Phase folding of magnetic signals in tokamak plasmas

The magnetic signals detected by the poloidal Mirnov probe arrays in \mbox{DIII-D} indicate that the phase of the signals routinely exhibits strong reversal, or `phase folding', over the entire inboard region of high beta plasmas. The magnitude of the reversal can be up to 2π in phase. This phenomenon appears paradoxical as only helical MHD modes with a single handedness exist in fusion tokamak plasmas. This paper shows that the superposition of poloidal harmonic components of rotating helical modes can produce such phase foldings under the proper conditions. Strong phase foldings observed in magnetic signals from tokamaks are possible due to toroidicity, plasma shaping, change in measurement orientations, and non-uniform distances between the signal source and the measurement locations. These introduce poloidal modulation to the amplitude of each helical component, and can induce apparently `opposite' helicity components at the measurement \nobreak location. Phase foldings of poloidal Mirnov array signals can also occur from multiple helical components of constant amplitudes. Thus, phase foldings can even occur in circular cylindrical tokamaks. The strong phase foldings observed by a poloidal Mirnov array in DIII-D tokamak plasmas are verified numerically with highly localized plasma displacements on the outboard due to the high beta effect and result from a superposition of many MHD components with a kink-type ballooning structure. Although it is premature to interpret detailed plasma behaviour responsible for phase foldings, this paper resolves the seemingly paradoxical phase folding phenomenon observed in high beta tokamak plasmas, and may provide a better analysis of MHD behaviour in the future.