Cathodic arcs: Fractal voltage and cohesive energy rule

The noise of the burning voltage of cathodic arcs in vacuum was analyzed for a range of cathode materials (C, Mg, Ti, V, Ni, Cu, Zr, Nb, Ag, Hf, Ta, W, Pt, Bi, and Si). Cathodic arcs were generated in a coaxial plasma source, and the voltage noise was measured with a broadband measuring system. Each measurement was analyzed by fast Fourier transform, revealing a power spectrum $∼1∕f2$ for all cathode materials over several orders of magnitude of frequency $f$ (brown noise). The absence of any characteristic time down to possible cutoffs at high frequencies supports a fractal model of cathode spots. The amplitude of colored noise scaled approximately linearly with the cathode’s cohesive energy, which is another manifestation of the cohesive energy rule.