Measurement of ionisation and attachment coefficients in SF6

Measurements of prebreakdown ionisation currents in SF6 at pressures of up to 400 torr have been carried out over a range of E/p (E being electric-field strength, and p the gas pressure referred to 20°C) for 115 <E/p ≪ 200 V/cm torr and values of the primary ionisation α, secondary ionisation γ and attachment μ coefficients obtained. Within the acccuracy with which the values are known, α̅/p (where α̅=α – η), α/p, η/p and γ are functions of E/p only, and thus similarity is obeyed. The experimental limitations are fully surveyed and the effect of field nonuniformity on the current growth is discussed.