Experimental investigation on the characteristics of the plasma jet of a low-current vacuum arc in axial magnetic fields

Abstract

In this paper, the characteristics of the plasma jet of a low-current vacuum arc with a single cathode spot (CS) in an external axial magnetic field (AMF) up to 150 mT is investigated experimentally, at a constant arc current ranging from 20 A to 60 A. The experiments are conducted with Cu butt contacts in a demountable vacuum chamber. Images of the plasma jets are photographed with a high-speed digital camera with an exposure time of 2 μs. The uniform constant AMF (Bn) within the inter-contacts region is supplied by Nd–Fe–B permanent magnets. The influence of the external AMF on the shape of the jet near the anode surface as well as in the arc column is mainly investigated. A luminous ‘spot’ is observed on the anode surface facing the position of the CS under a relatively strong AMF. The mechanism of the appearance of the luminous ‘spot’ is proposed to be connected to the secondary plasma originating from the anode. Moreover, with the increase in the strength of the AMF, the spreading angle of the cone-shaped plasma jet in the arc–column region decreases gradually. The plasma jet, subjected to a relatively strong AMF (120 mT and 150 mT), becomes cylindrical in shape in the arc–column region and conical in shape in the near-electrode regions. The overall geometry of the plasma jet looks like a dumbbell.