# Characteristics of 2.45 GHz Surface-Wave-Sustained Argon Discharge for Bio-Medical Applications
## 1 Introduction
The electromagnetic waves in the microwave region may also have unacceptable biological effects, which need further investigation. For the direct treatment of biological samples, the plasma must come out of the discharge tube, i.e., we need a plasma torch (Figure 1). The surface-wave argon plasma torch can be produced in a gas flow regime by increasing of the wave power so that the electromagnetic surface wave propagation along the plasma-dielectric interface continues along the plasma-air boundary. The part of the plasma outside the tube is not an afterglow, but an active discharge region with high enough wave power for sustaining plasma, which leads to a higher temperature. At the same time, for bio-medical application, it is important to keep the gas temperature of the sustained plasma below 40 \({}^{\circ}\)C.  
We found some regimes of operation allowing a microwave plasma torch to be applied for the treatment of biological systems without thermal damage. The investigations show high bactericidal [24] and virucidal [26] effects at very short treatment time (5-30 s). This treatment time is much shorter than the typical 2-5 min of MicroPlaSter [23]. Promising results are also obtained for the decontamination of seeds, plants, and fruits at a similarly short treatment time by a microwave plasma torch [28; 29; 30]. Some preliminary experiments  
Figure 1: Plasma torch produced by a surfatron wave launcher. on wound treatment on mice [27] also stimulate the investigations in these directions. The reactive oxygen and nitrogen (RONS) species important for bio-medical plasma effects are produced, even when the working gas is pure argon as a result of the interaction of the plasma torch with ambient air [25]. The biggest problem of the microwave plasma torches sustained by a traveling wave that needs systematic investigation is that by using the same (low) wave power and gas flow, in some cases, the temperature at the plasma torch tip is low enough to be touched (Figure 1) and in other cases, can be above 100 \({}^{\circ}\)C [25]. In some conditions, only one stable filament exists, while at the same wave power and gas flow, a second filament can appear. Another important difference between the surface wave plasma torch and the MicroPlaSter device [23] is connected with the microwave field used for the plasma ignition. The microwave electric field in MicroPlaSter is applied between electrodes inside a metal container, and the plasma is transported outside the active region by the gas flow. In this way, the treated sample is prevented from the microwave radiation, but only long-living active plasma particles can be used for the treatment process. The surface wave plasma torch is produced by an electromagnetic wave traveling along the plasma with the wave power decreasing along the torch. At low and intermediate pressures, the end of the plasma column is at a position where the wave power becomes zero and the wave cannot propagate anymore. At high and atmospheric pressures, the wave power for sustaining plasma is much higher, and the plasma column ends when the wave power is not enough for sustaining plasma, but usually is not zero. The plasma used for the samples treatment is a part of the active discharge region with all short- and long-living particles produced in the plasma (which can be the reason for obtaining good results at very short treatment time), but also the microwave field there may be too high. It is important to investigate the microwave radiation at the position of the treated sample in order to avoid any possible damage during the treatment. The purpose of this work is to investigate the parameters of the surface-wave-sustained plasma torch and to find the well-reproducible discharge conditions at which the plasma is appropriate for bio-medical applications. The dependance of the plasma torch length, the temperature of the treated samples, the microwave radiation at the samples on the wave power and discharge tube geometrical parameters are investigated and presented in the paper.