Carbon cathode spot plasma flux distributions in low pressures of hydrogen: some evidence for the C++H2→CH++H reaction

Measurements of the wall current collected from the expanding cathodic arc plasma are reported as a function of angle to cathode plane normal, with hydrogen gas pressure in the arc chamber varying between 10 ^-4 torr and 10 torr and distance R to the cathode varying between 10 and 80 mm. The angular distribution of the collected current is not seen to be affected by the gas pressure in the chamber. This flux decreases with distance to the cathode as R^-(1.4) in vacuum and R^-4 in the 1-10 torr pressure domain. Flux intensity evaluated as a function of NR, where N is the number density of hydrogen in the chamber, leads to an evaluation of an effective collision cross section, σ_e, of 0.2×10^-16 cm² for the C⁺-ion/H₂ molecule interaction. This value can be attributed to the reaction C⁺+H₂→CH⁺ +H and agrees with a velocity of the C⁺ ions following a shock wave interaction model