A hollow cathode arc in neon: simultaneous laser probing and molecular-beam sampling of metastable atoms as a plasma diagnostic

The mean free path of metastable rare-gas atoms in a plasma can be determined by simultaneous sampling of a beam of metastable atoms through an orifice in the wall and optically pumping of the metastable atoms in the plasma with a cw dye laser beam perpendicular to the atomic beam axis. This method gives very accurate results for the absolute value and the velocity dependence (using a time-of-flight detection technique) of this mean free path. With a simple model the contributions of collisions with electrons and with heavy particles to the experimental mean free path can be determined from the velocity dependence. We have applied this method to the external plasma column of a hollow cathode arc in neon. The experimentally determined mean free path of 2-10 mm points unambiguously to a surprisingly low degree of ionization of 2% or less. At these conditions the plasma column is fully transparent for the background atoms in the vacuum chamber. This all means that the character of the plasma of a hollow cathode arc in neon is totally different from argon and helium, which is probably caused by the isotope composition of neon