A retarding-potential-energy analyser with mass spectrometer has been developed to investigate the velocity (energy) distribution of an ion swarm in a drift tube. The energy distributions of Cs+ ions in Ar are measured in a steady stage ion stream over the range of E/N from 85 to 480 Td at N between 3.8*1015 and 4.8*1016 cm-3 at Tg=300K. The measured distributions at the axis of the velocity space are in good agreement with the distorted shifted Maxwellian for 85<or approximately=E/N<or approximately=300 Td, though the skewness in the distribution is observed at E/N=480 Td. The drift velocity and the longitudinal diffusion coefficient, derived from the mean value and the variance of the distribution, agree well with those from the conventional measurements of the arrival-time spectrum in the drift tube experiments.
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