An arc chamber liner has first and second surfaces and a hole having a first diameter. A liner lip having a second diameter extends upwardly from the second surface toward the first surface and surrounds the hole. An electrode has a shaft with a third diameter and a head with a fourth diameter. The third diameter is less than the first diameter and passes through the body and hole and is electrically isolated from the liner by an annular gap. The head has a third surface having an electrode lip extending downwardly from the third surface toward the second surface. The electrode lip has a fifth diameter between the second and fourth diameters. A spacing between the liner and electrode lips defines a labyrinth seal to generally prevent contaminants from entering the annular gap. The shaft has an annular groove configured to accept a boron nitride seal.