Investigation of the parameters of a nonstationary arc spot on a copper cathode by the thermospectroscopic method. I. Current density

Abstract

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...

Arc spot formation conditions and influencing factors of a micro multi-electrode technology

“…At high current (up to several kA) and long discharge distance, several cathode spots exist simultaneously. Bublievskii and Kesaev [17] found that when the current exceeded a threshold current, the single arc spot split into multiple cathode spots of branch-like character. Martin et al [18] studied the reason for the movement of the split spots.…”