SAWTEETH AND TRANSPORT IN TOKAMAKS

Sawtooth activity occurs in ohmically heated tokamak plasmas even at high values of edge safety factor. This is not consistent with the predictions of transport calculations in which the resistivity is classical and the thermal conductivity is uniform. In that case q falls to unity in the centre only if the edge q is less than 3.3. Sawteeth can persist to higher values of edge q if the neoclassical enhancement to resistivity is included or the thermal conductivity increases with radius. In JET, sawteeth persist to higher values of q than would be expected even using neoclassical resistivity demonstrating that the thermal conductivity increases with radius.