The influence of rotating and locked magnetic islands on edge transport in Tokoloshe tokamak.

Abstract

The turbulence and fluctuation induced transport in the edge plasma of the Tokoloshe tokamak was studied using a Langmuir probe array. In this thesis three separate experiments are presented, each of which examines a particular aspect of the edge plasma. In the first experiment measurements of edge plasma parameters are presented. These include standard parameters (such as Ne, Op , Te, etc.) as well as features such as the velocity shear, T(t) during periods of both high and low Mirnov activity, Te/Te and Q. These are compared with results from other machines as well as predictions of several turbulence theories. It was found that many of the results are very similar to those obtained on other machines and that, since the operating parameter space on Tokoloshe is well within the parameter space described by drift wave theories, resistivity-driven gradient driven turbulence theories do not describe the edge turbulence. In the second experiment external windings are used to produce fields which can slow and lock magnetic islands in the toroidally rotating plasma. Edge parameters are again presented and these results compared with those from the so-called 'reference' plasmas, i.e. ones in which no locking occurred. During locking some parameters are dramatically altered, e.g.Te/Te Standard transport theory ignores the effect of Te/Te since they are usually small in reference discharges. During the locked phase, however, certain measurements used to deduce T and Q are greatly affected by increases in Te/Te. As a result, certain assumptions regarding these measurements are no longer valid. Comparison of results for different island positions (produced by different coils) indicates that the assumption of poloidal and toroidal symmetry of edge conditions is invalid. The third experiment investigates the high frequency (~60 kHz), low amplitude, magnetic oscillation which characterises the locked phase and which exhibits some small degree of correlation with the fluctuations observed on (e.g.) Of'. Since over 80% of the spectral power of Te/Te lies below 70 kHz and since Of /Te depends strongly on Te/Te , it is suggested that the magnetic mode and these large variations in Te, may be due to a similar physical process.