Tokamaks: turbulence and first test of an ergodic magnetic limiter

The characteristics of the ubiquitous low-frequency turbulence in tokamaks have been examined in an extensive set of experiments using far infra-red scattering to measure the full spectrum, S(ksub(perpendicular),ω), of density fluctuations throughout the plasma in the TEXT tokamak and using sets of probes to measure the structure of both density and potential fluctuations at the edge of TEXT and continuing into the interior in the smaller Caltech tokamak. Scattering experiments reveal a broad frequency spectrum of fluctuations which show clear dispersion in ksub(theta), propagating in the electron diamagnetic direction with phase velocity approx. 2-3vsub(De), but no ksub(r) dispersion. The wavelength spectrum is peaked at ksub(perpendicular)rhosub(s)<0.5. The fluctuations show marked spatial asymmetries. The amplitude rises toward the outside; probes show n-tilde/n of 20-80% at the periphery. Probes reveal complex turbulent structures with statistical properties matching those from scattering which reverse apparent propagation direction at the limiter. Density and potential fluctuations are related to drive an outward radial flux. The Ergodic Magnetic Limiter (EML) on TEXT, created with eight modular coils which impose a helical magnetic perturbation, m/n, of 7/2 or 7/3, is designed to explore the effects of an EML and study heat transport in an ergodic layer. Experiments have demonstrated that a small resonant perturbation with a fractional amplitude of approx. 10-3 creates a stable ergodic magnetic layer and substantially modifies the heat flow pattern to the limiter. (author)