Measurement of ion flux as a function of background gas pressure in a hot refractory anode vacuum arc

The hot refractory anode vacuum arc (HRAVA) is a metallic plasma source in which plasma expands radially from the interelectrode gap and may deposit substrates circumferentially disposed around the electrode axis. The dependence of copper ion flux expanding from the HRAVA interelectrode gap was determined as a function of background gas pressure. The fraction of the ion flux in the radially expanding plasma flux was obtained by measuring the ion current and the film thickness. Experiments were conducted with arc currents of 145-250 A, a molybdenum anode, and an electrode separation of about 10 mm. The saturation ion current was measured with a circular flat probe with 10-mm diameter biased at -30 V with respect to the anode. It was found that the collected ion current in vacuum was almost constant during the first 30 s of the arc- ∼2.5 mA/cm² at a distance of 110 mm from the arc axis, with an arc current of 200 A, and increased to a steady-state value in the developed HRAVA (t > 40 s) of ∼5.5 mA/cm². The measured ion current in argon, nitrogen, and helium environments and the deposition rate in nitrogen remained approximately constant with background gas pressure up to some critical pressure and, then, decreased with pressure eventually reaching zero. The critical pressures were 2, 4, and 10 Pa for argon, nitrogen, and helium, respectively. The critical nitrogen pressure for the deposition rate was 2 Pa in contrast with 4 Pa for the ion current. The ion fraction in total deposition flux was 0.6 in vacuum and decreased with nitrogen pressure, except that a local maximum of ∼0.8 was observed at ∼13 Pa.