## 1 Introduction  
Nowadays, to obtain qualitative permanent joints of heterogeneous materials the methods of welding in a solid state are widely used. The most prevalent of these is a diffusion bonding. The wide nomenclature of compounds creates a complex of specific requirements for diffusion bonding's energy sources. These requirements are mainly related to the acceptability of a wide range of materials and shapes of products, the accuracy of the specific heat capacity control and the ability of the wide regulation of the sample's temperature [1]. The distributed plasma of a glow discharge burning in a rarefied gas atmosphere at a pressure of 0.1-10 kPa is widely used in processes accompanied by the direct action of charged electric particles on the treated or welded materials. Known works consider the possibility of application of gas discharges technique in the field of thin films deposition, metals surfacing, treatment and modification of metals before welding [2, 3, 4, 5]. Still, as practice has shown, diffusion welding [6] and thermal and chemical-thermal surface treatment [7] are the most appropriate. This is due to the high technological capabilities of the glow discharge, which in these processes can serve both a processing tool and a source of thermal energy for their implementation simultaneously. Additionally, a glow discharge has high technical, economic, and environmental indicators, for instance, high heating productivity, energy savings, and last but not least the absence of environmental pollution [8]. However, during surface treatment or welding of metals, the instabilities of samples temperature (discharge cathode in these processes), the pressure of the working gas, the voltage of the power supply, and a number of other factors, can lead to the transition of a glow discharge into another form of gas discharge--an electric arc [9] (Figure 1). In this case, the distributed glow discharge transforms into a contracted form. Its cathode spot narrows to a very small size, increasing the energy concentration dramatically. A prolonged action of a concentrated arc discharge inevitably leads to the local melting and destruction of samples. The probability of arcing increases with the rising of the total and specific power of the discharge. The energy characteristics of the ionic treatment processes are mainly determined by the magnitude of the discharge current and the gas pressure in the working chamber. The development of processes' productivity leads to the necessity of increasing their values. That, in turn, entails rising of average current density (j) in the cathode spot of the discharge and the average specific volumetric power (jE) in the discharge plasma [10]. However, with increasing of discharge energy characteristics in the interelectrode gap, the short-term local arc breakdowns with a duration of one or two half-periods of the rectified current can form (Figure 2). Figure 1: The view of the glow discharge on the cathode surface in stable (**a**) and unstable (**b**) burning modes during the ion treatment (the bright spots to the right—arc breakdowns). In [11; 12], the glow discharge's instabilities are closely associated with its contraction (compression) and transition to a cord form of discharge with the increasing of volumetric power (JE) accordingly. The gas in the cord discharge heats up extremely while the burning voltage decreases and eventually the cord switches into an arc electric. Such a mechanism of glow discharge's instability is associated with volumetric processes in positive column of the discharge plasma, and therefore is more characteristic of extended discharges of the laser type [13], where the length of the interelectrode gap is 0.5-1.0 m. In the processes of ion treatment and welding of metals, a stationary DC glow discharge burns between the electrodes with a limited distance of 0.005-0.05 m. In these conditions, processes in the near-electrode regions of the discharge can affect negatively its stability.