# Characteristics of 2.45 GHz Surface-Wave-Sustained Argon Discharge for Bio-Medical Applications
## 3 Results
### Temperature at the Sample Position  
For this investigation, the argon flow rate is varied from 2 L/min to 15 L/min for the three tubes. The temperature is measured by a contact thermometer and by the Mica Plate and infrared thermo camera system. Figure 8 illustrates the latter experiments and obtained images used for the temperature estimation at an argon flow rate of 10 L/min. Similar images are obtained at all the other discharge conditions.  
Figure 7: Axial distribution of argon atomic ion Ar\({}^{+}\) (balck lines) and two molecular ions Ar\({}_{2}\)\({}^{+}\) (red lines) and Ar\({}_{3}\)\({}^{+}\) (blue lines) along the plasma column in the three discharge tubes: (a) 8 mm/2 mm; (b) 8 mm/3 mm; (c) 8 mm/4 mm. The dots correspond to the same wave power as in Figure 5a.  
Figure 9 demonstrates the effect of the discharge tube inner diameter on the temperature at fixed gas flow rate. In Figure 9a, the gas flow rate is 5 L/min and in Figure 9b it is twice higher, 10 L/min. The temperature measured by the contact thermometer is always higher than that obtained by the IR camera. We observe that the temperature of the plasma is lower than the temperature of the not ionized Argon in the tube and ambient gas outside it. By the contact thermometer, the average temperature of the plasma and ambient gas is measured, while infrared thermography determines only the average plasma temperature, ignoring the carrying gas temperature.  
Both methods show that the lower temperature is obtained by using the discharge tube with a bigger inner diameter, i.e., 8 mm/4 mm. For this tube, even at 20 W wave power, the temperature does not exceed 45 \({}^{\circ}\)C, measured by the contact thermometer. Similar results are obtained for all the other gas flow rates, i.e., the best performance is shown for the 8 mm/4 mm discharge tube. More detailed dependance of the temperature on the gas flow rate for the 8/4 discharge tube is shown in Figure 10.  
Figure 8: Images from Mica Plate and infrared thermo camera system.  
Figure 9: Temperature measured at the position of the sample as function of the wave power for three inner diameters of the discharge tubes. Solid symbols and lines are the results obtained by the IR camera and the empty symbols and dot lines by contact thermometer. The argon flow rate is fixed at (**a**) 5 L/min; (**b**) 10 L/min.  
At a higher gas flow rate, the temperature is lower, but this effect is less than 5 \({}^{\circ}\)C at a low wave power and less than 10 \({}^{\circ}\)C at 30 W. Comparing the results presented in Figures 9 and 10, one can conclude that for decreasing the temperature at the sample, it is more important to choose a discharge tube with a bigger inner diameter than to increase the gas flow and gas consumption. In addition, if the sample is solid, its surface is not much affected by the high gas flow, but this is not the case when the treated sample is liquid. In the latter case, the gas flow cannot be too high. For the discharge tube of 8 mm/4 mm, the temperature below 45 \({}^{\circ}\)C can be achieved by a gas flow rate of 5 L/min or 8 L/min at a wave power of up to 20 W, which could be the optimal operational regime.