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 beta (N) for NTM is found to be proportional to the normalized ion gyrorad ius rho* and independent of the normalized collisionality nu* for a wide ra nge of nu*. This scaling is in accordance with both polarization current an d chi perpendicular to/chi parallel to model if for the latter the heat flu x limit on parallel heat conductivity is taken into account. Analysis of th e structure and dynamics of NTMs validates the negative Delta'. Complete st abilization using ECCD has been demonstrated at beta (N) = 2.5 and with abo ut 10% of the total heating power. The results are in good qualitative agre ement with modelling using the Rutherford equation and in quantitative agre ement with a two dimensional non-linear cylindrical tearing mode code. A pr ecise positioning of the ECCD microwave beam, achieved so far by feedforwar d variation of B-t, is required for efficient stabilization.