V. Vekselman

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.

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.

Two-dimensional model of orificed micro-hollow cathode discharge for space application

In this paper, we describe results of self-consistent two-dimensional (x-z) particle-in-cell simulations, with a Monte Carlo collision model, of an orificed micro-hollow cathode operating in a planar diode geometry. The model includes thermionic electron emission with Schottky effect, secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform xenon background gas density in the cathode-anode gap. Simulated results showing behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and orifice walls, are discussed. In addition, results of simulations showing the effect of different Xe gas pressures, orifice size, and cathode voltage, on operation of the micro-hollow cathode are presented.

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.

Effect of explosive emission on runaway electron generation

The results of numerical simulations of the generation of runaway electrons in a nitrogen-filled coaxial diode with electron emission governed by field emission that transfers to explosive emission with a variable time delay are presented. It is shown that the time when the explosive emission turns on influences significantly the generation of runaway electrons. Namely, an explosive emission turn-on prior to the formation of the virtual cathode leads to an increase in the current amplitude of the runaway electrons and a decrease in its duration. Conversely, an explosive emission turn-on after the formation of the virtual cathode and during the high-voltage pulse rise time does not influence the generation of runaway electrons significantly. When the explosive emission turns on during the fall of the high-voltage pulse and after the virtual cathode formation, one obtains additional runaway electron generation. Finally, a comparison between electron energy distributions obtained with and without explosive e...

Time-resolved investigation of nanosecond discharge in dense gas sustained by short and long high-voltage pulse

The results of experimental and numerical studies of the generation of runaway electrons (RAE) in a pressurized air-filled diode under the application of 20 ns, 5 ns and 1 ns duration high-voltage pulses with an amplitude up to 160 kV are presented. It is shown that with a 1 ns pulse, RAE with energy ≥20 keV reach the anode prior to the formation of the plasma channel between the cathode and anode. Conversely, with 20 ns or 5 ns pulses, RAE with energy ≥20 keV were obtained at the anode only after the formation of the plasma channel. In addition, the high- and low-impedance stages of the development of the discharge were found. Finally, a comparison between experimental and numerical simulation results is presented.

High-current carbon-epoxy capillary cathode

The experimental results of a promising pulsed plasma source, producing electron beams with a current density of up to 10 kA/cm² are presented. The beam duration was up to ~1 μs, in an accelerating voltage of up to ~350 kV, without shorting of the cathode-anode gap by the cathode plasma. This plasma electron source is proven to sustain hundreds of pulses without degradation in its emission properties. The cathode plasma electron density n_e ≤ 10¹⁵ cm^-3, electron T_e ≤ 13 eV and ion T_i ≤ 4 eV temperatures, and expansion velocity V_pl 1.6 × 10⁶ cm/s were determined using time- and space-resolved spectroscopy and light emission.

A concept of ferroelectric microparticle propulsion thruster

A space propulsion concept using charged ferroelectric microparticles as a propellant is suggested. The measured ferroelectric plasma source thrust, produced mainly by microparticles emission, reaches ∼9×10−4N. The obtained trajectories of microparticles demonstrate that the majority of the microparticles are positively charged, which permits further improvement of the thruster.