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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 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.
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.
Numerical studies of the nonlinear evolution of MHD-type tearing modes in three-dimensional toroidal geometry with neoclassical effects are presented. The inclusion of neoclassical physics introduces an additional free-energy source for the nonlinear formation of magnetic islands through the effects of a bootstrap current in Ohm's law. The neoclassical tearing mode is demonstrated to be destabilized in plasmas which are otherwise[Delta]' stable, albeit once an island width threshold is exceeded. The plasma pressure dynamics and neoclassical tearing growth is shown to be sensitive to the choice of the ratio of the parallel to perpendicular diffusivity ([Chi][parallel]/[Chi][perpendicular]). The study is completed with a demonstration and theoretical comparison of the threshold for single helicity neoclassical MHD tearing modes, which is described based on parameter scans of the local pressure gradient, the ratio of perpendicular to parallel pressure diffusivities[Chi][perpendicular]/[Chi][parallel], and the magnitude of an initial seed magnetic perturbation.
Numerical studies of the nonlinear evolution of coupled magnetohydrodynamic - type tearing modes in three-dimensional toroidal geometry with neoclassical effects are presented. The inclusion of neoclassical physics introduces an additional free-energy source for the nonlinear formation of magnetic islands through the effects of a bootstrap current in Ohm's law. The neoclassical tearing mode is demonstrated to be destabilized in plasmas which are otherwise[Delta][prime] stable, albeit once a threshold island width is exceeded. A possible mechanism for exceeding or eliminating this threshold condition is demonstrated based on mode coupling due to toroidicity with a pre-existing instability at the q= 1 surface.
Linear tearing instability is studied in the banana collisionality regime in tokamak geometry. Neoclassical effects produce significant modifications of Ohm's law and the vorticity equation so that the growth rate of tearing modes driven by .delta.' is dramatically reduced compared to the usual resistive MHD value. Consequences of this result, regarding the presence of pressure-gradient-driven neoclassical resistive interchange instabilities and the evolution of magnetic islands in the Rutherford regime, are discussed. 10 refs.
MSE measurements reveal hybrid-like flux-pumping associated with 2/1 NTM-ELM coupling. Analysis of MSE signals using digital lock-in amplifiers shows the strength of the flux-pumping is more than twice that of typical hybrid discharges. This flux-pumping maintains the minimum safety factor above unity, thereby avoiding sawteeth. The strength of the flux-pumping and ELM-NTM coupling have a clear upward dependence on normalized beta and NTM-pedestal proximity. The size of the island does not appear to effect flux-pumping, except that the mode must be present, suggesting the island chain serves as a radial pivot surface around which poloidal flux is pumped from the core to the edge. This result implies that higher normalized fusion performance (lower q95 and higher beta) may be achieved in hybrid discharges that contain a partially suppressed 2/1 NTM. ELM-NTM coupling consists of an Alfvénic timescale drop in the island width followed by a resistive recovery. The recovery phase is successfully modeled using the modified Rutherford equation. The depth of the drop in island width increases as the size of the ELM increases. To aid in the design of a highly resolved MSE pedestal measurement, full spectral analysis was preformed on existing edge channels. This analysis has revealed that coherent core MHD oscillations cause interference with present dual PEM polarimeters. Avoiding this interference requires a dedicated pedestal polarimeter with second harmonic frequencies greater than those of MHD fluctuations.