VARIOUS WAYS OF CATHODE PLASMA FORMATION TO PRODUCE UP TO 200 A CM(2)OF H- AND 0.1-1 A/CM(2) OF C-, F-, I-, AND PB- IONS/

Various ways of initiating cathode plasma for producing high-current d ensities of negative ions were tried at the P. N. Lebedev Physical Ins titute. The first was prepulse induced electrical breakdown on the sur face of a dielectric inserted between two metallic parts of a cathode. A second method was surface discharge of a perforated dielectric slee ve on a metallic cathode rod, which yielded acceptable plasma paramete rs for larger prepulse voltages than in the first case. A circuit was developed and tested to supply an externally applied prepulse with cha ngeable 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 h ydrogen ions were obtained as with the first method of plasma initiati on. A new more widely applicable way of cathode plasma initiation was proposed recently and studied experimentally. This method allows one t o produce negative ions of various chemical elements in high-current p ulsed diodes. It utilizes cathode bombardment by positive ions extract ed from near-anode plasma injected at the outer surface of the anode a nd which penetrates partially through anode holes into the diode. Plas ma parameters can be easily varied by changing plasma gun voltage and time delay between plasma gun pulse and diode voltage pulse. Current d ensities 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 o f cathode plasma production. The merits and disadvantages of all metho ds of cathode plasma production are discussed.