A MASS-SPECTROMETRIC STUDY OF POSITIVE AND NEGATIVE-ION FORMATION IN AN SF6 CORONA .2. INFLUENCE OF WATER AND SF6 NEUTRAL BY-PRODUCTS

The effects of water and SF6 by-products on both positive and negative ion formation in an SF6 Corona have been examined. For a positive corona for 'pure' SF6 at p = 6.5 kPa the primary ions observed are SF5+, SF3+ and SF2+, with water addition (in the range 0-640 ppm) ion conversion and clustering occurs leading to H+(H2O)n, (n = 1-4), SF5+(H2O)n, (n = 0-2), SOF+3(H2O)n, (n = 0-2). For a negative corona for 'pure' SF6, the primary ions observed are SF6-, SF5- and F-, with water addition ion conversion and clustering also occurs leading to F-(HF)n, (n = 0-4), OH-(H2O)n, (n = 0-2), SOF5-(HF)n (n = 0, 1), SF6-(HF)n, (n = 0, 1), SO2F- and SO2F2-, where HF is a neutral by-product of the SF6/H2O corona. Of these ions OH-(H2O)n is considered a possible 'seed' ion for collisional electron detachment in an SF6/H2O corona. It is shown that by-product formation such as the production of SOF4, even when present at trace levels, will lead to significant modifications to the negative ion content.