*3.1. Electrode Preparation*

The switching contacts in real vacuum interrupters are manufactured under special conditions in a clean atmosphere [1]. These conditions are typically not available in research facilities. Therefore, some cleaning procedure must be applied prior to a study of arc properties in order to remove adsorbed

micro particles, lubricant from the machining processing, dielectric layers, water, etc. The cleaning procedure comprises processing in an ultrasonic bath with distilled water and a degreasing fluid, surface cleaning with isopropanol, and application of low-current arc discharges (<400 A) of less than 30 ms duration for final conditioning of the surface after the mounting inside the vacuum chamber and pumping to ultra-high vacuum. First shots with non-conditioned electrodes show instabilities in the voltage behavior (Figure 6), which is related to stochastic spot formation on the electrode surface at the positions of adsorbed impurities. The spot formation is accompanied by voltage jumps (blue curve in Figure 6) in agreemen<sup>t</sup> with different arc voltages for cathode spots I and II [24]. The high-speed videos confirm the unstable arc root positions in this case. After typically 10 shots, the voltage curve becomes much smoother (green curve in Figure 6), indicating the end of the conditioning process.

**Figure 6.** Electrical signals during the conditioning process. Red curve—arc current; blue and green curves—arc voltage during two different shots; blue curve before conditioning and green curve after conditioning.
