A model for mercury orificed hollow cathodes - Theory and experiment

The model outlined here provides a useful qualitative description of the basic physical processes taking place within a mercury orificed hollow cathode and can predict, to first order, important cathode operating parameters such as emission length and insert temperature. The analytical formulation of the model is based on the concept of an idealized production region/' which is defined as the volume circumscribed by the emitting portion of the insert. The energy exchange mean free path for primary electrons is used as a criterion for determining the length Le of this region. An ion production region aspect ratio D/Le of 2 is suggested as a design criterion for minimizing the keeper voltage. The model accounts for electrons produced in the ion production region by surface emission and volume ionization. Surface and volume energy balances are used to predict plasma density and plasma potential in this region. An empirical relation is presented that can be used to estimate cathode internal pressure (a necessary input to the model) from the discharge current and cathode orifice diameter. Calculations based on the model are compared with experimental results.