Abstract
A review is given on recent results on electrode spot plasmas in electric arcs, in particular for arc cathode spots at medium currents. Ignition and nature of the spots are described for various gas pressures and electrode temperatures. Experiments with high resolution in space and time yielded new insight into the nature of the spot plasma, viz. parameters, structures, configurations, and ignition processes. The paper starts with a description of electrode spots in gaseous arcs with special attention to the glow-to-arc transition. For the latter it is shown that anode spots precede the cathode spots in the glow phase, thus creating favourable conditions for field electron emission at the cathode due to enhanced plasma density. After that, high resolution pictures of cathode spots in vacuum are presented that demonstrate micrometer substructures and configuration changes on time-scales of 10 ns − 100 μs. Also, the influence of magnetic fields is treated, including the retrograde cathode spot motion at reduced pressures.
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