Drift waves in non-circular tokamak geometry

Licentiate thesis, 2000

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 tokamaks has been achieved. It is at present an important issue to investigate the stability of drift waves in realistic tokamak geometries. Among the drift wave candidates for explaining the anomalous transport are the toroidal etai-modes in the core and the resistive etai-modes in the edge. In this thesis, the etai 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. The effects of plasma shaping on magnetohydrodynamic (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 elonagtion is favourable. In the core plasma a stabilization of the etai modes with increasing elongation is found. It has been found that the spectrum of the unstable etai-modes is shifted toward shorter wavelengths with increasing elongation. In the edge (or rather for peaked density profiles) the effects of elongation on the etai-mode are, however, stabilizing. For edge parameters a stabilizing effect of elongation on the resistive modes is also found and the collisional effects on the etai-modes are rather weak. It is shown that the effects of ion temperature fluctuations on the resistive ballooning modes are stabilizing and this may further enhanced by elongation effects. In particular, it is found that the resistive ballooning mode is stabilized by finite Larmor radius effects already for etai < 3. However, to draw conclusions of the effects of elongation on the confinement time a more extensive study using a transport code which treats the edge and core processes self-consistently is needed.