*3.2. Arc Dynamics*

Figure 7 presents the typical evolutions of arc voltage and current along the discharge with spectrally filtered images. The ignition instant in the considered example (the start of electrode separation) was about 2.2 ms (Figure 7b,c). The ignition position in present study was defined by mechanical surface modification, as described in Section 2.3. After the ignition, the arc spreads fast over an area, which is larger than the ignition spot and remains diffuse in the present example up to about 3.6 ms (Figure 7d,e). During the diffuse stage, the arc is formed by a nearly homogeneous distributed cathode spots, uniformly glowing anode and a diffuse arc column. The spatial distribution of atoms and ions is very similar at this stage. The bridge explosion gives enough material, which fills a significant volume due to small electrode distance. The arc becomes constricted around the last contact spot (ignition point) (Figure 7f,g) after a certain amount of time, which is necessary to build up enough electromagnetic force (Lorentz force) for constriction process. Notice that even after constriction, the cathode spots cover nearly the whole electrode surface. Their distribution becomes inhomogeneous with higher spot density around the position of constricted arc attachment. Opposite to this, the anode attachment is localized and moves in accordance with actual electromagnetic force direction. Thus, the arc rotation means moving group of cathode spots, opposite anode spot and the arc column, which connects both attachments. The arc trajectory (moving position of highest intensity) is close to the outer electrode boundary. The frames (h–k) show the instants of time when the arc was passing through the ignition position. The spatial distributions of atoms and ions start to differ from each other. At this stage, the atomic radiation is more constricted and has higher intensity near to the cathode surface, while the ionic radiation is distributed wider and more homogeneously over the gap region. This qualitative picture changes as soon as the arc starts to rotate, i.e., after about 3.6 ms in the present example. During rotation, the arc column is dominated by the ions. Their spatial distribution is homogeneous within the emitting area. The atoms follow the ions with some delay. The radiation remains most intense near the cathode surface and is less pronounced in the arc column. The area of atomic radiation near the cathode is much larger compared to that of ionic radiation. The cathode surface is nearly completely melted as will be shown below. Notice, that an arc constriction does not imply that the electrode surface outside of the constriction area is not active anymore. Detailed analysis of high-speed images shows that some weak cathode spots continue to exist on the broader electrode area, and thus represent an intense source of atoms. In addition, the atoms, which are evaporated by melted surface, can become excited due to reaction kinetic processes, like e.g., ion recombination, de-excitation cascades, or charge transfer reactions between copper species. Detailed explanation requires a space- and time-dependent collisional-radiative model, which is missing due to its complexity.

**Figure 7.** (**a**) Temporal evolution of arc current and voltage. Vertical lines show the instants for which the arc images are presented. (**b**–**k**) Spectrally filtered arc images at different time instants. Images (**b**,**d**,**f**,**h**,**j**) show Cu I emission, (**<sup>c</sup>**,**e**,**g**,**i**,**k**) that of Cu II. Electrode type A.

Figure 8 presents the arc rotation frequency along with the arc current shape. It was determined from the arc images by evaluation of the instants at which the arc column passed the same spatial position on the electrode surface. The rotation frequency depends on several factors, like. e.g., arc current value, magnitude of magnetic field flux and the electrode distance. At the beginning, the rotation is slow. The frequency reaches its maximum value, which is about 4 kHz in current example, around the peak current. Then, the frequency follows the current, decreasing up to the instant of about 6.5 ms. The rotation becomes slightly faster for the later time instants, due to accumulated e ffects of phase shift between the arc current and induced magnetic flux and increasing electrode distance. The initial arc position has no influence on rotation frequency taking into account the accuracy of applied method. In the case of position B (which is closer to the electrode centre), the arc starts to move closer to the electrode axis and reaches the outer rotational position slightly later, comparing to ignition position A. The analysis of the high-speed images clarifies that most of the electrode surface becomes molten (or at least close to the molten state) after several rotation cycles. Thus, for the same arc duration a comparable molten surface area will be reached later for the case with ignition position B.

**Figure 8.** Rotation frequency of the arc determined from high-speed images. Red triangles—contact system of type A; green circles—those of type B. Arc current evolution is presented by dashed curve.

### *3.3. Anode Surface Temperature*

Figure 9 presents the temporal evolutions of local anode surface temperature after the current zero crossing at the position of arc ignition, i.e., at localized spot of about 1 mm in diameter. In case of electrodes of type A, the position shown in Figure 2a was used, in case of the electrodes of type B, the position shown in Figure 2b. Notice that the temperature in the other regions of the anode surface can significantly di ffer from obtained values. However, it is expected that measured values give an estimate for the maximum anode temperature after current interruption, since the accumulated arc residence time in those region is higher than in surrounding regions. For type A electrodes, the initial temperature tends to increase with the arc duration. Its initial value was around 1300 K. Longer arcing time leads to slower temperature decay. Since the arc is rotating, the measurement point could be slightly cooled down between two subsequent rotations. However, with increasing arc duration the amount of melted material becomes bigger. This is clearly visible in the videos. Consequently, the melted pool needs more time for cooling down. The type B electrodes show less variability in the temperature. The position at which the temperature was measured is in this case closer to the electrode axis. The arc residence time in that region is much shorter comparing to the outer region. Consequently, the temperature is lower, its value is about 1200 K. Slower temperature decay in this region is mainly caused by contact design. The cooling path for the surface is going toward the stem. The electrode temperature is obviously higher in the outer regions (Figure 9). Thus, those regions act as a heat source for the inner part of the electrode after current interruption. Therefore, a temperature stagnation occurs in those regions.

**Figure 9.** Evolution of surface temperature measured at ignition position after the arc extinction. Arc duration time is indicated. Uncertainty of the numerical method is shown. Total measurements uncertainty is estimated as ±50 K [14].

### *3.4. Cr Density after Current Interruption*

Figure 10 shows the results of absorption measurements for Cr I vapor density after current zero crossing. Due to low spectrally resolved radiation intensity, the acquisition of only one spectrum per shot is possible. Therefore, several measurements (up to five for each contact pair) at the same peak current value of 28 kA have been performed. Due to the presence of many mechanical parts in the driving and in the switching systems it is not possible to ge<sup>t</sup> exactly the same arc duration in different shots and the same instant of absorption spectrum acquisition. On the other hand, the reproducibility of plasma characteristics seems to be quite stable, when the same arc duration occurs. Thus, different shots give an estimation of temporal evolution after current zero crossing. The temporal region within first 500 μs is of special importance for interruption process. During this time period, the fast increasing transient recovery voltage can force a breakdown if the species densities are high enough. Atomic species do not follow the electric field and have enough residence time between the electrodes to ge<sup>t</sup> ionized. The density of neutral species has the highest value direct after the current zero crossing instant and shows a nearly linear decay within the first microseconds after current interruption [20,21].

The arc duration has an insignificant influence on Cr density only. Initial temperatures at current zero do not differ much for the same contact type (cf. Figure 9). The density slightly increases with longer arc duration. Stronger deviations in the density were found when the arc ignition position was varied. A higher electrode temperature leads consequently to higher density of chromium vapor in the case of the electrodes with position A. It amounts to about 1.1 × 10<sup>18</sup> m<sup>−</sup><sup>3</sup> for starting position A immediately after current interruption, while the value for position B is about 9.5 × 10<sup>17</sup> m<sup>−</sup>3, even though the arc duration was longer. The results are similar for other time instants.

**Figure 10.** Cr I density measured after current zero crossing for di fferent shots with contact systems of type A (red symbols) and B (green symbols). The numbers indicate the arc duration in ms.
