Coherent and turbulent fluctuations in TFTR

Classification of the sawteeth observed in the TFTR tokamak has been carried out to highlight the differences between the many types observed. Three types of sawteeth are discussed: ''simple'', ''small'', and ''compound''. During the enhanced confinement discharges on TFTR, sawteeth related to q=1 are usually not present, but a sawtooth-like event is sometimes observed. Beta approaches the Troyon limit only at low qcyl with a clear reduction of achievable βn at high qcyl. This suggests that a βp limit, rather than the Troyon-Gruber limit, applies at high qcyl in the enhanced confinement discharges. These discharges also reach the stability boundary for n → ∞ ideal MHD ballooning modes. Turbulence measurements in the scrape-off region with Langmuir and magnetic probes show strong edge density turbulence, n-tilde/n=0.3-0.5, with weak magnetic turbulence, B-tildeθ/Bθ>5x10-6, measured at the wall, but these measurements are very sensitive to local edge conditions. (author)