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 arisesfrom 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 EOMPclass shows unusual magnetic phenomena including up/down asymmetry inpoloidal intensity variation that cannot be generated by MHD modes alone. Thecontributions of MHD modes in plasma edge regions are too small toexplain the magnitude of the observed EOMP perturbations. At leasttwo-thirds, and possibly nearly all, of the magnetic perturbations in atypical EOMP originate from sources other than MHD modes. An EOMP hasa unity toroidal harmonic number and a poloidal harmonic number closeto the q value of the discharge edge. It produces insignificant temperaturefluctuations, except possibly in edge regions. The KLM class producestemperature fluctuations, mostly confined within the q = 1 surface withan ideal-mode-like structure, but generates only insignificant external magneticperturbations. The TM class generally conforms to expectations fromMHD modes. It is proposed that current flowing in the scrape-off layer(SOL) plasma is a possible origin of EOMPs.
Multimode observations and 3D magnetic control of the boundary of a tokamak plasma