## 2 Materials and Methods
### Plasma-Chemical Reactor and Plasma Source  
The plasma treatment process was investigated in a volume reactor consisting of a metal case lined inside with a layer of refractory and heat-insulating materials, as described in [17; 26]. Such a reactor is suitable for the destruction of extremely hazardous solid substances owing to the relatively long exposure time of the material in a high-temperature zone. The authors also designed a novel experimental plasma volume reactor to create a steady non-transferred plasma environment (Figure 2). The shield of this reactor is made of metal with a high-temperature ceramic lining. It consists of a flat graphite plate used as the anode and a cooling PT that works as a cathode. A cooling graphite flat plate was mounted with thermal sensors and thermocouples with an external diameter of 4 mm. The reactor's body and graphite plate were then grounded. During waste processing, a  
Figure 2: The scheme of the plasma-chemical reactor. 1—raw material input, 2—plasma torch, 3—graphite plate (anode), 4—cooling flow inlet, 5—Chromel–Alumel thermocouple, 6—cooled probe for gas sampling. PT generates the high-temperature flow necessary for destroying the chemical bonds in hazardous materials [13; 18]. The highly intense energy flux initiated by PT is powerful enough to degrade or detoxify hazardous waste into atoms, molecules, and radicals. Combined heat and mass transfer occur in the electric arc plasma generator owing to the interaction of the arc, gas, and walls. When the electric current flows into an ionized gas, the electric arc energy transforms into heat, which is transferred to the flow, channel walls, and electrodes. However, only the energy transferred to the flow was beneficial. Generally, the arc length depends on the construction of the PT and processes occurring in the reaction chamber. It is most affected by the phenomenon of electric discharge, called arc bypass, which can be significant (discharge occurs between the arc column and the anode wall) or negligible (discharge occurs between individual parts of the arc). During high-amplitude shunting, the parameters of the electric arc and, simultaneously, the gas flow change and instability increase. However, in the presented reactor, the arc length is regulated by changing the distance between the plasma source and graphite plate, that is, between the anode and cathode. It can be achieved by employing a specific mechanical device. The gas entering the reaction chamber flows inside the PT into the arc, and the gas temperature increases, which increases the thickness of the positive column of the arc in the direction of gas flow. During the intensive cooling of the PChR, the arc cannot reach its wall, so there is always a layer of electrically non-conductive gas between the wall and the arc. When broken, it undergoes bypass processes that determine the length of the arc. The current density in the arc was \(10^{4}\)-\(10^{7}\) A/m\({}^{2}\), that in the anterior part of the arc was \(10^{8}\)-\(10^{10}\) A/m\({}^{2}\), and that in the anodic part was \(10^{6}\)-\(10^{9}\) A/m\({}^{2}\). The length of the pre-cathodic zone of the atmospheric pressure plasma arc was approximately \(10^{-6}\) m, which is equal to several particle-free path lengths. It indicates that the thermoelectric emission of electrons from the cathode to the gas and the PChR wall is possible. The PT hafnium cathode and copper body were intensively cooled using water (Figure 3). The sensors were electrically insulated from the anode by fluorine-plastic liners and ceramic backfilling. The anode moved across the arc; thus, the sensors measured the heat flux distribution along the spot of the arc. The values of the PT parameters used in the experiments are listed in Table 1.  
The heat generated by the PT brings the waste material to temperatures that are sufficient to melt and destroy it entirely. An experimental plasma torch was constructed to determine the energy transfer peculiarities from the free electric arc to waste.