*4.1. General*

An ablation-dominated arc of 5 kA peak current was operated in a model circuit breaker with a CO2 atmosphere. The application of a slit over the full radius of the PTFE nozzle enabled a direct investigation inside the nozzle. This was substantial progress in comparison with previous measurements that potentially suffered from an influence of turbulent gas flow in the heating channel [17]. Sealing by thin quartz plates proved to be a useful method to obtain reproducible conditions of discharges without significant changes of material flow or plasma conditions. Consequent exchange of the plates after each shot yielded high window transmission with moderate blackening. An averaged overall transmission of about 50% was estimated along the observation axis made of the quartz plate in the nozzle and windows of the MCB chamber as well as the high-pressure vessel. The shot-to-shot variation was usually about 10 %. Nozzle ablation caused a widening of the nozzle diameter and thus a slow reduction of maximum pressures over many shots. Therefore, the PTFE nozzles were exchanged regularly. Only very few cases of reduced sealing quality occurred, easily noticeable after discharges due to increased blackening at the plate's corners. These shots had be repeated with renewed sealing.

The assets of different methods for optical investigation were demonstrated. Using high-speed cameras the general arc behavior was investigated, e.g., revealing rotational symmetry over the whole nozzle diameter in the high-current phase but not close to current zero when the stabilization by ablation of nozzle wall was lost and the arc was constricted and out of the nozzle center. Besides, the HSC allowed an investigation of many successive points of time within the same shot. Thus, reproducibility was not demanded and dynamic changes, their rising and falling times, and shot-to-shot variation could be easily investigated. Although parameter variation was not the focus of this work, such investigations are rather comfortable using this technique. Furthermore, a combination of HSC with double frame optics was introduced, filtering one channel only for radiation from CO2 (O I at 777 nm) and the other channel only for emission from PTFE (F I at 675 nm), which allowed us to gain knowledge about the temporal evolution of the plasma composition. In combination with according CFD simulation, gas flow behavior could be analyzed, including the exact determination of the point in time when flow reversal occurred before current zero. In the experiments described here, only qualitative analysis could be carried out. For a quantitative description, more knowledge about plasma composition and absolute intensity calibration would be mandatory [23].

### *4.2. OES Using HSC*

Deeper information was obtained from spatially- and temporally-resolved video spectroscopy using HSC. That comprised of the different phases of discharge and the occurrence of Swan band emission from C2 molecules that are treated in an accompanying publication [20]. Radial temperature profiles have been determined until 400 μs before current zero. Assuming a plasma composition of 100% C2F4 at atmospheric pressure, a broad temperature profile has been obtained with a maximum of 9400 K in the arc center and about 9000 K at a radial position of about 4 mm, i.e., 2 mm away from the nozzle wall. Several fluorine lines were applied for comparison, yielding similar results. A "dark window" without detectable emission was observed due to low intensities caused by cold gas flow and low current on the one hand and limitations in the sensitivity of high-speed cameras on the other hand, starting in a best case about 100 μs before CZ. Furthermore, it could be stated for the high current and setup (a) that the arc plasma needs about 0.5–1.0 ms for both the ignition phase as well as the current zero crossing to be completely dominated by the ablated wall material. The influence of the surrounding gas can be neglected, at least under ambient conditions of one bar air.

### *4.3. OES with ICCD*

The sensitivity of OES was increased by application of OES with an intensified CCD camera, allowing single-shot measurements until a few μs before current zero. Two lines were used for the determination of temperature profiles of the arc plasma, whereas the ionic carbon line C II at 658 nm has a normal maximum around 22,000 K and therefore better sensitivity concerning temperatures around 18,000–20,000 K, the atomic oxygen triplet O I at 777 nm has its normal maximum around 16,000 K and higher sensitivity at lower temperatures. Off-axis maxima of the radial emission coefficient of the O I triplet were found, indicating temperatures in the arc center above and in the arc fringes below the normal maximum. Hence, the normal maximum can be used for the calibration of the emission coefficient according to the Fowler–Milne method. In addition, the absolute intensity calibration by a radiation normal has been used for verification. The emission coefficient of the C II line was evaluated with absolute intensity calibration only.

As experimental uncertainties of the determined emission coefficients, in particular the window transmission (estimated to 50% ± 10%), adjustment of slit width (50 ± 5 μm), performing absolute intensity calibration (uncertainty up to 20%), and pressure measurement (uncertainty up to 10%) have to be considered. It is an advantage of the applied method for the O I triplet that the normal maximum is independent from influences by transmission, absorption, and absolute intensity calibration. Therefore, the uncertainty of the temperature determined from O I is low around the normal maximum of 16,000 K as well as in the range of 10,000–14,000 K (up to 10%) due to the exponential intensity rise with temperature. However, reliable temperatures above 20,000 K cannot be determined from the O I triplet. An important factor is the remaining uncertainty of the gas composition and the partial pressure of carbon and oxygen. Pressure measurements showed a shot-to-shot variation between 0.2 and 0.4 bar, and the fitting of the emission coefficients of the C II line and the O I triplet with respect to the corresponding normal maximum in pure CO2 lead to the different pressures 1.2 and 2 bar. But considering the relatively low variation of the temperatures at the normal maxima with pressure, the uncertainty of the plasma temperatures at least around the normal maximum is below ±200 K. For times closer than 100 μs to CZ, where the emission coefficient of the O I triplet could be evaluated by absolute intensity calibration only, the uncertainty depends on the rise of line emission coefficient with temperature. For temperatures below 11,000 K, the emission coefficient is possibly underestimated by factor 2 in maximum, which causes an underestimation of temperature by 900 K.

It has been found that the arc was rather broad at 1000 μs and 400 μs before CZ in agreemen<sup>t</sup> with the observations of wall-stabilization. In the following, arc constriction was observed. Despite the uncertainties discussed above, it can be stated that the maximum temperature decreased from above 18,000 K at 300 μs to about 11,000 K at 10 μs before CZ.
