Characteristics of Energetic Ions Emitted from Hollow Cathodes

The behavior of a hollow cathode operated within an ion thruster discharge chamber is characterized using a remotely located, charged-particle analyzer. A very complex structure of the ion energy distribution function is revealed that is in general agreement with previous observations from many earlier works. Observations, which are in general agreement with the literature, include the measurement of ions that have “through anode” energies and beyond, that have higher ion energies occurring as flow rate is reduced, and that have higher discharge currents inducing more energetic ion production. All of these observations are also in line with results from long term life tests of hollow cathode-equipped ion thruster systems, which show erosion of hollow cathodes and components located nearby that is presumably caused by sputtering due to energetic ion bombardment. Most studies of hollow cathodes have concentrated on devices operated outside of discharge chambers where they are more readily accessible. This study is different in that it was performed with the hollow cathode located within a specially designed, 30-cm diameter discharge chamber. The measurements were made using a collimated electrostatic energy analyzer (ESA) that was sighted through a slot cut in the discharge chamber wall and pseudo grid surface. The hollow cathode/discharge chamber system could be rotated about an axis centered at the cathode orifice, and this enabled measurement of the ion energy distribution functions at zenith angles ranging from 0 to 90 with respect to the hollow cathode centerline. The ESA lateral position was also varied to look at regions located [in a line-of-sight (LOS) sense] on either side of the hollow cathode. Results are presented which show the effects of zenith angle variation, discharge current, and cathode flow rate on the ion energy distribution. Very interesting results obtained in recent experiments at high discharge currents are presented that show the ion energy distribution varying widely when measured at different zenith angles and at different positions near the hollow cathode. Specifically, few high-energy ions are seen on-axis, but many are seen at moderate (~20° and higher) zenith angles and from regions up to 5 cm from the cathode. This observation is different than those made on hollow cathodes operated outside of a discharge chamber.