Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part I. Measurements of Magnetic Islands and Delta Prime
Authors: Z. Chang, E.D. Fredrickson, S.H. Batha, M.G. Bell, R.V. Budny, F.M. Levinton, K.M. McGuire, G. Taylor, M.C. Zarnstoff, and TFTR Group
Tearing-type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from the electron cyclotron emission (ECE) diagnostic, and an experiment where the plasma major radius is rapidly shifted ("Jog" experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing-mode eigenfunction mapped from the jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory.