ABSTRACT
The perturbed ion temperature and toroidal flow were measured in rotating neoclassical tearing modes (NTM) in a tokamak for the first time. These toroidally and radially resolved profiles were obtained by impurity ion spectroscopy in a 2,1 NTM in DIII-D. In agreement with drift-kinetic simulations, the electron temperature profile is flat, while the ion temperature gradient is restored across the magnetic island O point in the presence of fast ions; the perturbed flow has minima in the O points and maxima at the X points. These measurements provide the first confirmation of the theoretically expected ion temperature and flow response to a magnetic island needed to predict the NTM onset threshold scaling for ITER and other future tokamaks.
ABSTRACT
A new drift-kinetic theory of the ion response to magnetic islands in tokamak plasmas is presented. Small islands are considered, with widths w much smaller than the plasma radius r, but comparable to the trapped ion orbit width ρ_{bi}. An expansion in w/r reduces the system dimensions from five down to four. In the absence of an electrostatic potential, the ions follow stream lines that map out a drift-island structure that is identical to the magnetic island, but shifted by an amount â¼ few ρ_{bi}. The ion distribution function is flattened across these drift islands, not the magnetic island. For small islands, wâ¼ρ_{bi}, the shifted drift islands result in a pressure gradient being maintained across the magnetic island, explaining previous simulation results [E. Poli et al., Phys. Rev. Lett. 88, 075001 (2002)PRLTAO0031-900710.1103/PhysRevLett.88.075001]. To maintain quasineutrality an electrostatic potential forms, which then supports a pressure gradient in the electrons also. This influence on the electron physics is shown to stabilize small magnetic islands of width a few ion banana widths, providing a new threshold mechanism for neoclassical tearing modes-a key result for the performance of future tokamaks, including ITER.
