**1. Introduction**

Vacuum technology provides environmentally compatible and emission-free solutions for switching applications in power grids [1,2]. Some advantages of vacuum circuit breakers include a high number of operations under standard load conditions, safe and reproducible short-circuit current interruption capability, and maintenance-free operation. Further development of such devices requires fundamental knowledge about the switching process and interaction of materials used with the working medium—the vacuum arc plasma.

The vacuum arc is ignited by the electrode separation under the current load. While the arc appears as di ffuse glow for currents typically below 10 kA [1–3], constriction e ffects dominate at higher currents. The disadvantage of a constricted arc is that it causes a localized thermal load on the electrode, which leads to enhanced electrode erosion due to melting and evaporation. Therefore, various measures are applied in switching devices for arc control. The application of external magnetic fields helps either to hold the arc in di ffuse stage (axial magnetic field (AMF)) or causes the rotation of the constricted arc (radial magnetic field (RMF)) or transverse magnetic field (TMF)) [1].

The RMF/TMF contacts have been in use for many decades. In the pioneering works [4–6], the two di fferent contact shapes were suggested. Corresponding principles are used in the modern contact systems in vacuum interrupters and give inspiration for improved contact geometries [7]. The choice of contact design depends on the electrical and thermal load in a certain application.

In case of successful current interruption, the arc is terminated immediately after the current zero (CZ) crossing. However, there are various factors that could prevent the current interruption. When the density of neutral metal vapor is high enough, ionization due to appearance of transient recovery voltage can lead to arc reignition and disconnection failure [8]. The anode surface is the main source of neutral vapor. The anode activity is particularly high if high-current anode modes occur [9]. In case of RMF contacts, the arc movement prevents formation of stationary anode spots. However, the arc needs certain time to maintain the magnetic field for the start of arc rotation. During this stage, the arc position is fixed, which leads to a localized electrode heating. In later stages, the same position could be repeatedly heated, due to a high rotation speed. Since thermal dissipation is a relatively slow process, the position of arc initiation can potentially reach higher temperatures compared to the surrounding material and thereby influence the performance of the contacts. Thus, studying the influence of arc ignition position on the arc dynamics, spatio-temporal evolutions of electrode surface temperature, and vapor density after current interruption is of grea<sup>t</sup> practical importance for the understanding of switching behavior and development/improvement of the contact design.

Optical diagnostics offer numerous methods for the characterization of the arc plasma and electrodes. High-speed camera techniques are widely used for observation of arc dynamics, such as appearance of certain high-current modes, mode transitions, arc constriction, rotation speed, etc., [10–13]. Various high-speed cameras are used for acquisition of temporal evolution of light emitted by the arc (mainly in visible range). Typically, an acquisition frequency of 5 k–100 k fps is applied.

Optical emission spectroscopy (OES) can determine the surface temperature. A big challenge for surface temperature measurements is the presence of the arc in front of the electrode. The arc radiation is quite strong during the whole arcing phase and disappears immediately after current zero crossing. This behavior of plasma radiation gives the opportunity for undisturbed access to the electrode surface after the current zero. The corresponding measurements techniques have been reported in [14–16]. One of the possibilities is to use a compact near infrared spectrometer [14], which is adjusted to the position of interest by an optical system. The method works as long as the surface emits enough thermal radiation.

Evaluation of the ground state density of neutral species (Cu and Cr atoms) can be performed by optical absorption spectroscopy (OAS) [17–21]. The absorption measurement setup typically consists of a light source, optical system for beam adjustment and a spectrograph for registration of absorption spectra. Usually, broadband light sources are applied since they provide enough power in the visible spectral region and, thus, allow for a choice of various wavelength ranges of interest. The measurements are of feasible complexity as long as they are performed after the current zero crossing, when the emission from arc plasma is negligibly small [17–20]. Investigations in the active phase, however, require much more effort, e.g., arrangemen<sup>t</sup> of a second optical path for registration of plasma radiation from the probed region only [21]. This technique is very complicated and works well in the case of non-moving arc only. Therefore, a setup with single optical path was used for measurements after current interruption.

In the present study, the influence of the arc ignition position on local surface temperature of the anode and vapor density after current zero crossing was investigated in a model vacuum interrupter using the above-mentioned optical diagnostics under realistic operation conditions.

### **2. Materials and Methods**
