Investigation of neoclassical tearing mode detection by ECE radiometry in an ITER-like tokamak via asymptotic matching techniques

Abstract The TJ toroidal tearing mode code is used to make realistic predictions of the electron cyclotron emission (ECE) signals generated by neoclassical tearing modes (NTMs) in an ITER-like tokamak plasma equilibrium. In the so-called ‘outer region’, which comprises the bulk of the plasma, helical harmonics of the magnetic field with the same toroidal mode number as the NTM, but different poloidal mode numbers, are coupled together by the Shafranov shift and shaping of the equilibrium magnetic flux-surfaces. In the ‘inner region’, which is localized in the vicinity of the NTM rational surface, helical harmonics whose poloidal and toroidal mode numbers are in the same ratio as those of the NTM are coupled together nonlinearly to produce a radially asymmetric magnetic island chain. The solutions in the inner and outer regions are asymptotically matched to one another. The asymptotic matching process determines the overall magnetic structure of the NTM, as well as the global perturbation to the electron temperature caused by the mode. A simulated ECE diagnostic is developed that accounts for the downshifting and broadening in frequency of the signal due to the relativistic mass increase of the emitting electrons. It is estimated that ECE can reliably detect NTM island chains in ITER whose widths exceed 1% of the plasma minor radius (in the absence of noise).