Phase folding of magnetic signals in tokamak plasmas

Abstract

The magnetic signals detected by the poloidal Mirnov probe arrays in\\mbox{DIII-D} indicate that the phase of the signals routinely exhibits strongreversal, or `phase folding', over the entire inboard region of highbeta plasmas. The magnitude of the reversal can be up to 2π inphase. This phenomenon appears paradoxical as only helical MHD modeswith a single handedness exist in fusion tokamak plasmas. This papershows that the superposition of poloidal harmonic components of rotatinghelical modes can produce such phase foldings under the properconditions.Strong phase foldings observed in magnetic signals from tokamaks arepossible due to toroidicity, plasma shaping, change in measurementorientations, and non-uniform distances between the signal source andthe measurement locations. These introduce poloidal modulation to theamplitude of each helical component, and can induce apparently`opposite' helicity components at the measurement \ obreak location. Phasefoldings of poloidal Mirnov array signals can also occur from multiplehelical components of constant amplitudes. Thus, phase foldings can evenoccur in circular cylindrical tokamaks.The strong phase foldings observed by a poloidal Mirnov array in DIII-Dtokamak plasmas are verified numerically with highly localized plasmadisplacements on the outboard due to the high beta effect and resultfrom a superposition of many MHD components with a kink-type ballooningstructure. Although it is premature to interpret detailed plasmabehaviour responsible for phase foldings, this paper resolves theseemingly paradoxical phase folding phenomenon observed in high betatokamak plasmas, and may provide a better analysis of MHD behaviour inthe future.