## 3 Results  
### Visual and Thermal Imaging  
The plasma torch length is proportional to the applied MW power, as demonstrated by Figure 2.  
The total length of the plasma torch is an important parameter with respect to the treatment because of active particles' distribution [32]. The low limit of power convenient for the plasma treatment is the plasma torch length: plasma outside the discharge tube should be at least 4-5 mm long in order to be in contact with the sample. The increase of MW power leads to the extension of the torch and the dependence on the torch length is directly proportional to the applied power (see Figure 2). However, the wave power increase also influences the gas temperature of the plasma torch, which increases accordingly. The upper limit of power is determined by the maximum temperature achieved at the sample surface. To determine this parameter, infrared camera measurements were carried out. In this way, the minimum wave power for the fruit treatment is set to 5 W and the maximum power is 9 W. (In some cases 5 W is not enough power for good contact between the plasma torch and the sample; 7 W is the lower power limit in these experiments.) The infrared camera images of cherries' and blueberries' surface temperatures during the plasma treatment and a short period after it are presented in Figures 3 and 4, respectively. The treated surface was positioned at the tip of the plasma torch (i.e., 9 mm away from the quartz capillary end), perpendicular to the torch axis. The maximum temperatures for both fruits investigated during the plasma treatment are presented in Figure 5, where the average value of 10 pieces treatment is shown. It is clearly visible in both cases that the temperature of the treated fruit at the contact point with the plasma torch increases and the affected area with the elevated temperature increases with the treatment time duration. The total thermally affected area is comparable for both fruits, but due to the smaller volume of the blueberries in comparison with the cherries it is bigger with respect to the total fruit surface. The maximum temperature of the treated area achieved on the cherries' surfaces is lower than the temperature on the blueberries' surfaces (10 to 15 \({}^{\circ}\)C), while the initial temperature was more or less the same. This can be expected, keeping in mind that the blueberries' mass is smaller than the cherries' mass, while the thermal energy transferred from the plasma to each fruit is almost the same for a fixed treatment time and power. The orientation of the fruits and their sizes do not play a role in heating generally, while a reflection of the gas flow from the Petri dish can be much higher and thus is a potential reason for the temperature increase. Additional measurements should be done to confirm this hypothesis. The surface temperature relaxes quite fast when the plasma treatment is finished. The highest temperature obtained is still low (obtained for a short time) to initiate any  
Figure 2: Visual plasma torch length as a function of MW power ((a) 5 W, (b) 7 W and (c) 9 W) for the fixed argon flow of 7 L/min with 4 L/min of air cooling. The torch length dependence on MW power in region of 4-12 W (**d**). The end of the quartz discharge tube is 2 mm outside the surfatron resonator and the presented length is measured with respect to its end. visually remarkable changes at the fruit surface. The fast surface cooling reflects the fact that there is little heating of the fruit volume. Figure 4: Thermal images of blueberries’ treatment at different times with MW power of 9 W, the argon flow of 7 L/min and the cooling air flow of 4 L/min. Treatment is realized by the edge of the discharge. Red triangle—maximal temperature; blue triangle—minimal temperature. Figure 3: Thermal images of cherries’ treatment at different times with MW power of 9 W, the argon flow of 7 L/min and the cooling air flow of 4 L/min. Treatment is realized by the edge of the discharge. Red triangle—maximal temperature; blue triangle—minimal temperature.