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Results from MAST concerning the effect of toroidal rotation driven by neutral beam injection (NBI) on sawteeth are presented. The sawtooth period is shown to increase as the co-NBI power, and thus the toroidal plasma rotation, is increased. As the counter-NBI power is increased, the sawtooth period decreases to a minimum that is shorter than in Ohmically heated plasmas, before lengthening at high toroidal flows. The sawtooth period reaches a minimum when the sawtooth precursor changes the direction of rotation. This can be interpreted as the counter-NBI inducing a toroidal flow which balances the intrinsic ion diamagnetic rotation of the plasma, at which point the mode precursor changes direction. Magnetohydrodynamic stability analyses of the ideal n = 1 internal kink mode with respect to toroidal rotation at finite ion diamagnetic frequency show good accordance with the experimental results. When MAST discharges are modelled at the experimental ion diamagnetic frequency, , the ideal n = 1 internal kink mode is stabilized completely by co-rotation approaching the ion sound speed. When the rotation is oriented in the counter- direction, the mode is initially driven more unstable, before being stabilized by high toroidal flows. Experimentally, the radial extent of the sawtooth crash, and hence the q = 1 surface, varies with respect to the sawtooth period. Stability analyses show that the marginally stable radial location of the q = 1 surface reaches a minimum at approximately the same counter-toroidal rotation as that which minimizes the sawtooth period experimentally. The magnitude of the toroidal rotation at the q = 1 surface determines the stabilizing effect of the toroidal flow upon the ideal n = 1 internal kink mode.

Joint experiments have been conducted on JET and DIII-D to study m = 2, n = 1 neo-classical tearing modes (NTMs). Very similar instability behaviour is observed on both machines and the scaling of the mode island width with β and the observation of n = 1 islands at q = 2, strongly points to these modes being driven by neo-classical effects. Identity experiments using closely matched non-dimensional parameters (plasma shape, aspect ratio, q, ρ* and collisionality) on JET and DIII-D show similar global β-limits for the NTM. Further scalings, which combine broader data sets from JET and DIII-D, show an approximately linear scaling, or slightly stronger, with ion gyro-radius and a relatively weak dependence on collisionality.

The theory of the marfe stability in a plasma is generalized by using the equilibrium temperature as the independent variable. The description incorporates both perpendicular and parallel transport

A new scenario to delay or prevent neoclassical tearing mode (NTM) onset is presented. By active sawtooth destabilization, short period and low amplitude sawteeth are generated, such that the sawtooth produced NTM seed island is reduced and the threshold normalized plasma pressure for triggering of NTMs, beta(Nonset), is increased. The scenario has been explored experimentally in the Joint European Torus (JET). Ion cyclotron resonance frequency (ICRF) waves tuned to the 2nd harmonic H-minority resonance have been used for sawtooth control. Whereas ICRF waves generally induce sawtooth stabilization, favouring the triggering of NTMs and reducing beta(Nonset), the present experiments show that by toroidally directed waves, ion cyclotron current drive is produced, and that sawteeth can be destabilized by careful positioning of the 2nd harmonic H resonance layer with respect to the sawtooth inversion radius. As a result, NTM onset is delayed and beta(Nonset) is increased above its value obtained in discharges with additional heating from neutral beam injection alone.

The transport, flow shear, and linear growth rates of microturbulence are studied for a Joint European Torus (JET) plasma with high central q in which an internal transport barrier (ITB) forms and grows to a large radius. The linear microturbulence growth rates of the fastest growing (most unstable) toroidal modes with high toroidal mode number are calculated using the GS2 and FULL gyrokinetic codes. These linear growth rates, gamma (subscript lin) are large, but the flow-shearing rates, gamma (subscript ExB) (dominated by the toroidal rotation contribution) are also comparably large when and where the ITB exists.

Linear stability properties of the ideal internal kink mode in toroidal geometry are calculated for equilibria in which the q(r) profile is very flat and q ≈ 1 in the core region of the plasma. Marginal stability criteria and growth rates are calculated analytically in the large aspect ratio limit and compared with numerical results from the FAR code. The theory is developed for m = n = 1 modes and for higher m (= n) modes. The temperature perturbation due to adiabatic expansion in the linear phase of the 1/1 mode is calculated and compared with experimental data, showing that the predictions from ideal MHD theory cannot account for the observed temperature increase on JET. Comparison with collisionless theory suggests that the observed temperature increase can be accounted for by compression of the trapped particles.

The non-linear development of the m = 0 resistive 'g' mode has been studied in the reverse-field pinch. A saturation process is shown to exist in which the perturbations cause a quasi-linear flattening of the equilibrium pressure near the m = 0 resonant surface. The mechanism for this flattening is discussed.