J. Z. Gleizer

Numerical simulations of runaway electron generation in pressurized gases

The results of a numerical simulation of the generation of runaway electrons in pressurized nitrogen and helium gases are presented. It was shown that runaway electrons generation occurs in two stages. In the first stage, runaway electrons are composed of the electrons emitted by the cathode and produced in gas ionization in the vicinity of the cathode. This stage is terminated with the formation of the virtual cathode, which becomes the primary source of runaway electrons in the second stage. Also, it was shown that runaway electrons current is limited by both the shielding of the field emission by the space charge of the emitted electrons and the formation of a virtual cathode. In addition, the influence of the initial conditions, such as voltage rise time and amplitude, gas pressure, and the type of gas, on the processes that accompany runaway electrons generation is presented.

Characterization of the plasma on dielectric fiber (velvet) cathodes

An investigation of the properties of the plasma and the electron beam produced by velvet cathodes in a diode powered by a ∼200kV, ∼300ns pulse is presented. Spectroscopic measurements demonstrated that the source of the electrons is surface plasma with electron density and temperature of ∼4×1014cm−3 and ∼7eV, respectively, for an electron current density of ∼50A∕cm2. At the beginning of the accelerating pulse, the plasma expands at a velocity of ∼106cm∕s towards the anode for a few millimeters, where its stoppage occurs. It was shown by optical and x-ray diagnostics that in spite of the individual character and nonuniform cross-sectional distribution of the cathode plasma sources, the uniformity of the extracted electron beam is satisfactory. A mechanism controlling the electron current-density cross-sectional uniformity is suggested. This mechanism is based on a fast radial plasma expansion towards the center due to a magnetic-field radial gradient. Finally, it was shown that the interaction of the elec...

Space- and time-resolved characterization of nanosecond time scale discharge at pressurized gas

The phenomenon of ultra-fast electrical gas breakdown was investigated. Nanosecond high-voltage pulses with durations of 1 and 5 ns and amplitudes of 100 and 200 kV, respectively, were used to study the parameters of the discharge in a pressured (1-7) × 105 Pa air-filled gap. The development of the discharge and the plasma propagation velocity was examined using optical fast frame imaging. The generation of runaway electrons in the breakdown process was confirmed by electron imaging and time-resolved x-ray diagnostics. Runaway electron beam energy distribution was obtained for a 1 ns duration high-voltage pulse. The origin and the role of runaway electrons in the discharge initiation are also discussed.

Pulsed plasma electron sources

There is a continuous interest in research of electron sources which can be used for generation of uniform electron beams produced at E≤105 V/cm and duration ≤10−5 s. In this review, several types of plasma electron sources will be considered, namely, passive (metal ceramic, velvet and carbon fiber with and without CsI coating, and multicapillary and multislot cathodes) and active (ferroelectric and hollow anodes) plasma sources. The operation of passive sources is governed by the formation of flashover plasma whose parameters depend on the amplitude and rise time of the accelerating electric field. In the case of ferroelectric and hollow-anode plasma sources the plasma parameters are controlled by the driving pulse and discharge current, respectively. Using different time- and space-resolved electrical, optical, spectroscopical, Thomson scattering and x-ray diagnostics, the parameters of the plasma and generated electron beam were characterized.

Plasma characterization in a diode with a carbon-fiber cathode

Results of optical and spectroscopic studies of the plasma formation at the surface of two types of carbon-fiber cathodes in a diode powered by an ∼200 kV accelerating pulse are presented. It was found that during the pulse, generation of the plasma occurs in a form of several millimeter size plasma spots. In the vicinity of the cathode surface the average plasma density and temperature were found to be ∼3×1014 cm−3 and ∼5 eV, respectively, for an electron current density of ∼22 A/cm2. The plasma expansion velocity toward the anode was found to be ∼1.5×106 cm/s during the first 150 ns of the accelerating pulse duration.

