Gennady A. Mesyats

On the mechanism of emission from the ferroelectric ceramic cathode

This article suggests an alternative mechanism for electron emission from the ferroelectric ceramic cathode. This mechanism involves the concept of vacuum breakdown initiation from metal‐dielectric cathodes that comprises two stages: the field emission through metal‐ dielectric‐vacuum conjunction followed by the processes resulting in an uncompleted surface discharge. The electron emission thus occurs from the low‐density surface plasma.

The RADAN series of compact pulsed power Generators and their applications

This paper presents results of development of a compact pulsed power high-voltage generators and high-current electron accelerators of the RADAN series. The basic high-voltage units of RADAN instruments are built around coaxial pulsed forming lines and efficient charging device represented by a Tesla transformer. The fields of applications in science and in practice are rather wide and include formation of nanosecond and subnanosecond voltage and ultrawideband RF pulses, high-power microwave generation, X-ray radiography, radiation physics, chemistry, and biology. The designed technique provided achievements of outstanding specific parameters of dense e-beams, microwaves, and ultrawideband pulses.

Ecton mechanism of the vacuum arc cathode spot

A new mechanism for the operation of a cathode spot in a vacuum arc, based on ecton processes, is proposed. An ecton is formed by the explosion of the tip of a jet of molten metal as it interacts with plasma. The time of ecton operation is assumed to be limited by the thermal conductivity of the liquid metal. For copper electrodes, the theoretical expressions are derived for the specific mass removal, ion erosion characteristics, current density, and the diameters of craters. The results agree well with the experimental data available.

The current density and the specific energy input in fast electrical explosion

The electrical explosion of wires is considered as a method for studying the behavior of materials under the conditions of fast heating. A fast electrical explosion occurs subject to the conditions that the heating time is shorter than the time required for capillary and magnetohydrodynamic instabilities to develop and that this time is longer than the time required for the current to expand throughout the wire cross section. Corresponding similarity criteria have been derived for each of the processes disturbing the uniform heating of a wire. The conditions for fast electrical explosion have been experimentally realized. The current density was varied from 10/sup 7/-10/sup 9/ A/cm/sup 2/ resulting in heating rates between 10/sup 10/ and 10/sup 13/ J/(g.s). Exploded wires of copper, nickel, tungsten, and molybdenum were investigated. These experiments have shown that the energy density introduced into the wire material depends on the heating rate (i.e., the current density). Fourfold overheating of the materials in the condensed state has been attained. It has been found that the specific current action also increases with increasing current density but to a lesser degree than the energy input.

Repetitively pulsed high-current accelerators with transformer charging of forming lines

This article describes the principles of operation and the parameters of the SINUS setups designed at the Institute of High-Current Electronics, Siberian Division, Russian Academy of Science, over the period from 1990 to 2002. A characteristic feature of accelerators of the SINUS type is the use of coaxial forming lines (in particular, with a spiral central conductor) which are charged by a built-in Tesla transformer to produce the accelerating high-voltage pulses. This ensures a reasonable compactness and long lifetime of the setups. The range of parameters of the SINUS setups is as follows: ○ Voltage amplitude at the cathode: 200-2000 kV ○ Electron beam current: 2-20 kA ○ Equivalent load impedance: 30-180 Ω ○ Accelerating pulse duration: 4-130 ns ○ Pulse repetition rate: up to 400 Hz ○ Pulse amplitude instability (RMS): 0.7-2.5% A number of setups of this type use a three-electrode controllable gas gap switch. This has made possible on-line electronic control (from pulse to pulse) of the output voltage pulse amplitude. The control band width δ = ΔU/U max was up to 75%. Studies have been performed on the lifetime of explosive-emission cathodes. At current densities of 25-30 A/cm 2 , a pulse duration of ∼20 ns, and a pulse repetition rate of 100 Hz, the metal-dielectric cathode in a planar geometry retained its emissivity within 10 8 pulses. The SINUS accelerators are traditionally employed for producing high-power microwave radiation in various systems with a coaxial electron beam in a longitudinal magnetic field. For this purpose, magnetic systems with a solenoid powered from the bank of molecular capacitors have been designed. The duration of a quasistationary magnetic field was 1 s at a maximum solenoid power of 365 kW. The possibility has been shown to exist for a self-contained power supply of the accelerator from the bank of molecular capacitors in the batch mode. With an average power consumption of about 120 kW, the setup produces pulses in a batch of duration 2.5 s at a pulse repetition rate of 200 Hz.

