The physics of neoclassical tearing modes and their stabilization by ECCD in ASDEX Upgrade

An understanding of the physics of neoclassical tearing modes (NTMs) gained through experiments and modelling at ASDEX Upgrade is presented. The onset βN for NTM is found to be proportional to the normalized ion gyroradius ρ* and independent of the normalized collisionality ν* for a wide range of ν*. This scaling is in accordance with both polarization current and χ⊥/χ|| model if for the latter the heat flux limit on parallel heat conductivity is taken into account. Analysis of the structure and dynamics of NTMs validates the negative Δ'. Complete stabilization using ECCD has been demonstrated at βN = 2.5 and with about 10% of the total heating power. The results are in good qualitative agreement with modelling using the Rutherford equation and in quantitative agreement with a two dimensional non-linear cylindrical tearing mode code. A precise positioning of the ECCD microwave beam, achieved so far by feedforward variation of Bt, is required for efficient stabilization.