## 2 Materials and Methods  
### Testing Electrode System of the Nonthermal Plasma Jet  
Tests of several electrode systems (single-barrier discharge with needle electrode, double-barrier discharge and glow discharge) to produce nonthermal plasma flows showed high efficiency and electrical safety of a double-barrier discharge (DBD) electrode system design (Figure 1). High voltage applied to the ring electrodes 2 ignites the gas discharge plasma inside the tube 1, and the gas flow 6 pushes the plasma out in the form of a jet 5. Two power supplies were developed to compare the short-pulse and sine excitation regimes of inert gas. The short-pulse power supply was equipped with a MOSFETs-based bridge inverter and a TR1 step-up transformer (Figure 2a). The transistors switched the DC line voltage to the primary winding of TR1 so that high-voltage pulses (2 \(\upmu\)s duration)  
Figure 1: Electrode system of plasma-jet-based DBD: 1—glass pipe, 2—ring electrodes, 3—pulsed power supply, 4—gas discharge, 5—plasma jet, 6—gas flow. with long pauses and alternating polarity were generated at the electrodes. Sine voltage power supply is a standard resonant generator (Figure 2b), where the L2-C6-TR2 circuit of elements works as a resonant circuit. Transistors VT5 and VT6 have a duty cycle of 50%, at a frequency close to that of the resonant circuit. The high efficiency and long lifetime (up to 8000 h) of XeCl* and KrCl* UV radiators driven by DBD have been well described [12; 13]. These results are obtained due to high-frequency short-pulse excitation [14] and a special quartz bulb processing technology [15]. The electrode voltage and gas discharge current were measured with a PPE6 KV high-voltage sensor (LeCroy Corp.), a 50 Ohm shunt resistor, and a TEKTRONIX TDS2024C oscilloscope. Helium flow rate did not affect the voltage and current oscillograms. The flow rate was controlled by a PVQ13 proportional valve and PFMV530 flow sensor (SMC Corp.). Plasma gas temperature was measured with a thermocouple thermometer. At the first stage, we tested the influence of tube diameter, ring electrode length, and interelectrode gap on gas temperature. Optimal parameters of the electrode system in terms of minimum values of plasma jet gas temperature were the same for both short pulse and sine voltages: 5.5 mm tube diameter (glass thickness 0.5 mm), 3 mm width of ring electrodes, and a 5 mm interelectrode gap. A decrease in helium flow rate led to an increase in gas temperature of the plasma jet (Figure 3). A particularly rapid increase in gas temperature was recorded at the flow rate of less than 1 L/min. However, for excitation with short pulses, this value was much smaller. Thus, for 0.5 L/min, gas temperature was 34 \({}^{\circ}\)C for short-pulse excitation and 53 \({}^{\circ}\)C for sine excitation. The dependence shown in Figure 3 was obtained at the frequency of 33 kHz and excitation power of 4.2 W for both excitation methods. The excitation power was calculated using the technique described in [16]. Voltage and current of plasma jet at short pulse excitation and at sine voltage excitation shown in Figure 4.  
With plasma excitation by microsecond pulses of alternating polarity it was possible to obtain a diffuse plasma jet without contracted channels (Figure 5a). In this case, the gas temperature of the plasma jet was 34 \({}^{\circ}\)C at a helium flow rate of 0.5 L/min. Obviously, the larger the contact area of plasma with the treated surface, the less time it will take to treat it. In order to increase the diameter of the plasma flow, a multi-channel electrode system was tested. To implement this system, electrode systems absolutely identical to the construction demonstrated in Figure 1 were installed parallel to each other in one housing case (Figure 5b). The electrodes were supplied from a single power source. By means of a splitter, the helium flow was supplied to the channels at the same flow rate. The tests showed that all channels of the plasma jets could operate in parallel without affecting each other.