High voltage performance degradation of the 14ud tandem accelerator

The 14UD at the Australian National University's Heavy Ion Accelerator Facility (HIAF) operated at a maximum voltage of 15.5 MV after the installation of tubes with a compressed geometry in the 1990s. In recent years, the performance of the accelerator has shown a gradual decline to a maximum operation voltage of ∼14.5 MV. There are some fundamental factors that limit the high voltage performance, such as SF6 gas pressure, field enhancement due to triple junctions and total voltage effect. In addition, there are non-fundamental factors causing high voltage degradation. These are: operation with faulty ceramic gaps; operation at inappropriate voltage and SF6 pressure combinations; SF6 leaks into the vacuum space; use of SF6 and O2 as a stripper gases; poor electron suppression in the high energy stripper and frequent use of highly reactive ions such as sulphur and fluorine. In this paper we will discuss factors that limit the high voltage performance. The main outcomes of a preliminary investigation of titanium (Ti) electrodes removed from the accelerator after a few decades of operation will be reported. The investigation confirmed contamination of Ti electrodes with unstable films containing traces of oxides, sulphur and fluorine. The rehabilitation strategies for the accelerator will be discussed.