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Theory of magnetic islands in tokamaks with accenting neoclassicaltearing modes

Key aspects of existing theory of magnetic islands in tokamaks are overviewed. The threshold models of neoclassical tearing modes (NTMs) are discussed. It is found that though the NTM theory is often used for interpreting MHD data observed in experimental devices and for designing next‐step machines, the existing NTM theory is imperfect and the use of it is questionable. The main reason for such conclusion is a tangle of problems related to polarization current effect. Analytical approaches in the magnetic island theory are presented.