G. Schaefer

A review of opening switch technology for inductive energy storage

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.

Concepts for Optical Control of Diffuse Discharge Opening Switches

Optical control of diffuse discharges is discussed as opening mechanism for rep-rated switches. Diffuse discharges can be sustained or terminated by making use of optogalvanic effects, that means resonant interaction of laser radiation with diffuse plasma. Independent of control mechanisms, the performance of diffuse discharge opening switches is strongly affected by such fill gas properties as attachment and electron mobility.

A model of dc glow discharges with abnormal cathode fall

A model for an abnormal glow discharge, including a self‐consistent analysis of the cathode fall, was developed. It combines microscopic particle simulation by means of Monte Carlo methods with a fluid model of the gas discharge. The model allows calculations of the steady‐state electrical field distribution, the charged‐particle densities, and the current densities along the axis of the discharge. The model was used to simulate a glow discharge in 80% He and 20% SF6 at a pressure of 8 Torr with a current density of 1 A/cm2. The computed discharge voltage agrees well with measured values. The computer code can easily be modified to describe the charged‐particle densities and energies not only in the cathode fall region, but in any plasma boundary layer.

Photodetachment as a Control Mechanism for Diffuse Discharge Switches

Photodetachment is considered as a control mechanism for diffuse discharge switches. Experiments have been performed on photodetachment of ions in the flowing afterglow of a dc glow discharge in oxygen. Experiments with different laser wavelengths and the dependence of the optogalvanic signal on the laser energy flux indicate that O- is the dominant negative ion. For an energy flux of 35 mJ/cm2, 50percent of the O- ions can be photodetached.

Field-enhancement calculations for a field- distortion triggered spark gap

We present the results of numerical field calculation which supplement a recent article in which we described a new design concept for field‐distortion triggered spark gaps. The calculations verify the shielding and field enhancement assumptions made in the article, and they provide insight into the interaction of the design tradeoffs associated with simultaneously maximizing the holdoff voltage and the triggering capability of the gap.

Monte Carlo calculations on the influence of attachment on the thermalization of secondary electrons in an electron‐beam‐sustained discharge

Monte Carlo calculations have been performed on the relaxation of initial secondary electrons in electron‐beam‐sustained discharges at low values of the reduced electric field strength, E/N. In pure nitrogen, thermalizing electrons contribute to a secondary maximum of the distribution function in the energy range between 3.5 and 8 eV where the cross section for inelastic collisions in nitrogen has a minimum. This maximum is not found if the secondary electrons are produced at low energies. Admixtures of attachers with an attachment cross section in this energy range cause significant attachment of the initial secondary electrons. The calculations also demonstrate that even in molecular buffer gases, the inelastic cross sections of the attacher admixture have to be considered to gain correct distribution functions and attachment rates.

CO2-laser-enhanced electron attachment in externally sustained diffuse gas discharges containing vinyl chloride

Photon‐enhanced attachment can be used as a conduction‐control mechanism for diffuse discharge opening switches. This paper presents experimental results on photon‐enhanced attachment based on vibrational excitation of vinyl chloride using a line‐tunable CO2 laser. Strong resistance increases and discharge turnoff have been observed in low‐current, externally sustained dc discharges. The influence of experimental parameters such as discharge pressure, attacher concentration, discharge voltage, and laser wavelength are presented.

A new design concept for field distortion trigger spark gaps

A common field distortion triggered spark gap utilizing geometric field enhancement at sharp edges usually operates in a cascade mode via the trigger electrode. A new trigger concept is proposed allowing strong field enhancement and direct breakdown between the two main electrodes. A test setup was designed to prove the feasibility of this concept. Experimental results on delay and jitter depending on percent breakdown voltage are presented. Best results achieved are a delay of 9 ns and a jitter of 2 ns at a self‐breakdown voltage of 15 kV.

Pulsed Hollow Cathode Discharge with Nanosecond Risetime

This paper reports the operation of a cylindrical hollow cathode discharge with current risetimes of a few nanoseconds at current densities at the entrance of the cathode in the range of 50-560A · cm-2 and at voltages of 280-850 V. Time-dependent measurements of the impedance of the discharge are presented. They allow for the evaluation of discharge quantities such as risetime, delay time, discharge voltage, and current, depending on the operation parameters as applied voltage, pressure, and preionization. The power density in the active region of the hollow cathode exceeded 200 kW · cm-3.

Magnetic Control of Diffuse Discharges

By application of a crossed magnetic field, the electron energy distribution in a gas discharge can be shifted to lower energy values, as demonstrated by means of Monte Carlo calculations for electrons in He:SF6 mixtures. Consequently, through the change in the rate coefficients for ionization and attachment, the sustaining field in the discharge plasma is increased. This magnetically induced voltage rise was studied in a low-pressure glow discharge. The cathode fall was found to be the dominant component in determining the characteristics of this magnetically controlled discharge. The drastic rise of the cathode fall above a threshold value could be utilized in operating a glow discharge as an opening switch for an inductive energy storage system.