# Characteristics of 2.45 GHz Surface-Wave-Sustained Argon Discharge for Bio-Medical Applications
## 1 Introduction
Another type of plasma source operating at atmospheric pressure and a microwave frequency of 2.45 GHz, the surface-wave-sustained discharge (SWD) was investigated for possible applications in biology, medicine, agriculture, and the food industry [24; 25; 26; 27; 28; 29; 30]. The specific way of plasma produced and sustained by an electromagnetic wave traveling along the plasma-dielectric interface (surface wave) determines the features of these discharges. Surface-wave-sustained discharges operating at low and intermediate pressure have already a successful history after several types of wave launcher were introduced by [PERSON] in 1974 [31; 32; 33; 34; 35]. The good stability and reproducibility of these electrodeless discharges in a wide range of discharge conditions are very important characteristics for applications and the reason for intensive investigation during the past decades. A detailed review of these investigations can be found in [36; 37]. For these discharges, the operating frequency can vary from MHz to GHz. The discharge tube diameter, thickness and dielectric permittivity play a very important role for the characteristics of the plasma sustained inside it because the tube is not only a container for sustaining the plasma, but is an important part of the waveguide structure for the electromagnetic wave propagation. The plasma itself is also a part of the same waveguide, and its properties can be easily controlled by appropriate selection of the discharge tube geometric parameters and permittivity. The plasma density (electron number density) decreases almost linearly from the wave launcher to the plasma column end. The plasma can be much longer that the size of the wave launcher, and its length depends on the wave power. The theoretical and experimental investigations of SWD at this period are at low and intermediate gas pressure [37; 38]. It is shown that at these discharge conditions, the plasma is a strongly non-equilibrium one: the electron energy distribution function (EEDF) is non-Maxwellian; because of this, the electron temperature \(T_{\rm e}\) is defined as 2/3 of the mean electron energy \(<\)_u\(>\)_ obtained from the calculated EEDF (\(T_{\rm e}\) = 2\(<\)_u\(>\)_/3); and the electron temperature is much higher than the gas temperature (the temperature of the heavy particles in the plasma), \(T_{\rm e}\)\(>>\)\(T_{\rm g}\). The situation is more complicated for SWD operating at atmospheric pressure. A stable atmospheric pressure plasma inside the discharge tube or going outside it (plasma torch) can be sustained by high wave power, which leads to high gas temperature. In [39], stable argon plasma with an electron density of about 3 \(\times\) 10\({}^{20}\) m\({}^{-3}\) and gas temperature of 2500 K is produced by 70 W wave power at 915 MHz and gas flow from 0.2 to 17 L/min. It is mentioned there that at a wave power above 200 W, the surfatron wave exciter needs external water cooling. The further surface wave plasma torch development is directed toward devices operating at high power and with high gas temperature (4000-8000 K) for applications in cutting, welding, material processing [40], carbon dioxide elimination [41] and many others [42]. Being electrodeless, the corrosion of electrodes and plasma contamination is avoided, and a precise, clean composition of working gases and gas mixtures can be achieved. Producing a stable and well-reproducible surface-wave plasma torch at low wave power with low gas temperature is a tricky challenge. The filamentation in pure argon is one of the problems at atmospheric pressure SWD [43]. By adding a small amount of nitrogen, the filamentation can be eliminated [25; 44] but the gas temperature of the Ar + N plasma is much higher than is appropriate for bio-medical applications. It is possible to obtain a single filament discharge in pure argon when the discharge tube is with a small radius, but this requires a higher wave frequency in the microwave range, e.g., 2.45 GHz.