Research School of Physics and Engineering, Australian National University, Canberra, Australian Capitol Territory, Australia
The 14UD at the Australian National University’s Heavy Ion Accelerator Facility maintained a maximum operational voltage of 15.5 MV since the 1990s after the installation of tubes with a compressed geometry. In recent years, the performance of the accelerator has shown a gradual decline to a maximum operation voltage of ~14.5 MV. There are fundamental factors that limit the high voltage performance, such as the 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 or 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 and an investigation of titanium (Ti) electrodes removed from accelerator after operation for few decades. This confirmed contamination of Ti electrodes with unstable films containing traces of oxides, sulphur and fluorine.