Pulsewidth limitation in the relativistic backward wave oscillator

Spontaneous pulse shortening occurring in a relativistic backward wave oscillator (BWO) at gigawatt power levels is studied in experiment and theory. It is experimentally demonstrated that this phenomenon is accompanied by formation of an explosive-emission plasma at the surface of the corrugated slow-wave structure (SWS). Termination of microwave emission is explained by the increase of the BWO starting current from the absorption of the operating electromagnetic wave by electrons emitted from the plasma, whereas the intensity of the absorption radically increases offing to the presence of positive ions emitted from the plasma. Application of oil-free vacuum and electrochemical polishing of the SWS surface in an X-band BWO allowed generation of 3-GW, 26-ns microwave pulses with an energy of /spl sim/80 J, thereby demonstrating pulse lengthening by a factor of four.

Production of short microwave pulses with a peak power exceeding the driving electron beam power

This article presents results of theoretical and experimental studies on the production of ultrashort ~a few RF cycles duration! microwave pulses of gigawatt peak powers based on superradiance from high-current electron beams. With the Cherenkov backward-wave‐electron-beam interaction in a low-dispersion slow-wave structure, microwave pulses with a peak power greater than the peak power of the driving electron beam have been produced for the first time. In an experiment using the SINUS-150 compact high-current electron accelerator, with a 2.6-kA injected beam current and a 330-kV electron energy, microwave pulses of 1.2 GW peak power and;0.5 ns duration ~FWHM! were generated in the X-band. Production of superradiance pulses in a repetitive regime ~3500 Hz! in the Ka-band has been demonstrated using a compact hybrid SOS-modulator. The effect of spatial accumulation of microwave energy in extended slow-wave structures with substantially nonuniform coupling has been demonstrated. In an experiment using the SINUS-200 compact accelerator, X-band pulses of ;3 GW peak power and 0.6‐0.7 ns width~FWHM! were produced with a power conversion efficiency of 150‐180% and an energy efficiency of ;15%.

On the Nature of Picosecond Runaway Electron Beams in Air

Results of experiments on the generation of picosecond runaway electron (RE) beams in air gaps with a strongly nonuniform electric field distribution are discussed. The mechanism and conditions for the field emission initiation of beams of this type and possible reasons for the limitation of the beam duration are considered. REs are supposed to be the dominant factor in the development of an ionization wave in the gap. Estimates of the mean velocity of propagation of the ionization wave are given.

Ecton Mechanism of the Cathode Spot Phenomena in a Vacuum Arc

In this paper, we review the state of the art in studying the physical processes that occur in the cathode spots of vacuum arcs. The now available experimental data are interpreted in the context of the ecton mechanism of the operation of vacuum arc cathode spots. Central in this mechanism is the explosive electron emission, a phenomenon discovered by the author and his co-workers in the mid-1960s while studying high-voltage pulsed vacuum breakdown. In the light of the ecton mechanism, the cathode spot of a vacuum arc consists of individual cells which are explosive emission sites each emitting a portion of electrons termed an ecton. The cathode spot processes are cyclic in nature due to the finiteness of the ecton lifetime. It is shown that an arc is self-sustained due to the explosive emission processes initiated on the interaction of the cathode plasma either with nonmetal inclusions present in the cathode surface (first-type spots) or with liquid metal jets ejected from the zone of an active cathode spot (second-type spots). Attention is focused on the physical processes occurring during the operation of a cathode spot cell. A statistical model of a vacuum arc is used to interpret the effect of the spontaneous extinction of an arc. It is shown that an increase in the arc current is accompanied by a slight increase in the number of simultaneously operating ectons; therefore, as observed in the experiments, the parameters of a vacuum do not greatly depend on the current up to the kiloampere level.

Desktop subnanosecond pulser: research, development, and applications

The studies on production of high-power high voltage pulses of subnanosecond duration having been performed at the Laboratory of Electron Accelerators are reviewed. The subnanosecond pulser we have created is a supplementary device for the earlier developed nanosecond repetitive pulsed power source RADAN 303. The principle of operation of the pulser is successive formation of the leading and trailing edges of the high voltage pulse with the use of high-pressure gas switches. Design versions with a different number of electrodes in the peaking spark gaps have been examined. With a four-electrode spark gap, a pulse of peak voltage 170 kV and FWHM duration 150 ps was produced across a 50 (Omega) load. The processes occurring in transportation of high voltage pulses through the coaxial section of the pulser have been analyzed. The data on the control of the output pulse parameters and typical jitter values are presented. The fields of possible applications of the pulser are discussed. The pulser has been proved in short-run tests at a pulse repetition rate of 100 pps.