D. Yarmolich

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.

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.

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.

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.

Microparticle flow generation by a ferroelectric plasma source

The paper presents the experimental results of the investigation of microparticle emission from a Ba–Ti-based ferroelectric sample subjected to a driving pulse with a slow rise time and a fast fall time. It was found that the formation of incomplete discharges on the surface of the ferroelectric is accompanied by the emission of an intense positively charged microparticle flow. This microparticle flow (density, velocity distribution, momentum and average microparticle charge) was studied using electrical and optical diagnostics at different distances from the front surface of the ferroelectric sample and at several time delays with respect to the fast fall of the driving pulse. It was shown that microparticles are positively charged at about 6×10−15 C; they have an average size of about 5 μm, a density of about 7×104 cm−2 and an average velocity of about 6×103 cm s−2. The mechanism of the formation of these microparticles and the application of microparticle flows as a propellant for a small thruster are discussed.

Electron beam and plasma modes of a channel spark discharge operation

Parameters of a modified pulsed channel spark discharge (CSD), operating at a repetition rate up to 100 Hz at Ar gas pressures of 10−3 and 10−4 Torr and of the generated electron beam, were studied using different electrical, optical, and x-ray diagnostics. It was shown that efficient (up to ∼74%) transfer of the initially stored energy to the energetic electron beam is realized only at the pressure of 10−4 Torr. Conversely, at the pressure of 10−3 Torr, less than 10% of the stored energy is acquired by the energetic electrons. It was found that the energetic electron beam generation is limited by the expansion of the cathode and anode plasmas and by the formation of plasma inside the gap between the CSD capillary output and the anode. It was also found that the plasma, which acquires the hollow cathode potential, is already formed at the beginning of the CSD operation inside the capillary, and the electron emission occurs from the capillary output plasma boundary. Finally, it was shown that the electron ...

Self-pulsing 104 A cm−2 current density discharges in dielectric barrier Al/Al2O3 microplasma devices

Excitation of Al/Al2O3 microplasma devices with 50 μs, 800 V pulses produces, in Ar/H2 gas mixtures at 600 Torr, ∼6 A current pulses with a duration of ∼30 ns. Corresponding to peak current and power densities of ∼104 A/cm2 and ∼2.5 GW/cm3, respectively, these pulses are generated in a 10 μs burst in which the voltage self-pulses at a repetition frequency of ∼3 MHz. Analysis of the Hα, Hβ, and Ar II emission line profiles yields a plasma density of ∼1017 cm−3, and the emission of O IV ions suggests the presence of energetic electrons. Images of the microplasma indicate that the plasma is initiated by surface flashover and extends ∼200 μm outside the microcavity.

Optimization of a low-pressure hollow-anode electrical discharge for generation of high-current electron beams

We report results on the optimization of the design of a high-current hollow-anode (HA) electrical discharge electron beam source that is triggered by a ferroelectric surface discharge. For the ferroelectric sample, we used a BaTi solid solution with a large dielectric constant (e 1600). Three different electric schemes for ignition and sustainment of the HA discharge were investigated. The studied HA designs allow reliable ignition and sustainment of the discharge with current amplitude of up to 1.2 kA and pulse duration of up to 2 x 10 -5 s, with and without gas flooding. It was found that the rise time of the discharge current monotonically decreases from ∼10-7.5 μs with the increase of the background pressure from 4 x 10 -5 to 7 x 10 -5 Torr. Generation of high-current electron beams was demonstrated under an accelerating voltage of up to 300 kV and ∼400 ns pulse duration. It was shown that the use of an optimal resistor which supplies an auto-bias potential to the HA output grid eliminates almost entirely the plasma pre-filling of the accelerating gap prior to the application of the accelerating pulse. In addition, it was found that within a certain range of time delays (12.5-15.5 μs) of the application of the accelerating pulse with respect to the beginning of the HA discharge, the amplitude of the diode current remains practically unchanged in spite of a considerable decrease in the amplitude of the discharge current.

Characterization of Deposited Films and the Electron Beam Generated in the Pulsed Plasma Deposition Gun

The channel spark discharge was used as a high-current density (up to 30 kA/cm2) relatively low-energy (<20 keV) electron beam source in a pulsed plasma deposition (PPD) gun. The PPD gun was used for the deposition of thin films by pulsed ablation of different target materials, at a background gas pressure in the 10-3–10-5 Torr range. The parameters of the electron beam generated in the modified PPD gun were studied using electrical, optical, and X-ray diagnostics. It was found that a higher background pressure stimulates a denser plasma formation between the gun output and the target, that restricts the energy delivery to the beam electrons. Namely, the efficient (up to ~74%) transfer of the initially stored energy to the electron beam is realized at the background gas pressure of 10-4 Torr. Conversely, at a pressure of 10-3 Torr, only ≤10% of the stored energy is acquired by the energetic electrons. It was shown that the modified PPD gun, owing to the extremely high energy density delivered by the electrons to the target, may be applied for the deposition of a wide variety of different insulators, semiconductors, and metals. A selection of materials such as diamond-like carbon (DLC), cadmium telluride (CdTe), cadmium sulphide (CdS), zinc oxide (ZnO), tungsten, and tungsten carbide (WC) have been deposited as thin films and the properties and deposition rates of the deposited thin films are discussed.