Tokamak plasmas with dominant electron cyclotron heating; evidence for electron thermal transport barriers

In the Rijnhuizen Tokamak Project [RTP, A. J. H. Donné, the RTP Team, Plasma Phys. Rep. 20, 192 (1994)], experiments have been carried out with Electron Cyclotron Heating (ECH) at four to seven times the Ohmic power, with a deposition width <10% of the minor radius. Central deposition leads to sharp electron temperature (Te) gradients near the q=1 surface. Off-axis deposition results in steady-state hollow Te profiles. A radial scan of the ECH deposition reveals a discrete, stepwise response of Te profile. The transitions occur for a displacement of the deposition by less than 1 mm. Each transition is due to the loss of a transport barrier. The transitions occur when the minimum value of q passes through the values 1, 4/3, 5/2, 2, 7/2 and 3. A numerical model featuring transport barriers near those q values gives good reproduction of the experimental data. The possible role of the topological properties of the magnetic field is discussed.