The effect of cleaning and metallisation on high voltage performance of alumina insulators in vacuum

Reliable high-voltage performance of ceramic insulators in vacuum is essential to a range of technical applications, such as vacuum tubes, high-energy particle accelerators and fusion reactors. The study of the electrical performance of ceramic insulators in vacuum is one of the vital tools in understanding the flashover phenomenon, which limits the performance of insulators in high-voltage devices. This study is concerned with the high-voltage vacuum electrical breakdown of alumina (Al₂O₃) ceramics, measured in terms of I-V characteristics, Fowler-Nordheim plots and spark rates. A systematic study of eight non-metallised (raw and chemically cleaned) and six metallised samples was carried out in electric fields up to 15 MV/m. From the complete performance history of each sample (including electro-optical threshold voltage, maximum leakage current and spark rate) a comprehensive comparison of the electronic behaviour from the ceramics was obtained. Clear distinctions were observed between the characteristics of metallised and non-metallised samples. The distinct behaviours of the two types are discussed in relation to contemporary physical models of the pre-breakdown surface-flashover of the solid insulator in vacuum