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

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 ppm_v, 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.