gas tubes, SCRs, and welder arcs all have negative incremental resistance. This means when conduction begins, the resistance drops like a crowbar and is sustained until the current drops below the holding current.

thus a typical ambient breakdown voltage for a sharp point of 1kV/mm is the same as 1V/um if the surface is smooth and clean.

once an arc strikes the path length may increase, by adding inductance a break in current causes the voltage to increase and be sustained

to maximize power transfer you want plasma resistance equal to the metal resistance but the metal R increases with temperature x12? at 3000'K and higher with white arcs so controlling the gap is crucial to a good TIG weld.