Various ways of cathode plasma formation to produce up to 200 A/cm2 of H− and 0.1–1 A/cm2 of C−, F−, I−, and Pb− ions

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Various ways of initiating cathode plasma for producing high-current densities of negative ions were tried at the P. N. Lebedev Physical Institute. The first was prepulse induced electrical breakdown on the surface of a dielectric inserted between two metallic parts of a cathode. A second method was surface discharge of a perforated dielectric sleeve on a metallic cathode rod, which yielded acceptable plasma parameters for larger prepulse voltages than in the first case. A circuit was developed and tested to supply an externally applied prepulse with changeable parameters. The third method employed laser illumination of a cathode dielectric surface to initiate cathode plasma. No prepulse was used in this case and nearly the same current densities of negative hydrogen ions were obtained as with the first method of plasma initiation. A new more widely applicable way of cathode plasma initiation was proposed recently and studied experimentally. This method allows one to produce negative ions of various chemical elements in high-current pulsed diodes. It utilizes cathode bombardment by positive ions extracted from near-anode plasma injected at the outer surface of the anode and which penetrates partially through anode holes into the diode. Plasma parameters can be easily varied by changing plasma gun voltage and time delay between plasma gun pulse and diode voltage pulse. Current densities of up to 30 A/cm2 of H− and about 1 A/cm2 of C−, 0.3 A/cm2 of F− and 0.1 A/cm2 of I− and Pb− were recorded using this last method of cathode plasma production. The merits and disadvantages of all methods of cathode plasma production are discussed.