## 3 Experimental Results on Plasma Jet  
With increasing voltage amplitude, the discharge development in helium exhibits four steps. First, at a small voltage, luminous spots on a tip of the powered electrode appear. Then a glow spreads over the gap between the powered electrode and capillary insert. With further increasing \(U\), the glow propagates inside the capillary and finally at some critical voltage \(U_{cr}\) the plasma jet starts to propagate beyond the dielectric tube over the inert gas flow in surrounding atmosphere. We found that in argon \(U_{cr}\) is essentially higher than in helium for the same gas flow rate v and diameter of capillary insert d. For example, for v = 2 L/min and d = 2.3 mm, \(U_{cr}\) is 2.6 kV for helium and \(U_{cr}\) = 3.5 kV for argon. Note that \(U_{cr}\) does not depend on the flow rate for \(d>2.3\) mm. The plasma jet length can be up to 60 mm in helium and only 20 mm in argon. An optimal geometry for the argon plasma jet is with the capillary placed at the edge of dielectric nozzle. The experimental study of plasma jet generation with the planar one slit design source (see Figure 1b) shows the CAP formation picture which is similar to the coaxial geometry case. The feature of the planar design is that the presence of a multi-tip structure on the plane powered electrode is critical for the plasma jet formation. Please note that the critical voltage of CAP jet generation in helium for the planar source \(U_{cr}\approx 6\) kV, which is much higher than \(U_{cr}\) for the cylindrical design. The developed plane design of the CAP source allowed us to enlarge the zone exposed to the plasma jet. However, in our experiments with the cancer cell treatment discussed below we used the cylindrical plasma device to provide better uniformity of irradiation of wells with the media and cells without touching the plastic edge of wells. In Figure 3, the waveforms of the voltage and current are shown for helium working gas. It is seen that the current increases at a positive half-wave of voltage as was observed in Ref. [4]. For all considered conditions of the CAP generation the discharge current does not exceed 10 mA. The examples of typical emission spectra of helium and argon CAP are shown in Figure 4. In addition to helium and argon lines in the plasma jet, there are lines of molecular nitrogen N\({}_{2}\), molecular nitrogen ions N\({}_{2}^{+}\), and nitric oxide NO. In the UV range, weak O\({}_{2}\) and O\({}_{2}^{+}\) lines are observed, as well as the Balmer series of the hydrogen line. Figure 3: Discharge voltage and current with time in helium for \(U\) = 5 kV. As seen in Figure 4, hydroxide OH appears in both spectra of helium and argon CAPs. In our experiments, the OH-radical production with CAP jet over the plasma-target interface was studied for the cases with and without the grounded substrate beneath the plate with the media and cells. As discussed below in Section 4, the presence of the grounded substrate must amplify the plasma-target interaction. The spectra were measured in gas phase near the interface of plasma and liquid media. A variation of the intensity of OH-peak at \(\lambda\) = 309 nm in spectra was recorded for different voltages and gas flow rates. Please note that the peak at \(\lambda\) = 309 nm is always dominant in a group of OH-lines over our range of plasma parameters. A part of spectrum with OH-lines and the peak at 309 nm is shown in Figure 5 for the cases with and without the grounded substrate for \(U\) = 4.8 kV. It is seen that the intensity of OH-lines unexpectedly increases by an order of magnitude after the installation of the grounded substrate. An arrow in Figure 5 shows the OH-line at \(\lambda\) = 309 nm used for our analysis. In the experiment, the spectra were measured within a range of the voltage from 2.5 kV to 6.5 kV. The intensity of the OH-peak depending on the voltage is given in Figure 6a. This intensity and consequently, the production rate of OH-radicals start to increase quickly for \(U>4\) kV.