**5. Conclusions**

The main challenge concerning optical investigations of ablation-dominated high-current arcs close to current zero is a low intensity of the emitted radiation. Therefore, different techniques of high-speed camera (HSC) imaging with or without filtering and optical emission spectroscopy (OES) were introduced and their assets and limitations were discussed. It was shown that some important effects can be analyzed even with rather simple nozzle experiments, applicable to other groups and setups.

Two setups were used: With the first setup, experiments with a long, tubular nozzle were applied on to study the CZ transition including new re-ignition of the arc. Using OES with HSC, a dark period of 200 μs around CZ was observed and the differences in spectra before and after CZ were discussed. Radial temperature profiles could be obtained until 400 μs before CZ. Here, a typical case of a well-established arc was found with a broad temperature profile, characteristic for an arc stabilized by ablation of the nozzle wall.

For the second setup, a model circuit chamber in CO2 atmosphere was used to study more realistic, praxis-relevant features including flow reversal, arc behavior around CZ, and arc before vanishing. Using OES with an intensified CCD camera, higher sensitivity was realized allowing the determination of temperature profiles. Whereas ionic carbon lines were applied mainly for quantitative characterization at higher temperatures until 100 μs, atomic oxygen lines delivered quantitative profiles until a few microseconds before CZ with a higher sensitivity at lower temperatures. Generally, a transition was observed in the arc behavior. Until several hundred microseconds before CZ, the arc was wall-stabilized with a broad and rather flat temperature profile. After vanishing of wall stabilization and inflow of cold gas, a highly dynamic arc appeared that was constricted and asymmetric moving out of center. During the transition, the maximum temperatures in the core increased to yield higher current densities for the constricted arc.

In future work it will be important to combine the experimental results and modeling concerning temperature profiles, composition calculation for a determination of current density.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/13/18/4714/ s1, Video S1 (DFO-fig4.mp4) : High–speed imaging using double frame optics according to Figure 4. Video S2 (OES-fig5.mp4):Video S2: High–speed imaging optical emission spectroscopy according to Figure 5. Video S3 (OES-fig7.mp4):Video S3: High–speed imaging optical emission spectroscopy around the O I triplet at 777 nm with higher spectral resolution according to Figure 7.

**Author Contributions:** Conceptualization, R.M. and D.U.; methodology, validation, and formal analysis, R.M.; investigation with setup (a), R.M. and A.K.; investigation with setup (b), R.M., A.K., and N.G.; writing—original draft preparation, R.M. and D.U.; project administration, D.U. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Deutsche Forschungsgemeinschaft gran<sup>t</sup> numbers UH 106/13-1 and SCHN 728/16-1.

**Acknowledgments:** The authors would like to thank Steffen Franke for experimental help and fruitful discussions. The calculation of plasma composition was realized by Sergey Gortschakow (all Leibniz Institute for Plasma Science and Technology).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

*Energies* **2020**, *13*, 4714
