Abstract. Drift velocities were measured for ions of both polarities in unpurified cylinder SF₆, at pressures ranging from 50 to 700 Torr. The ions were produced in electron avalanches occurring in the high-field region close to a protrusion on one electrode of a uniform-field discharge gap, and the drift velocities were measured in the uniform-field region, where the electric field strength was below the minimum value required for avalanche growth. Negative ions were studied over the E/p range from 33 to 100 V cm^-1 Torr^-1: their behaviour suggests a possible transition in the nature of the ion in the range 64-82 V cm^-1 Torr^-1, but this is not entirely clear. Positive ions were studied over the E/p range from 40 to 120 V cm^-1 Torr^-1 and their drift velocity was found to vary linearly over this range. Empirical equations are given for the measured values. It appears that these may safely be extrapolated to higher pressures provided that the compressibility of the gas is taken into account. The presence of variable amounts of impurities did not affect the results significantly.

Print publication: Issue 6 (14 June 1989)