Fourier Analysis of Fast Vacuum Arc Parameters

Vacuum arcs are known for their noisy character: all parameters show more-or-less rapid fluctuations. This is of particular concern to those who would like to utilize the exceptional properties of the vacuum arc plasma in ion sources and other devices. Among these properties are the very high degree of ionization and the presence of multiply ionized species. The rapid fluctuations are known to be ultimately caused by the explosive nature of the perpetual spot ignition and plasma production. We present a systematic application of the fast Fourier transform to the fluctuating arc voltage and current. New possibilities have arisen with the advent of fast digital oscilloscopes and low cost computation of large amounts of data. We show that the spectral power density (SPD) scales with a power law in log-log presentation for all frequencies below 50 MHz, supporting a fractal description of the cathode spot phenomena below this limit. At higher frequencies, the result is less clear. The fractal behavior seems to be cut off as indicated by material dependent fluctuations which are above the level found for the power law at lower frequencies. This might be associated with approaching the region of individual explosive processes, also known as ectons. However, the nonlinear behavior of the SPD contains peaks that are the same for different materials, indicating that resonances in the measuring circuit play a role, too. Hence, fractal behavior may be possible for frequencies faster than 50 MHz