Speaker
Description
In order to investigate the mechanisms behind vacuum arc formation, it is beneficial to use as many diagnostic tools as possible at a wide temperature range for different metals. The cryogenic HV pulsing system in FREIA laboratory is experimentally following this line of research. We study vacuum arc breakdowns and surface conditioning using high-repetition rate DC pulses at a wide range of temperatures, from room temperature, down to 4K. In our experiments, we have tested different materials commonly found in the accelerator technology like copper, niobium and titanium.
It has been previously shown that vacuum breakdowns are initiated by field emission (FE). In order to better understand the mechanisms behind vacuum arc breakdowns and the conditioning process, we are measuring FE currents at different temperatures during the conditioning process. The behavior of the FE current in function of the field at cryogenic temperatures give very clean data that follow almost perfectly theoretical Fowler-Nordheim theory despite large emission area (high field area of >1200 mm^2). The extracted Fowler-Nordheim parameters can therefore very accurately describe the current state of the surface and allows us to follow the changes due to conditioning process.
We have observed and quantify for instance the decrease of the field enhancement factor as well as the change in the emitted area during conditioning. We have also observed that the field emission current can decrease during long field emission measurements or when the system is slightly heated up. A rest gas analyzer (RGA) was added to the system to look for changes in the partial pressures of trace gases that correlate to these variations in current.
We will present the results of the conditioning process, together with the field emission and RGA measurements for different materials. We hope that the results shown in this talk will be useful for improving our current understanding of vacuum arcs.
