# Characteristics of 2.45 GHz Surface-Wave-Sustained Argon Discharge for Bio-Medical Applications
## 2 Materials and Methods  
### Experimental  
The experimental set-up is schematically presented in Figure 2a. A simple surfatron-type wave launcher is used for plasma sustaining. The surfatron (Sairem, SURFATRON 80) is connected with a solid-state microwave generator at 2.45 GHz (Sairem, GMS 200 W) by a coaxial cable. The plasma is produced inside the discharge tube situated at the surfatron axis. Various quarts tubes with different outer and inner diameters (Table 1) are used. For all of them, the real and imaginary parts of the dielectric permittivity are \(\varepsilon_{\mathrm{r}}=5.58\) and \(\varepsilon_{\mathrm{i}}=0.036\), respectively. The discharge tubes end is fixed at 2 mm out of the surfatron, and the plasma torch is formed out of the discharge tube in the air. The working gas is argon 5.0 (purity of 99.999%), and its mass flow is controlled by Omega FMA-A2408 mass flow controller. The system is vertically installed with the gas flow from top to down.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Tube Notation** & **Outer Diameter** & **Inner Diameter** \\ \hline
8/2 & 8 mm & 2 mm \\
8/3 & 8 mm & 3 mm \\
8/4 & 8 mm & 4 mm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Outer and inner diameters of quarts tubes used.  
As it was mentioned above, the propagating surface wave produces strongly non-equilibrium plasma not only at low and intermediate, but also at atmospheric pressure. Under such conditions, the assumption that the translational (gas) temperature is equal to the rotational temperature cannot be used a priori. This is shown for other types of non-equilibrium discharges in [45] and discussed in detail in [46]. Approaching carefully this problem, it is shown in [25] that the rotational temperatures calculated from the nitrogen second positive 0-2 band is in the interval 1500-3000 K, while it is between 600 K and 1000 K when using the OH (A \(\rightarrow\) X) 0-0 band. No information about the gas temperature of the plasma torch is obtained in this way. In [25], the maximum temperature on the treated sample (fresh pork skin, 10 s treatment) measured by a thermo camera when the plasma torch tip touches the sample is about 110-120 \({}^{\circ}\)C. Although the area on the sample surface with such a temperature is very small and the average temperature is lower, such conditions are not acceptable for bio-medical applications.  
It is also observed that around the plasma filament in the discharge tube, there exists hot argon gas, which flows outside close to the plasma torch. Its temperature is usually higher than that of the plasma torch, but it does not radiate in the visible region and is not registered by optical emission spectroscopy.  
Because of the above reasons, the plasma temperature is obtained by two independent methods: contact thermometry at the plasma tip and via thermal emission measured by Mica Plate and an infrared thermo camera system. (Figure 2b). The used contact thermometer is a mercury thermometer with fused quartz with a range of up to 400 \({}^{\circ}\)C. The temperature determined in this way is the average temperature of the plasma column and of the gas surrounding the plasma at the position where the treated sample usually is placed.  
We used a Testo 865 thermal imager for the infrared thermography of a standard Mica Plate inclined at 45 degrees to the plasma torch axis and the camera-to-plate center direction. The distance between the plasma column end and Mica Plate was 29 cm and distance of the camera-plate was 55 cm. The ambient temperature was 21 \({}^{\circ}\)C, and the relative humidity was 48%. The method of infrared thermography determined the average plasma jet temperature, ignoring the carrying gas temperature.  
Microwave radiation measurements from the Surfatron device were done by half-wave dipolar antenna attached to HF ANALYSER HF59B by Gigahertz Solutions(r) or wattmeter \(\Omega\)2M-66. Measurements were done with use of 20 dB attenuator and antenna at the level of the plasma column end.