A mass spectrometric study of positive and negative ion formation in an SF6corona. I. Sources of sulphur-fluoride ions

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Abstract
The mass spectra of positive ions generated in a positive high-voltage point-to-plane corona and negative ions generated in a negative high-voltage point-to-plane corona in high-purity SF6 are reported. In the absence of added impurities the major positively- and negatively-charged ions are SF5+, SF3+, SF2+, SF+, SF6-, SF5- and F-. Evidence for the formation of SF6 corona discharge neutral by-products, SF4 and SF2, is observed from the presence of cluster ions, SFx+(SF4) and SFx+(SF2), x=2, 3. Based on analyses of predicted ion intensities from cross-section data, SF2+ and SF+ ions are attributed to SF2 in the discharge, while most of the SF3+ observed is attributed to SF4. In the case of negative ions, analyses indicate that F- is formed in the discharge at high-density normalized electric field, E/N, from both dissociative attachment of SF6 and ion conversion processes, while SF6- and SF5- are likely to be formed outside the glow. Other ions observed resulting from the presence of trace contaminants include SOF3+, SOF2+, SOF+, SOF5- and F-(HF)2. In this paper (part I) the various sources of ions produced in 'pure' SF6 corona are discussed while in part II the effects of water and SF6 neutral corona by-products on ion formation are discussed.