Breakdown localization in the fixed gap system

Accurate localization of breakdowns in vacuum could help shed light on breakdown related processes that are not yet fully understood. At the DC spark lab at CERN, an instrument called the Fixed Gap System (FGS) has been developed partially for this purpose. Among other things, the FGS has four built-in antennas, which are intended for breakdown localization. The capability of this aspect of the FGS was explored in this report. Specifically, the feasibility of using a method similar to that which is used in cavity Beam Position Monitors (BPMs) was investigated. The usable frequency range of the current experimental setup was also studied. Firstly, a modal analysis of the inner geometry of the FGS was done in HFSS. This showed that the two first modes to be expected in the spark gap quite differ from those of the ideal pillbox – both in field pattern and in frequency ( 4 and 6 GHz vs. 0.2 and 3 GHz). Secondly, S-parameters of the system were measured. These showed that the coupling between antennas is weak below 13 GHz, which is due to the high cut-off frequency of the waveguides in which the antennas are located. The breakdown signal was also measured using an oscilloscope connected to the antennas. However, it was determined that the detected signal was picked up from outside of the system, rendering it useless for localization purposes. It was concluded that either a new approach has to be adopted or the current system must be modified.