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 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 length and analysis reveal that these end effects are generally more pronounced for ion-neural 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 transverse ion current density profile with drift length.