2.4.2. Arc Images

Several high-speed cameras registered the arc dynamics (Motion Pro Y4, IDT) with a repetition/frame rate of 20,000 fps and an exposure time of 1 μs. Standard camera lenses were used. One camera monitored the ignition position and general arc dynamics. In addition, two cameras equipped with narrow-band metal-interference filters transmitting at 521 nm with FMHW 1 nm (Cu I line) and at 494 nm with FWHM 4 nm (several Cu II lines) separated the dynamics of copper atoms and ions during the arcing.

### 2.4.3. Determination of Surface Temperature

Near infrared spectroscopic measurements (NIR in Figure 1) have been used for evaluation of the local anode surface temperature after current interruption (after the time instant 10 ms in Figure 4b) at the points of arc ignition. A NIR spectrometer (C1142GA, Hamamatsu) with a spectral range of 900–1650 nm and a temporal resolution of 1.25 ms (exposure time 200 μs) was used for spectral measurements. After the acquisition, the spectra were processed according to the routine presented in [14] to determine the anode surface temperature. Figure 4 summarizes the information about NIR diagnostics. The red spot in Figure 4a shows the region, from which the anode surface radiation was collected. The measurement spot was about 1 mm in diameter and focused on the arc ignition position, i.e., in case of electrodes of type A, the position shown by red circle in Figure 2a was used, in case of the electrodes of type B, the corresponding position shown in Figure 2b. The measurements started around the current maximum with an acquisition time of 200 μs and frequency of one spectrum each 1.25 ms.

**Figure 4.** (**a**) Measurement position in case of NIR spectroscopy (red circle). Anode on the top, cathode in the bottom. (**b**) Acquisition instants of NIR spectroscopy related to the current shape. (**c**) Sample of NIR spectrum suitable for temperature evaluation: instant of time 1.25 ms after current interruption.

Figure 4c shows an example of temperature evaluation for 1.25 ms after current interruption (point around 11.25 ms in Figure 4b) for the contact pair of type B. The surface temperature was calculated using diagnostic techniques described in [14]. A tungsten strip lamp was used for determination of wavelength dependent windows transmission, as well as for spectral calibration of NIR spectrometer. The temperature arises from the shape of measured intensity (Figure 4c) by comparison with a Planck radiator with given temperature. Details of the methods are described in [14].

### 2.4.4. Absorption Spectroscopy for Species Density Determination

Broadband optical absorption spectroscopy (OAS) determined the chromium vapor density after extinction of arc plasma (after current interruption). This technique is based on evaluation of absorption spectra in the wavelength range where the resonance lines of material of interest are present.

A pulsed DC high-intensity Xenon lamp acted as a background radiation source. It emits a Planck-like radiation of 12,000 K with a maximum power of 1 MW [23]. The Xenon lamp is positioned on the right-side window of the vacuum chamber (Figure 1). Its radiation is directed through the electrode system and coupled to the spectrometer entrance slit that is placed at the opposite window. By using a deflecting and a focusing mirror, the electrode gap is observed along a line parallel to the electrode surfaces (dashed line in Figure 5a). The slit width was about 50 μm. The spatial position of spectra acquisition was about 1 mm away from the anode surface. The lamp and the spectrograph start at a desired instant (Figure 5b). The Xe lamp started at about 9.5 ms (green curve in Figure 5b) and reached its maximum intensity close to the instant of acquisition time (grey curve denoted as OAS in Figure 5c). The radiation is spectrally dispersed using a Czerny–Turner type spectrograph (Shamrock 750, Andor Technology Ltd.) with a 0.75 m focal length equipped with an intensified charge coupled device camera (iStar, Andor Technology Ltd.). Absorption spectra were acquired immediately after the current zero crossing with a delay between 100 μs and 300 μs, which was necessary to study the decaying plasma. A spectral interval between 423–431 nm was chosen to study the Cr I density, which contains spectral lines of different resonance transitions of 425.43, 427.78, and 428.97 nm. Figure 4c presents an example of an acquired spectrum for Cr I 428.97 nm line. The density of absorbing species is proportional to the area under the curve. More details about the method and the corresponding theory are described in [20,21].

**Figure 5.** (**a**) Position of spectrograph slit for absorption spectra acquisition. Slit width was 50 μm; distance to the anode was about 1 mm. (**b**) Temporal evolution of current (red) and voltage (blue) along with sequence of control signals during the absorption spectroscopy measurements: driving current of Xe lamp (green) and exposure signal for spectrum acquisition (grey). (**c**) Example of Cr I 428.97 nm line fit procedure.

### **3. Results and Discussion**
