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Design of an electron source of triode type is described, which produces a radial converging electron beam employed for modification of the outer surface of fuel element claddings. Experimental investigations of the source showed that beam current magnitude, efficiency of beam focusing to the target, stability of source operation, and beam pulse duration become worse when the target diameter drops below certain values. A method for the calculation of the source operation was developed. Calculations showed that the reason of worse source operation is the initial angular spread of electrons when the target diameter is small. Because of the angular spread a part of electrons passes by the target and oscillates between the target and cathode. The space charge of the oscillating electrons lowers the emission current magnitude and leads to the formation of a virtual cathode in the grid-target gap despite the fact that the emission beam current is less than the limiting one. Formation of a virtual cathode is a reason for the unstable source operation and restriction of the beam pulse duration. Recommendations for the improvement of the source design are suggested.

The paper presents research data on positive and negative coronas in atmospheric pressure air in a highly inhomogeneous electric field. The data show that irrespective of the polarity of pointed electrodes placed in a high electric field (>200 kV/cm), this type of discharge develops via spherical streamers even if the gap voltage rises slowly (£0.2 kV/ms). The start voltage of first positive streamers, compared to negative ones, is higher and the amplitude and the frequency of their current pulses are much lower: about two times and more than two orders of magnitude, respectively. The higher frequency of current pulses from negative spherical streamers provides higher average currents and larger luminous regions of negative coronas compared to positive ones. Positive and negative cylindrical streamers from a pointed to a plane electrode are detected and successive discharge transitions at both polarities are identified.