File:Fig221 Hollow Cathode.PNG

Katz, I., Anderson, J.R., Polk, J.E., "One-Dimensional Hollow Cathode Model," Journal of Propulsion and Power, Vol. 19, No. 4, 2003, pp. 595-600.

Also shown here:

http://aoss-research.engin.umich.edu/sets/papers/JPC2004fuhrhoprev1.pdf

Summary
Diagram of basic hollow cathode technology