Temperature Gradient Instabilities and Transport in Toroidal Plasmas
This thesis focuses on temperature gradient driven modes and their implications for heat and particle transport in toroidal plasmas. Some of the consequences of including an ion impurity species in a model for low frequency electrostatic modes and fluctuation driven transport are discussed. A comparison between JET profile data and the predictions of a transport model, based on toroidal ion-temperature-gradient (ITG) modes and collisionless- trapped-electron (CTE) modes, is made.
The stability of the toroidal main and impurity ITG-modes is determined by using a higher-moment fluid description. The impurity effect on the background mode is found to be stabilizing for a large domain of parameters. However, the instability threshold for the impurity ITG-mode can be lower than that for the main ITG-mode. The effect of dissipative trapped electrons is investigated and found to change the stability thresholds.
The particle and thermal diffusivities are estimated, using quasi- linear theory and a saturation level approximation, and the scaling properties are studied. The magnitude of the ion thermal conductivity is shown to decrease due to the stabilizing influence of impurities. The radial variation of the predicted transport coefficients in a two-ion-component plasma is considered for some experimental equilibrium TEXTOR profiles. Impurity ions tend to diffuse outwards for typical tokamak conditions. The predicted and experimental heat fluxes i JET discharges are compared, using TRANSP to obtain the latter.
Magneto-hydro-dynamic (MHD) ballooning modes are studied in gyro-kinetic and reactive or dissipative fluid descriptions.