Cathode spot velocity in tangential magnetic field on cathode of copper-chromium composition in vacuum

Abstract

The paper presents measurement results for cathode spot (CS) velocity V under action of a tangential magnetic field on a clean surface (refined by preceding arcing) of the cathode made of cupper-chromium (CuCr30) composition in vacuum. Measurements were performed in the range of currents I = (30 -250) A in arcs of different lengths, h = 2 mm and 4 mm. The induction of tangential magnetic field varied in the range B = (0.02 -0.15 ) T. The dependence of the CS velocity on the magnetic field induction was found to be linear: V = K·B. The constant K depends on the arc current for currents I  2I 0 . (where I 0 is the current per spot at B = 0). The maximum value of K (at I > 2I 0 ) depends on the arc gap length. These results are similar to the results obtained for CS on cathodes made of pure metal, but are at variance with the result of the work by Y.H. Fu (1990) for the CuCr25 cathode. The obtained results are compared with similar results obtained previously in the experiments with different pure metals: Cu, Mo and W      

Revisiting Theoretical Description of the Retrograde Motion of Cathode Spots of Vacuum Arcs

“…Assuming as characteristic values R = 10 μm and t = 50 ns, one obtains 2R/t = 400 m s −1 . This value is much higher than speed of the retrograde motion observed in the experiment, which normally does not exceed 20 − 30 m s −1 , e.g., see [25]. Hence, the second case is the realistic one, that is, the distribution of probability is nearly isotropic.…”

“…On the other hand, a may be determined empirically. For example, the measurements [25] have given v r = 260B 0 m s −1 T −1 for the following conditions: I = 65...245 A, B 0 0.08 T, CuCr30 cathode, and the interelectrode gap of 2 mm. Assuming t = 50 ns as above, one finds a = 1.3 × 10 −5 m T −1 for these conditions.…”

A Simple Model of Distribution of Current Over Cathodes of Vacuum Circuit Breakers

“…Here, B t is the net tangential magnetic field (the superposition of the selfinduced and external tangential magnetic fields) and k 1 and k 2 are coefficients that depend on the cathode material. Although k 1 and k 2 may also depend on other parameters, in particular, on the state of the cathode surface, the arc current, and the interelectrode gap [11], this dependence was not taken into account in the modeling reported in this work and it was set k 1 = 230 ms −1 T −1 , k 2 = 19 ms −1 ; values corresponding to CuCr cathodes at an arc current of 30 A [29]. Note that the above-mentioned values of the coefficients D, k 1 , and k 2 correspond to CuCr cathodes with the usual surface roughness; fine effects, like the effect of nanostructured layers [19], remain beyond the scope of this work.…”