## 4 Conclusions  
The present study shows for the first time a direct comparison of an identical setup within an identical range of operation settings with helium and argon operation. The range of applied voltage ranged from 8 kV\({}_{pp}\) up to 20 kV\({}_{pp}\) for both He-CAP and Ar-CAP, while the resulting dissipated power rises linearly up to 5 W for both systems. An investigation of the electrical field amplitude for He-CAP and Ar-CAP showed peak values of 300 kV/m and 320 kV/m, respectively. As expected of such high power consumption, the temperature induced on a conductive surface ranges from 40 \({}^{\circ}\)C up to 130 \({}^{\circ}\)C for He-CAP and from 50 \({}^{\circ}\)C up to 85 \({}^{\circ}\)C for Ar-CAP. The temperature slope over distance is constant and further reduction can be achieved by parameter adjustment of e.g., gas flow. The patient leakage current for He-CAP ranges from 10 \(\mu\)A up to 200 \(\mu\)A over applied voltage and drops over distance. The leakage current measurement for Ar-CAP, however, shows a unique slope with a linear increase over applied voltage and a constant slope over distance. This was never observed in literature before and offers unique conditions for surface treatments with constant temperature and leakage values at changing treatment distance over a range of up to the investigated 20 mm. The production of reactive species was investigated by emission spectroscopy in the (V)UV-VIS-NIR range. Atomic nitrogen and oxygen could be detected in VUV for both systems; however, Ar-CAP showed much stronger signals in all spectral ranges. In addition, the nitrogen oxide emission in the UV range was only measured for argon operation. In accordance with the OES results, species densities of O\({}_{3}\) and NO\({}_{x}\) could only be measured for Ar-CAP, while, for He-CAP, they were below the detection limits. Plasma'spectroscopic temperature' values, rotational temperatures of OH and N\({}_{2}\), were estimated to be the same for Ar-CAP and He-CAP, around 420 K (160 \({}^{\circ}\)C), near the maximum values measured via FOT (130 \({}^{\circ}\)C). The vibrational temperatures N\({}_{2}^{+}\) revealed different values (higher for He-CAP than Ar-CAP), mainly due to different plasma ignition mechanisms. In addition, the higher T\({}_{p}\) in the case of helium plasma might be associated with lower production of NO\({}_{x}\) and O\({}_{3}\). At identical CAP settings of geometry and operation conditions, overall, the Ar-CAP shows the more promising parameters, a constant leakage current over distance and a significant species production. Meanwhile, He-CAP generates comparable values in power, electrical field, temperature and leakage current with no detectable reactive species density. It is an interesting observation and further investigations are clearly required to trace the pathways of the helium power consumption leading to the presented observations. In the meantime, the Ar-CAP provides the interesting property of constant leakage current over distance that indicates a voltage dependent current limitation of the conductive channel that is generated by the discharge. The He-CAP might be an interesting approach for a high electrical field application with no toxic species as byproducts. These findings may open a new direction in the plasma source development for Plasma Medicine. Nevertheless, more experiments should be made, like surface charge production, ultra-fast photography or mass spectrometry in order to have the whole understanding of the plasma source. Conceptualization, A.V.N. and T.G. ; methodology, A.V.N. and T.G. ; validation, A.V.N. and T.G. ; formal analysis, A.V.N. and T.G. ; investigation, A.V.N. and T.G. ; data curation, A.V.N. and T.G. ; writing--review and editing, A.V.N. and T.G. All authors have read and agreed to the published version of the manuscript. Not applicable. Raw data may be available, on reasonable request, from the authors.