Positive ions in spark breakdown of SF6

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Abstract
Positive ions formed by spark discharges and via reactions following the discharge in SF6 in the pressure range 13-67 kPa have been detected and identified using high-pressure quadrupole mass spectrometry. Results are presented for ions produced in 'unpurified' SF6, 'purified' SF6 and binary mixtures of 'purified' SF6 with N2 and O2. Without reduction of air and water impurities to the parts per million level the major ions detected at P>or approximately=40 kPa were N2O2F+, H2SOF2+, N2F4+, S2NF4+, S2NOF4+, S2NO2F4+, and S2NF8+. These were identified through additions of N2, O2, D2O, and the measurement of sulphur isotope ratios. After extensive purification of SF6, the predominant ions at these pressures were S2F7+ and SF3+. The effect of impurities is discussed in light of work performed by others which shows that the long-lived neutral products depend strongly on impurities.