Unusual low frequency magnetic perturbations in the TFTR tokamak

Low frequency magnetic perturbations (⩽30 kHz) observed in the Tokamak Fusion Test Reactor (TFTR) tokamak do not always conform to expectations from magnetohydrodynamic (MHD) modes. The discrepancy between observations and expectations arises from the existence of three classes of magnetic perturbation in TFTR: (1) edge originated magnetic perturbations (EOMPs), (2) kink-like modes (KLMs) and (3) tearing modes (TMs). The EOMP class shows unusual magnetic phenomena including up/down asymmetry in poloidal intensity variation that cannot be generated by MHD modes alone. The contributions of MHD modes in plasma edge regions are too small to explain the magnitude of the observed EOMP perturbations. At least two-thirds, and possibly nearly all, of the magnetic perturbations in a typical EOMP originate from sources other than MHD modes. An EOMP has a unity toroidal harmonic number and a poloidal harmonic number close to the q value of the discharge edge. It produces insignificant temperature fluctuations, except possibly in edge regions. The KLM class produces temperature fluctuations, mostly confined within the q = 1 surface with an ideal-mode-like structure, but generates only insignificant external magnetic perturbations. The TM class generally conforms to expectations from MHD modes. It is proposed that current flowing in the scrape-off layer (SOL) plasma is a possible origin of EOMPs.