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

Abstract

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.

Keywords

• ageing
• carbon compounds
• chromatography
• dissociation
• electric breakdown
• electrochemical electrodes