## 7 Conclusions  
The typical cold atmospheric plasma jet device with 40 kHz-driven AC voltage of 2.5-6.5 kV amplitude and with the cylindrical and plane designs were developed and tested. The inert gas (helium or argon) was pumping through the plasma device with the rate of 1-5 L/min. The cylindrical plasma device was used for the treatment of cancer cells in vitro.  
In this work, for the first time, the grounded substrate was installed beneath the plate with the cells and media for the intensification of the plasma jet treatment. Previously, a strong dependence of the plasma jet characteristics near the surface of targets on their permittivities was reported (see, for example, Refs. [7; 8]). The metal grounded target was shown can essentially intensify the plasma jet characteristics near the surface. In the experiments and 2D fluid model simulations, the interaction of CAP jet with (a) the bio-target placed on the grounded substrate and (b) electrically isolated bio-target without the metal substrate have been studied and compared. The presence of the grounded substrate was shown to lead to the considerable increase of the electric field over the plasma-media interface. More elevated electron energy and density and OH-radical production rate were registered with the grounded substrate compared to the case without it.  
A range of optimal voltage and gas flow rate values for the plasma device for the efficient OH generation was obtained from the measurements of plasma spectra over the plasma-target interface. The intensities of OH-lines at \(\lambda\) = 309 nm were compared for different plasma jet characteristics and with and without the grounded substrate. When the plate with the media was placed on the grounded substrate, the OH-peak began to increase quickly at U \(>\) 4 kV and with further voltage it became 10 times higher than the OH-peak without the substrate. So, the voltage amplitude from 4.5 kV to 5 kV is optimal for the purpose of the OH-production. Please note that for a lower voltage, U \(<\) 3.5 kV, the occurrence of the substrate weakly affects the OH-peak in spectra. The measured effect of gas flow rate on the intensity of OH-line at \(\lambda\) = 309 nm showed that the gas flow rate ranging from 2.5 L/min to 4 L/min is optimal for the OH-radical production.  
A drastic effect of the occurrence of the grounded substrate beneath the plate with cancer cells and media have been observed in the measurement of viability of cancer cells after the indirect and direct CAP treatment. The time-dependent viability of human cell lines, A431 (skin carcinoma), HEK 293 (kidney embryonic cells) and A549 (human lung adenocarcinoma cells) was monitored with MTT and iCELLigence assays after the plasma treatment with different CAP jet parameters. It was shown that the direct cancer cell treatment with CAP jets with the optimal plasma parameters and the grounded substrate visibly increased the impact of CAP.  
Conceptualization, I.S.; Methodology, D.Z., I.S. and O.K.; Software, I.S.; Validation and Investigation, D.Z., O.K., I.S., P.G., E.G., O.T. and E.Y.; Writing-Original Draft Preparation, I.S.  
The authors gratefully acknowledge financial support from Russian Science Foundation, grant N 19-19-00255. The authors, [PERSON] and [PERSON], were partly supported financially by RFBR N 18-08-00510 for development of various designs of experimental setup.  
The authors declare no conflict of interest.