Topic/Type: 1.2 Fusion Plasmas (magnetic & inertial confinement), Invited
D. L. Shmelev1, T. Delachaux2
1 Institute of Electrophysics, Russia
2 ABB Switzerland Ltd Corporate Research, Switzerland
The paper focuses on the numerical modeling of constricted vacuum arcs driven by a transverse magnetic field (TMF). Such moving arcs are found in vacuum circuit breakers and in vacuum arc rail guns. The arc velocities range from several hundreds of meters per second in the case of interrupters up to several kilometers per second in the case of rail guns.
The present model describes the behavior of only constricted arcs during high current ( kA) arcing. The average arc current density for such kind of arc estimated from experimental observations exceeds A/cm. The resulting constriction of the arc is strong enough so that the heat flux to the arc-electrode attachment areas quickly heats the metal surface under the foot points up to temperatures exceeding the evaporation temperature. The metal vapor enters the inter-electrode gap, where it is ionized to provide the conducting medium for the current.
A 2D model to simulate the motion of high current constricted arcs under the influence of a TMF was developed. The model uses the MHD approach, together with the radiation transfer equation in the P1 approximation. An attachment model, developed especially for this purpose, showed that the cathode connection of the constricted arc can exist in the form of a solid extended area instead of multiple cathode spots. The location and extension of the attachment zones are determined from the surface temperature distribution. The attachment zones are defined as the areas of dominant electron and vapor emission at the cathode and the anode, respectively.
The model reveals two modes of arc motion. The first mode is a drift mode, which occurs when the arc is moving smoothly, approximately preserving the plasma column shape. The second mode is a step-wise mode, which appears when TMF exceeds a certain value. In this mode the new attachment zone appears at certain distance away from the old one and the arc motion resembles the series of jumps or steps.
Average arc velocities determined by numerical simulations vary from hundreds to thousands of meters per second, in reasonable agreement with experimental findings.
The work of the first author was partially supported by Russian Fundamental Research Foundation under awards No. 09-08-00178.