Gas decomposition and electrode degradation characteristics of a 20% C3F7CN and 80% CO2 gas mixture for high voltage accelerators

Abstract

Sulphur hexafluoride (SF6) is a potent greenhouse gas used in high voltage accelerators. As a promising alternative to SF6, the C3F7CN/CO2 gas mixture and its by-products are of great interest to ensure the safe operation of accelerators that will adopt any SF6-free solution. This work experimentally examines the electrical ageing characteristics of a 20% C3F7CN/80% CO2 gas mixture tested using spark gaps under a pressure of 7.2 bar (abs.). Gas samples were collected after 1000 DC breakdowns and analysed using gas chromatography mass spectrometry (GC-MS) with an estimated toxicity value of 54,459 ppmv, which indicates the aged mixture to be non-toxic. Subsequent investigation was conducted on the gas-solid interface after 500 breakdowns for both SF6 and the 20% C3F7CN/80% CO2 gas mixture. Aged electrodes were analysed using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques. Electrode surface analysis revealed the formation of metal fluorides on the electrode surface tested using the 20% C3F7CN/80% CO2 mixture, whereas metal fluorides and sulphides were detected for electrodes tested with SF6. The findings provide a reference on the toxicity and gas-solid interaction of the electrically aged 20% C3F7CN/80% CO2 gas mixture for potential retro-fill application in high voltage accelerators.