Experimental study of cathode spot motion and burning voltage of low-current vacuum arc in magnetic field

Abstract

This experimental study dealt with cathode spot (CS) motion and the burning voltage of a low-current vacuum arc with electrodes of HCOF Cu and with a composition of CuCr30, subjected to a magnetic field. Arcs with current of I/spl ap/30 A burned in axial and transverse fields as well as with the induction vector inclined at an angle 5/spl deg/</spl phi/<65/spl deg/ to the cathode surface. Butted-end electrodes were used with diameter 30 mm and gap h=4 mm. The field components, axial B/sub n/ and transverse B/sub t/, were varied independently over a wide range. The superposition of a transverse magnetic field on the arc was found to considerably increase both the constant component and noise level of the voltage U. If this arc is also superposed with an axial field, both the constant component and noise level decrease. With growing B/sub n/, the dependencies of the U(B/sub n/) at different values of B/sub t/ converge to a single curve that is typically V-shaped. This is similar to the dependence of the voltage across a high-current vacuum arc with fixed current in an external axial magnetic field on the magnetic field induction. The velocity of the retrograde CS motion grows approximately in proportion to the growth of the transverse field. The proportionality factor is /spl sim/60 m/(s/spl middot/T) for Cu and /spl sim/200 m/(s/spl middot/T) for CuCr. Increase of the axial field causes a slight reduction in the magnitude of the velocity vector and its rotation with respect to the retrograde direction through an angle /spl theta/, its value being proportional to the field inclination angle /spl phi/. The results obtained were compared with the data in references.