An experimental study of longitudinal end effects in transverse diffusion measurements of Na+ ion swarms in He gas

Abstract

The inevitable presence of longitudinal end effects occurring in drift-tube mass spectrometers, if not corrected for, can cause significant systematic errors in measurements of ion transport coefficients. In our transverse diffusion measurements of Na+ ions in He and several other ion-neutral pairs, appreciable longitudinal end effects were detected for sufficiently high values of E/N, the ratio of electric field to neutral gas number density. Systematic measurements at various drift lengths and analyses reveal that these end effects are generally more pronounced for ion-neutral pairs with very low or very high mass ratio. They can be either positive or negative and their magnitudes can be quantified by a simple linear relationship of the variance of the transverse ion current density profile with drift length. A systematic study of the Na+-He interaction pair, which is known to exhibit such end effects, establishes that it is possible to apply a methodical correction procedure to experimentally obtained values of DT/K to yield reliable results of an accuracy comparable to those obtained from systems known to be free from such end effects.