## 5 Conclusions  
In this work, we show that the gas temperature of nonthermal plasma flow depends not only on the configuration of the electrode system, the excitation power level, and the rate of inert gas pumping, but also on the form of the voltage applied to the electrodes. Applying pulses of microsecond duration with long pauses leads to low values of the plasma flow temperature at a minimum level of helium flow rate. We have also been able to demonstrate in principal the possibility of using digital holographic interferometry to register and observe refraction in pulsed plasma. We were able to see a change in the refractive index both with and without gas jet ionization. Further improvement of the technique will allow us to estimate the concentration of electrons in order to determine the dosage of plasma used in skin treatment. Conceptualization, D.V.S. and I.V.A. ; methodology, I.V.A. ; software, A.M.K. ; validation, D.V.S., I.V.A., A.S.I. and A.M.K. ; formal analysis, I.V.A. ; investigation, A.S.I., A.M.K. and I.V.A. ; resources, D.V.S. and I.V.A. ; data curation, A.M.K. and A.S.I. ; writing--original draft preparation, D.V.S. and I.V.A. ; writing--review and editing, A.S.I. ; visualization, A.S.I. ; supervision, A.M.K. and A.S.I. ; project administration, A.S.I. ; funding acquisition, D.V.S. and I.V.A. All authors have read and agreed to the published version of the manuscript. This research was partly funded by the Ministry of Science and Education of the Russian Federation under project #FZ-2020-0003, \"The study of new materials and methods for plasma and phototherapy of cancer, dermatitis and suppurated complications\". Not applicable. The study did not report any data. This work was supported by the Ministry of Science and Higher Education of the Russian Federation (Grant No. FZWM-2020-0003). The authors declare no conflict of interest. * _[PERSON] et al. (2015)_ [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON] KrCl Barrier-Discharge Excilamps: Energy Characteristics and Applications (Review). _Instrum. Exp. Tech._**2015**, _58_, 309-318. [CrossRef]
* _[PERSON] et al. (2002)_ [PERSON]; [PERSON]; [PERSON]; [PERSON] [PERSON] Influence of excitation pulse form on barrier discharge excilamp efficiency. In _Selected Research Papers on Spectroscopy of Nonequilibrium Plasma at Elevated Pressures_; International Society for Optics and Photonics: Bellingham, WA, USA, 2002; Volume 4460, pp. 38-45. [CrossRef]
* _[PERSON] et al. (2000)_ [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON] Lifetime of working mixtures of XeCl and KrCl exilamps. _Atmos. Ocean. Opt._**2000**, _13_, 286-288. * _[PERSON] (2001)_ [PERSON] Determination of energy input in barrier discharge exclamps. _Atmos. Ocean. Opt._**2001**, _14_, 1005-1008. * _[PERSON] (1979)_ [PERSON] _Holographic Interferometry_; John Wiley&Sons: Hoboken, NJ, USA, 1979; pp. 387-396. * _[PERSON] et al. (2018)_ [PERSON]; [PERSON]; [PERSON] [PERSON] Experimental investigation of gas flow rate and electric field effect on refractive index and electron density distribution of cold atmospheric pressure-plasma by optical method, Moire deflectometry. _Phys. Plasmas_**2018**, _25_, 043516. [CrossRef]
* [PERSON] (2005) [PERSON] _Handbook of Holographic Interferometry: Optical and Digital Methods_; WILEY-VCH Verlag GmbH & Co.: Hoboken, NJ, USA; Volume 370, 2005. * _[PERSON] (1986)_ [PERSON] Application of holographic interferometry for plasma diagnostics. _Sov. [PERSON]. [PERSON]_**1986**, _29_, 447. [CrossRef]
* _[PERSON] et al. (2001)_ [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] High sensitive pulsed digital holography for defect analysis using an in-built laser excitation. _Appl. Opt._**2001**, _40_, 100-103. [CrossRef]
* _[PERSON] (2005)_ [PERSON] _Digital Holography_; Springer: Berlin/ Heidelberg, Germany; Volume 64, 2005. * _[PERSON] et al. (1997)_ [PERSON]; [PERSON]; [PERSON] Simultaneous quantitative evaluation of in-plane and out-of-plane deformations by use of a multidirectional spatial carrier. _Appl.