Turbulent transport and flow effects on NTM evolution and trigger mechanisms

Abstract. We study two problems related to the excitation and nonlinear evolution of neoclassical tearing modes in a tokamak, namely, (i) the effect of a background of microturbulence generated by short scale length instabilities such as the Ion Temperature Gradient (ITG) mode or the Electron Temperature Gradient (ETG) mode and (ii) the effect of a toroidal sheared flow on the stability of the m = 1 resistive internal kink mode that can provide the seed island trigger for an NTM. A background of ITG turbulence generates an anomalous viscosity whereas an ETG microturbulence leads to both anomalous current diffusivity and resistivity effects. The concommitant changes in the linear and nonlinear characteristics of a single helicity NTM are investigated analytically and expressed in terms of modifications of the Rutherford equations for the island evolution. The effects of the toroidal sheared flow on the resistive internal kink mode are investigated numerically using an initial value fully three dimensional toroidal code (NEAR) that solves a set of generalized reduced MHD equations. Shear flow is found to significantly modify both the linear growth rate and the nonlinear saturated island widths of the m = 1 mode.