Hollow cathodes with cavity diameters of 1.9 cm have been operated in an MPD arc configuration at discharge currents from 0.25 kA to 30 kA. Current and potential distributions, measured with probes in the cathode cavity, show that a spontaneous mode of hollow cathode operation, wherein the current distribution penetrates upstream into the cavity, has been achieved for particular combinations of discharge current and mass flows. These measured distributions are consistent with a conduction model which suggests that maximum current penetration, and the more efficient cathode emission which accompanies it, occurs when the dominant mean free path for electron energy exchange is comparable to the cavity diameter.