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 impedance indicated the existence of a quasistationary cathode plasma boundary for electron current density ⩽20 A/cm2. We present the dependencies of the average emitted electron current density and of the time delay in the electron emission on the number of generator shots. We also present data of the vacuum deterioration as a result of the tested cathodes operation. The obtained data are discussed within the framework of plasma formation as a result of cathode surface flashover.
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 electron beam with the stainless-steel anode does not lead to the formation of an anode plasma.
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-cell size has approximately the same value as the thickness of the double layer formed between the plasma and the grid wires. Generation and characterization of a high-current electron beam with current amplitude of ∼1.2 kA was achieved under an accelerating pulse amplitude ⩽300 kV and ∼400 ns pulse duration.
We present results which show drastic changes in the parameters of a hollow anode (HA) which serves as a cathode in an electron diode. The HA has a ferroelectric plasma source (FPS) incorporated in it. The applied accelerating pulse causes increase of the potential of the plasma inside the HA up to 7 kV and switches the direction of the current of plasma electrons from the HA and its output grid towards the accelerating gap. It is shown that, in spite of the large positive plasma potential, electron emission occurs due to the dynamics of ion motion in the sheath near the HA grid.
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