Optical investigation of constricted vacuum arcs

The optical investigation of arcs in transverse magnetic field (TMF) vacuum interrupters is difficult due to the strong motion of the constricted arc in the magnetic field. In order to determine a first estimate of temperature and pressure distribution in such arcs, a non-moving constricted arc is created in a dedicated test geometry with two pin electrodes. Dimensions and currents in this setup are very similar to the TMF case; the arc is mostly cylinder symmetric. The plasma temperature for a plane inside the cathode jet is deduced from spatially resolved optical emission spectroscopy: Using an in-house code the expected spectrum from an arc of given profile is computed taking into account known emission lines in the observation window and (self) absorption. With this the arc profile can be fitted to the experimental spectrum. We find typical peak temperatures around 25 kK, the pressure depends on the current (e.g. 7 bar for 30 kA peak). Furthermore, spatial profiles of temperature and pressure are given.