AN IMPROVED EMPIRICAL-FORMULA FOR PUMPING SPEEDS OF ION PUMPS IN THE HIGH MAGNETIC-FIELD MODE

An improved empirical formula giving a pumping speed of ion pumps in the high magnetic field (HMF) mode is proposed based on Hartwig's empirical formula. Empirical formulae or theories derived so far have implicitly assumed the Penning cell with an infinite length along the axis. In this report, the total charge is calculated in the cell with a finite length using the finite element method (FEM), and the factor representing the geometrical dependence of the total charge is introduced. The improved formula is obtained by adding this new factor to Hartwig's formula. This improved formula is able to estimate the effect on the pumping speed of the cell radius, the cell height and the distance between the anodes and cathodes, and reproduces the experimental results very well.