Negative ion formation in SF6 spark by-products

When sulfur hexafluoride (SF6) gas is used as electrical insulation (e.g., in high voltage or switching applications), deterioration of the gas caused by sparks, corona, high electrical stress, or high temperature results in a buildup of by-products whose electron attaching properties differ from those of SF6. A number of long lived by-products formed by spark discharges in SF6 at pressures in the range 40 to 233 kPa are identified. The concentrations of the by-products SOF2, So2F2, SO2, SiF4, and SOF4 are measured quantitatively. These by-products are formed via the interaction of the multiplicity of transient short lived species in sparked SF6 with moisture, electrode material, and insulators. Each is studied individually in a time of flight mass spectrometer, and the negative ion yields as a function of electron energy in the range 0 to 12 eV are measured. Cross sections for electron attachment to SO2 are reported and compared with literature data.