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Investigation of arc roots of constricted high current vacuum arcs

The anodic and cathodic arc roots of constricted high current vacuum arcs were investigated with a fast framing CCD camera of 1 /spl mu/s exposure time. The experiments were performed on cup-shaped contacts, with sinusoidal currents of amplitudes of 20-100 kA and a halfwave duration of 11.5 ms. The arcs were drawn by contact separation, and accelerated by the Lorentz force between the arc current and the transverse magnetic field generated by the contrate contact. Both types of arc roots are elliptical, with a major to minor axis ratio of 1.4. The major axis points in the direction of arc propagation. Anodic and cathodic arc root cross-sectional areas as a function of current can both be described by a potential law with a common exponent of 0.76. For a current of 100 kA, mean current densities of 121 and 60 kA/cm/sup 2/ were found in anode and cathode arc roots.