Nonlinear wave decay of the rotating spoke oscillation in a partially magnetized plasma

Abstract

The rotating spoke oscillation is observed in experiments on a planar direct-current magnetron discharge. Previous studies on the same device found that the spoke rotation direction can reverse when the current increases. At low currents, the spoke disturbances propagate opposite the E×B drift direction. At higher currents, the spokes reverse to propagate in the E×B direction. In between, a chaotic state appears without coherent spokes. In this paper, we use higher-order spectral analyses to study the evolution of the oscillation spectrum in this intermediate state. The bispectrum shows a phase coherence between modes that points to nonlinear three-wave coupling processes. We solve for the quadratic power transfer functions using two-point measurements along the plasma circumference. The results show a period doubling process that takes place in the transition from a coherent spoke to a chaotic spectrum. Nonlinear wave interactions spread energy over a broad range of frequencies to form sidebands. Recovery of coherent +E×B spoke modes coincides with a reversal of the direction of quadratic power transfer back into the main frequency mode.