A. Krokhmal

Emission properties of different cathodes at E⩽105 V/cm

We present results of the investigation of different types of cathodes operating in an electron diode powered by a high-voltage generator (300 kV, 250 ns, 84 Ω, ⩽5 Hz). The cathodes which have the same emitting area of 100 cm2 are made of metal–ceramic, carbon fibers, carbon fabric, velvet, or corduroy. We also tested carbon fibers and carbon fabric cathodes coated by CsI. It was shown that for all types of cathodes the electron emission occurs from the plasma which is formed as a result of a flashover of separate emitting centers. The amount of the emitting centers and the time delay in the electron emission were found to depend strongly on the accelerating electric field growth rate. Experimental data concerning the uniformity of the light emission from the cathode surface and divergence of the generated electron beams are presented. Data related to the general parameters of the diode, namely its impedance, power, and energy are given as well. For all the cathodes investigated the observed diode impedan...

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...

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 energy and potential distribution in a diode with a ferroelectric cathode

We study the time and space resolved energy of charged particles emitted from the plasma formed on the surface of polarized and unpolarized ferroelectric cathodes under the application of driving pulses having either a positive or negative polarity. It is found that the energy of the emitted charged particles does not exceed the amplitude of the driving voltage and is independent of the initial polarization state of the ferroelectric. In addition, data concerning the energy of the electrons under the application of an accelerating pulse of 25 kV, and data concerning the electron beam uniformity and the time resolved potential distribution inside the anode–cathode gap of the electron diode are presented. We discuss our experimental results within the framework of the incomplete surface discharge model.

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...

Investigation of a hollow anode with an incorporated ferroelectric plasma source for generation of high-current electron beams

We report experimental results of operation of a high-current hollow anode (HA) with a BaTi ferroelectric plasma source (FPS) incorporated in it. It is shown that the application of the FPS allows one to significantly decrease the HA surface area, thus providing a compact electron source. Use of this HA as an electron source in a high-voltage diode for generation of high-current electron beams is described as well. It was found that the FPS allows reliable ignition and sustaining of the HA discharge with current amplitude ⩽1.2 kA and pulse duration ⩽2×10−5 s at N2 gas pressure of (1–3)×10−4 Torr. Also, it was found that the operation of the HA is characterized by plasma formation with density of ∼4×1012 cm−3, electron temperature of ∼5 eV, and that the plasma acquires a positive potential of ∼10 V with respect to the anode and of 50–70 V with respect to the autobiased HA output grid. It is shown that the autobiased HA output grid prevents plasma penetration towards the accelerating gap if the grid half-ce...

Grid-controlled electron emission from a hollow-anode electron source

We describe the operation of a hollow-anode electron source with a biased output grid in a diode powered by a 200 kV 400 ns pulse. The hollow anode had a ferroelectric plasma source incorporated in it. Three electrical schemes for the hollow-anode output grid bias were tested and compared. It is shown that the use of an autobias grid allows electron-beam generation with a current amplitude up to 1.2 kA in a plasma emission-limited mode and with insignificant plasma prefilling of the accelerating gap. The use of an externally biased output grid (either with a positive or negative potential) showed the possibility to control the emission properties of the hollow-anode plasma without changing the amplitude of the discharge current. Electron beams with an amplitude up to 2 kA and insignificant plasma prefilling of the accelerating gap were obtained. It was found that the application of the accelerating pulse leads to a drastic increase in the potential of the plasma up to several kV. It is shown that, in spit...

Spectroscopic investigation of the plasma in a hollow anode with an incorporated ferroelectric plasma source

Spectroscopic measurements are reported of the plasma formed inside a hollow anode (HA) with a ferroelectric plasma source (FPS) incorporated in it. The HA was used as a cathode in a diode supplied by an accelerating pulse (≤300kV, ≤400ns). It was found that the HA discharge (1.2kA, 10μs) is accompanied by the formation of a dense (≈8×1014cm−3) plasma layer at the surface of the FPS. This surface plasma serves as a practically unlimited source of electrons. In the bulk of the HA plasma the density is ≈3×1013cm−3 and it remains the same during the accelerating pulse whereas the plasma electron temperature increases from 4 to 11eV.

Study of electron diodes with a ferroelectric plasma cathode

We present results of studies of time- and space-resolved energy distributions of electrons and ions emitted from the plasma formed on the surface of poled and unpoled ferroelectric samples. Results of lifetime tests of different ferroelectric cathodes are also described. We studied the operation of a planar electron diode and a relativistic magnetron, both with a ferroelectric cathode under the application of a high-voltage pulse of /spl les/300 kV with a repetition rate of /spl les/5 Hz. In the planar diode, the energy of electrons, the uniformity of the extracted electron beam, and the potential distribution were studied. The obtained experimental data agree well with the model of plasma formation on the surface of the ferroelectric. Successful repetition rate operation of the magnetron with a PZT ferroelectric cathode was demonstrated.

Low-pressure, high-current hollow cathode with a ferroelectric plasma source

We report the parameters of a hollow cathode with a ferroelectric plasma source incorporated in it. It was found that this source allows the ignition and sustaining of a high-current discharge (⩽1.4 kA, ⩽2×10−5 s) at N2 gas pressure of (3–5)×10−4 Torr. It was shown that ∼85% of the discharge current is emitted by the ferroelectric sample. The plasma in the cathode acquires a positive potential (⩽50 eV) with respect to the anode and the plasma density and temperature are ⩽8×1012 cm−3 and ⩽18 eV, respectively. Generation of an electron beam (0.3–1.6 kA, 300 ns) was demonstrated under an accelerating pulse ⩽300 kV.