A new procedure of determination of the effective density of the current in a nonstationary arc spot with the use of thermophysical and spectroscopic measurements has been proposed and tested. The procedure is based on recording of the critical cathode temperature corresponding to the instant of sharp increase in the intensity of the CuI λ = 5218 A° atomic spectral line, which coincides with the beginning of intense emission of a copper vapor from the spot, according the hypothesis proposed. New results are compared to those obtained earlier by purely thermophysical methods.Introduction. The density of the current in an arc spot is one of the most important parameters determining the erosion and service life of the electrodes in electric-arc gas heaters (EAHs) and other arc devices. One traditionally calculates it from the data of measurements of the current and the dimensions of the arc spot, taking the symmetry of the latter to be circular, i.e., j = 4I/πd 2 . Two methods are usually used to measure the diameter: those of autographs and high-speed photorecording. In the autograph method, the dimension of the emitting region of the spot is taken to correspond to the size of the craters left by it, whereas in the method of high-speed recording, it is taken to correspond to the dimensions of the luminous region of the cathode plasma [1, 2].As has been revealed with the modern technique of optoelectronic recording with a high temporal and spatial resolution, the arc spot on a cold cathode has a very dynamic structure and consists of a large number of individual short-lived microspots that have a complex hierarchic structure and are in continuous motion. Moreover, as the recording technique is improved, this hierarchy increasingly expands toward revealing the finest details of the internal microstructure. Such a situation makes determination of the current-conducting zone of the spot very difficult and dependent on how the situation observed corresponds to the dimension of the zone [2]. In [3], an attempt was made to measure the average dimension of this zone by moving the spot slowly through an insulating gap of a special current sensor divided by this gap into two halves in recording a change in the current in each half. However, the presence of such a gap can substantially distort the shape and dimensions of the conducting zone; therefore, this method applies only to specific conditions of slowly moving spots with a size much larger than the gap size, because of which Szente et al. used it for a plasma cutting arc. In [4,5], traversal of a special linear magnetic sensor by the spot instead of the traversal of an insulated slot was used for such measurements in an electric-arc unit with a magnetic movement of the arc, which introduced no disturbances into the spot itself. However, such a method gives only the linear current density, which is difficult to convert to the real one without knowing the spatial current distribution. Therefore, we proposed that the concept of the effective current density in the arc spot...

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Indirect data on the features of the dynamics of a cathode spot in different erosion regimes have been obtained by investigation of the fluctuations of the intensity of radiation from the arc of the 5218 A° CuI copper spectral line modulated by the rotational frequency in a magnetic field. It has been revealed that the velocity of the cathode spot in the regime of intense radiation of this line in macroerosion is always lower than that in the regime of weak radiation in microerosion, whereas the variance of its distribution is, conversely, higher. A hypothesis for the relationship between the line-radiation intensity and the occurrence of a cathode copper-vapor jet affecting the mobility and dynamics of the spot and, in terms of them, the erosion has been proposed.Introduction. As has been reported in [1], to determine the density of the current in a cathode arc spot by the thermospectroscopic method we measured the intensity of the radiation of the 5218 A° CuI spectral line of copper from the arc rotating in a coaxial gap in a magnetic field. The beginning of an intense radiation of this line was taken as the beginning of macrofusion in the spot and the beginning of macroerosion, i.e., an intense form of erosion, instead of the low-intensity one called "microerosion." In [2,3], in experiments on a vortex electric-arc heater (EAH), An'shakov et al. observed the outflow of high-power cathode jets from the spot in passage of erosion from the lowintensity form to an intense one; these jets destroyed the structure of the vortex flow and disturbed regular rotation of the arc. This disturbance of the vortex stabilization of discharge was considered by An'shakov et al. to be predominantly responsible for the erosion becoming intense. Since we recorded the rotational velocity of the arc in both regimes -before the increase in the intensity of the copper spectral line and after it, which, presumably, corresponded to the regimes of micro-and macroerosion, the results of such recording also had to contain information on the features of the arc motion in them in the total absence of vortex stabilization of the discharge. To obtain this information we carried out special processing of the experimental results.Experimental Procedure. The experiments were carried out on a coaxial electric-arc unit with magnetic movement of the arc and axial feeding of a gas to the discharge zone. The unit had been described earlier (see [4,5]) in detail; therefore, we do not give its description here. In these experiments, the image of the entire coaxial interelectrode gap with an electric arc moving in its circle was projected onto the section of a light guide, after which the light signal was fed to the entrance slit of a spectrometer. A small segment of the aperture of the interelectrode gap was covered by an opaque screen. Owing to this, the light signal was modulated by the rotational frequency of the arc in the gap.Applying the fast Fourier transformation in the Origin program to processing of the signal of the line intensity recorded wit...