Unusual Low-frequency Magnetic Perturbations in TFTR
Authors: H. Takahashi, E.D. Fredrickson, and M.S. Chance
Date of PPPL Report: June 2001
Published in: Nuclear Fusion 42 (April 2002) 448-485
Low-frequency magnetic perturbations (less than or equal to 30 kHz) observed in the Tokamak Fusion Test Reactor (TFTR) tokamak do not always conform to expectations from Magneto-Hydro-Dynamic (MHD) modes. The discrepancy between observations and expectations arises from the existence of three classes of magnetic perturbations in TFTR: (1) 'Edge Originated Magnetic Perturbations' (EOMP's), (2) 'Kink-like Modes' (KLM's), and (3) Tearing Modes (TM's). The EOMP class has unusual magnetic phenomenon including up/down asymmetry in poloidal intensity variation that MHD modes alone cannot generate. The contributions of MHD modes in plasma edge regions are too small to explain the magnitude of observed EOMP perturbations. At least two-thirds, possibly nearly all, of 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 a discharge's edge q-value. It produces little 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 little external magnetic perturbations. The TM class conforms generally to expectations from MHD modes. We propose that current flowing in the Scrape-off-layer (SOL) plasma is a possible origin of EOMP's.