Ferroelectric plasma sources and their applications

We review our experimental studies of ferroelectric plasma sources and their applications. Using various diagnostics, it was shown that the source of the charged particle emission from the ferroelectric is surface plasma. This plasma is formed as a result of an incomplete discharge on the surface of the ferroelectric sample. The process of the plasma formation is accompanied by desorption of neutrals from the ferroelectric surface. It was shown that the parameters of the plasma and the neutral flow strongly depend on the polarization state of the ferroelectric material and on the parameters of the driving pulse. The lifetime of ferroelectric plasma sources was also studied. Electron beams with current amplitude of a few kiloamperes were generated with rep-rate up to 10 Hz under the application of accelerating pulses with amplitudes of 25-250 kV. Operation of the electron diode with and without plasma prefilling was demonstrated. Data concerning the uniformity of the extracted electron beam and the potential distribution in the diode are presented. In addition, we present data concerning an enhanced emission mode of the ferroelectric cathode and its application as a promising source of heavy ions. Results of applications of ferroelectric plasma sources in low-pressure high-current hollow-cathode discharge, as cathodes in relativistic magnetrons, as high-current switches and for generation of high-frequency modulated electron beams are presented as well.

Electron beam generation in a diode with a gaseous plasma electron source I: Plasma source based on a hollow anode ignited by a multi-arc system

We report on the operation of an electron diode with a cathode based on a hollow plasma anode (HPA) design. Six arc sources placed inside the anode cavity were used to produce a preliminary plasma. The latter was used to produce a high-current (up to 4 kA) gaseous discharge without formation of plasma spots at the anode wall and output grid. The plasma parameters inside the HPA were measured for different N2 and Xe gas pressures and discharge current amplitudes. It was found that the HPA operation is characterized by a negative anode potential fall and that the plasma density and temperature inside the anode are ≈6×1012 cm−3 and ≈9 eV, respectively. The characteristics of an electron diode and the generated electron beam were studied under an accelerating voltage amplitude ⩽250 kV and 400 ns pulse duration for different parameters of the HPA. It was found that in the beginning of the accelerating pulse the diode operates in a plasma prefilled mode while later the diode current is determined by the emissio...

Characterization of multicapillary dielectric cathodes

Parameters of the plasma and electron beam produced by a multicapillary cathode in a diode powered by a ∼200kV, ∼300ns pulse are presented. It was found that the source of electrons is the plasma ejected from the capillaries. Inside the capillaries this plasma obtains electron density and temperature of ∼8×1015cm−3 and ∼5eV, respectively. In the vicinity of the cathode, the density and temperature of the plasma electrons were found to be 2×1014cm−3 and 4.5eV, respectively, for electron current density of ∼40A∕cm2. It was shown that the plasma expansion velocity is in the range of (1–2)×106cm∕s for current density of >12A∕cm2.

Characterization of a channel spark discharge and the generated electron beam

We report on an experimental study of a channel spark discharge (CSD) and the generated electron beam. The CSD was operated at a discharge voltage Ud⩽30kV and a discharge current Id⩽3.5kA. The CSD system consists of a glass tube placed between a hollow cathode and a grounded anode electrode. The parameters of the CSD operation, the potential distribution along the glass tube, and the generated beam were studied by electrical, optical, and spectroscopic diagnostics in the Ar gas pressure range of P=0.005–2Pa. At P⩾0.5Pa, electrons with energy ∼eUd appeared prior to the start of the main CSD with a current amplitude ⩽10−4Id. These high-energy electrons are responsible for the initiation of the CSD inside the glass tube. The generation of the electron beam which was composed of low-energy electrons with a current amplitude up to 3kA occurred during a fast fall in the discharge voltage. Decreasing the Ar gas pressure below 0.1Pa allows one to increase significantly the beam duration and the part of the high-e...

High-current large-area uniform electron beam generation by a grid-controlled hollow anode with multiple-ferroelectric-plasma-source ignition

We report results on the generation of a large-cross-section (about 170 cm2) high-current (about 1000 A) uniform electron beam by a hollow anode (HA) plasma source at a pressure of approximately 8 × 10−5 Torr, in a diode with an accelerating pulse of 300 kV and approximately 300 ns duration. The HA discharge was sustained for about 10 μs by seven Ba–Ti-based ferroelectric plasma sources. The resistive decoupling of each plasma source produces a uniform plasma density distribution at the HA output grid at a discharge current of not more than 1000 A. It was found that the HA plasma is characterized by a density of about 1012 cm−3, an electron temperature of approximately 8 eV and a group of fast electrons with an energy of about 50 eV. It was shown that an increase in the HA output grid potential allows the plasma prefilling of the accelerating gap to be reduced significantly.