Drift Waves in General Toroidal Equilibria

Abstract: One of the main concerns in fusion research is to understand the anomalously high transport in magnetically confined plasmas. In recent years, substantial progress in the understanding of transport in terms of drift waves in fusion plasmas has been achieved. It is at present an important issue to investigate the stability of drift waves in realistic toroidal geometries.

Among the drift wave candidates for explaining the anomalous transport are the toroidal .äta._i-modes (.äta._i = L_n /L_Ti or ITG driven modes) in the core and the resistive .äta._i -modes and the resistive ballooning modes in the edge.

The effects of plasma shaping on magneto-hydrodynamic (MHD) modes have been thoroughly studied. However, the effects of plasma shaping on the drift waves are not well known. Empirically it is found that the overall effects of elongation on the energy confinement time is favorable with .tau._E .propto. .kappa.^0.5.

In this thesis, the .äta._i -mode and the resistive edge mode stability in a non-circular tokamak geometry are studied. In particular, the effects of elongation and Shafranov shift are studied. In the core plasma a destabilization of the .äta._i-mode with increasing elongation is found whereas a stabilization is found in the edge region (or rather for peaked density profiles).

Moreover, a comparison of the .äta._i growth rates in the tokamak and stellarator equilibria is made. The growth rates for the tokamak and stellarator cases are comparable whereas the modulus of the real frequency is substantially larger in the stellarator. In addition, a stronger stabilization of the ITG mode growth is found for large .epsilon._n(= L_n /R) in the stellarator case.

Finally, an analytical estimation of zonal flow generation including effects of elongation is presented. The results suggest that a strong excitation of zonal flows is obtained for peaked density profiles and close to marginal stability.

However, in order draw more detailed conclusions of the effects of elongation on the global confinement time, a more extensive study using predictive transport simulations, which treats the edge and core transport processes self-consistently will be needed.