## 4 Experimental Results  
### Vacuum Arc Behavior with Spiral-Type TMF Contacts  
Utilizing the experimental system described above, interruption experiments were carried out with currents of approximately 3.5 kA, using the type A and type B spiral-type TMF contacts. The typical vacuum arc appearance is shown in Figure 8 for a 2 ms arcing time. As shown in Figure 8a, at \(T=37.5\) us, a contracted arc developed, and the arc column diameter increased with the increase in the current. The arc in the contact anchored at the ignited point at the first arcing period of \(\sim\)0.1 ms. During this period, the arc columns went through an initial expansion process with a relatively brighter central core and the arc voltages increased to 27 V. At this early expansion stage, there were few or no cathode spots outside the central column. With the further expansion of the arc, one or two cathode spot groups were formed, with the cathode spots splitting from the arc column. Then, the cathode spots began to move around the arc column. The arcs moved, increasing the cross-sectional area, and new cathode spots were constantly formed, which moved to the edges of the contacts, driven by the ampere force. By the time \(T=1\) ms, the cathode spots had occupied almost the entire cathode surface; their diameter was approximately 3/4 of the diameter of the contact, and they did not expand further. This process was investigated in [29]. It was indicated that the arc column established between the separating electrodes (with the range of current limited to below 5 kA) and the observed columnar arcs became diffused. Thereafter, the movement of the cathode spots was not obvious. During this process, the arc was burning relatively stably. However, from \(T=1962.5\) us, the cathode spots moved to the edges of the slots of the spiral-type contacts, and began to burn intensively, developing into an intensive arc mode. With this locally intensive arc burning, large amounts of plasma and metal vapor were emitted into the contact gap, which is marked with an arrow in Figure 8a. When the arcing time was 2063.9 us, forced commutation was triggered, and the arc current began to decrease, flowing into the forced-commutation branch. With the decrease of the arc current, the brightness of the arc and cathode spots became less. At \(T=2108.2\) us, the arc current became zero. The bright cathode spots extinguished instantly, and the plasma began to diffuse between the contacts. At this time, the post-arc current reversed to \(I_{\mathrm{A}}\), while the plasma in the contact was not fully diffused. It could be seen that the plasma moved irregularly in the contact gap, and that its density became larger at the position of intensive arc burning. Figure 7: Vacuum interrupter with spiral-type TMF contacts. To better understand the formation of the intensive arc mode, tests were conducted with a type B contact for the same experimental parameters. A typical vacuum arc appearance was observed for the type B contacts under a 2 ms arcing time, and the results are shown in Figure 8b. The arc expanded gradually during the early stages and the arc voltage increased above 25 V. Then, with more cathode spots appearing outside the central core, a significant number of cathode spots began to appear outside the column, and they expanded to fill the cathode surface after a period of typically 0.17 to 1 ms. During this period, the arc transited to the column diffuse mode. After the diameter became the maximum, the arc started burning stably. Unlike the above-mentioned arc mode in the type A contact, no intensive arc was found in the whole arc period. The residual plasmas were obviously less after the current zero. The diameters of the diffuse columns at different current ratings in the DC-arcing stage are shown in Figure 9. In the early stages of column diffusion, the arc expands rapidly. The higher the current level, the faster the expansion will be. The motion of the cathode spot is the result of the interactions between the TMF and the self-generated magnetic field.