## 4 Conclusions  
The impact of a non-thermal atmospheric pressure plasma jet (APPJ) using different working gases, powered by an AC high-voltage source with a variable frequency reaching 60 kHz, and with a voltage ranging from 2.5 to 25 kV, has been determined. The optical and electrical characteristics of APPJ discharges for Ar and Ar/O\({}_{2}\) working gases have  
Figure 15: Dose rate of irradiance per unit area (μJ/mm\({}^{2}\)) of plasma jet discharge as a function of plasma jet length (**a**) for argon discharge with different flow rates and (**b**) for oxygen/argon admixture discharge for different oxygen ratios. been determined, such as: voltage-current waveform signals, gas flow rates, discharge plasma powers, lengths, widths, the axial temperature distribution, optical emission spectra, and irradiance. The discharge plasma power values of oxygen admixture discharges decrease, from 1.75 to 1.65 W, for oxygen admixture ratios increasing from 0.25% to 1.5%, and are lower than those of argon discharges, and the plasma power values increase from 1.95 to 2.4 W when the argon flow rate increases from 0.2 to 3 slm. This is useful in sterilization and inactivation processes, since it prolongs the interaction processes between the plasma jet and the sample. The width of the jet is an interesting parameter that depends on the plasma jet length and the applied flow rate of argon and oxygen. The width is a guideline for the maximum irradiance area emerging from the jet and reaching the sample. As the oxygen flow rates increase, the width of the plasma jet increases and elongates the axial length of the jet. In general, the temperature of the jet reaching the sample must not exceed the threshold of room temperature. As the oxygen ratio increases, the numbers of collisions among particles increase, and the jet temperatures decrease as a result of processes during discharge. For discharges with oxygen admixture ratios between 0.25% and 1.5% with argon, the emission spectra of APPJ give a behavior similar to that of pure argon discharges, but with low values of intensity due to the decrease in electron density and temperature and higher dissociation and ionization of the oxygen molecules in the discharges processes in the case of oxygen admixture. The optical emission spectra and the dose rate of irradiance of plasma jet discharge were investigated as a function of plasma jet length for dry argon discharge and for wet argon discharge. This investigation delivered data compatible with the International Commission on Non-Ionizing Radiation Protection (ICNIRP) for irradiance exposure limits of the skin, 30 ul/mm2. This limit is suitable for the disinfection of microbes on the skin without harmful effects. Our future work will involve an experimental study on the impact parameters affecting the microbial disinfection process of bacteria on skin in a wound area. The study will use different distances between the jet of the APPJ and the treated samples and the required knowledge to avoid toxic gas formation, damage, and heat radiation production in the sample. Moreover, the Reynolds numbers of argon and oxygen/argon admixture discharges for various gas flow rates will be considered. Conceptualization, methodology, validation, formal analysis, investigation, resources, data curation, writing (original draft preparation, review, and editing), visualization, supervision, project administration, and funding acquisition, all tasks done by the two authors. Both authors have read and agreed to the published version of the manuscript. The research work was funded by the Deanship of Scientific Research--Umm Al-Qura University, Makkah, Saudi Arabia, where the project was setup with funding No. 18-sci-1-01-0022. Not applicable. Data Availability Statement: Data are contained within the article. The authors acknowledge the support of the Deanship of Scientific Research-Umm El-Qura University-Makkah-Saudi Arabia where the project was set up with funding no.