current reversal.

Do Gas-Filled Switches Still Have a Future?

The first part of the paper is mainly focused on a comparison of the stat-of-the-art switching concepts with regard to most important parameters like hold-off voltage, peak current, rise time, pulse length, mean and total charge transfer, power loss, lifetime, repetition rate, availability and so on. Based upon the results of the previous part in the next part the pros and cons are summarized with the goal to comment the on-going process of substitution of gas-filled switches by solid-state devices. Finally the physical and technological limits are discussed together with an outlook to future chances with the development of both types of high power switches.