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 ...

Based on three-dimensional simulations of the Braginskii equations we show that for typical plasmaedge parameters the saturation of electromagnetic toroidal hi mode turbulence is controlled by the selfgeneration and subsequent annihilation of radial magnetic field perturbations. This should be contrasted with the electrostatic limit, where the growth of the linear hi mode is terminated by the o...

Linear properties of the reverse shear Alfvén eigenmode (RSAE) in a well-diagnosed DIII-D tokamak experiment (discharge #142111) are studied in gyrokinetic particle simulations. Simulations find that a weakly damped RSAE exists due to toroidal coupling and other geometric effects. The mode is driven unstable by density gradients of fast ions from neutral beam injection. Various damping and driv...

MHD properties of low-aspect-ratio (low-A) reversed field pinch (RFP) plasmas have been studied in RELAX machine with aspect ratio of 2. The MHD behavior appeared in edge magnetic field fluctuations shows strong dependence on toroidal field reversal. In standard or deep-reversal plasmas, the edge magnetic fluctuation has broad toroidal mode spectrum, while it is narrowed by reducing the toroida...

Global gyrokinetic particle simulation and nonlinear gyrokinetic theory find that electron temperature gradient sETGd instability saturates via nonlinear toroidal coupling, which is a nonlocal interaction in the wave vector space that transfers energy successively from unstable modes to damped modes preferentially with lower toroidal mode numbers. The electrostatic ETG turbulence is dominated b...

The linear stability of high-toroidal-number drift-ballooning modes in tokamaks is investigated with a model that includes resistive and viscous dissipation, and assumes the mode frequency to be comparable to both the sound and diamagnetic frequencies. The coupled effect of ion drift waves and electron drift-acoustic waves is shown to be important, resulting in destabilization over an intermedi...

The plasma response to resonant magnetic perturbation RMP and nonresonant perturbation fields is computed within a linear, full toroidal, single-fluid resistive magnetohydrodynamic framework. The response of resonant harmonics depends sensitively on the plasma resistivity and on the toroidal rotation. The response of nonresonant harmonics is not sensitive to most of the plasma parameters, excep...

Spontaneous toroidal rotation of impurity ions has been observed in the core of Alcator C-Mod plasmas with no external momentum input. The magnitude of the rotation ranges from −60 km/s (counter-current) in limiter L-mode discharges to +140 km/s (co-current) in ICRF heated H-mode plasmas. The core rotation in L-mode plasmas is generally counter-current and is found to depend strongly on the mag...

Recent observations of magnetic fluctuation activity in the Mega Ampere Spherical Tokamak (MAST) reveal the presence of plasmas with bands of both low and high frequency magnetic fluctuations. Such plasmas exhibit a spectrum of low frequency modes with adjacent toroidal mode numbers, for which the measured frequency is near the Doppler shifted rotation frequency of the plasma. These are thought...

The gyrokinetic toroidal code (GTC) capability has been extended for simulating internal kink instability with kinetic effects in toroidal geometry. The global simulation domain covers the magnetic axis, which is necessary for simulating current-driven instabilities. GTC simulation in the fluid limit of the kink modes in cylindrical geometry is verified by benchmarking with a magnetohydrodynami...