Downstream ion energy distributions in a hollow cathode ring cusp discharge

The presence of energetic ions produced by a hollow cathodes operating at high-emission currents (>10 A) has been documented in the literature. As part of an ongoing effort to uncover the underlying physics of the formation of these ions, ion efflux from a high-current hollow cathode operating in an ion thruster discharge chamber was investigated. With use of a spherical sector electrostatic energy analyzer located downstream of the discharge cathode, the ion energy distribution over a 0-60-eV energy range was measured. The sensitivity of the ion energy distribution function to zenith angle was also assessed at three different positions: 0, 15, and 25 deg. The measurements suggest that the majority of the ion current at the measuring point falls into the analyzer with an energy approximately equal to the discharge voltage. The ion distribution, however, was found to be quite broad. The high-energy tail of the distribution function tended to grow with increasing discharge current. Sensitivity of the profiles to flow rate at fixed discharge current was also investigated. A simple model is presented that provides a potential mechanism for the production of ions with energies above the discharge voltage.