Ohmic H-modes in the TCV tokamak

The TCV tokamak has obtained ohmic H-modes in virtually all diverted plasmas with the ion ∇B drift directed towards an X-point and in several elongated limiter plasmas. Troyon factors (βtor''alpha'' B/Ip) up to 2 and line average densities up to 2.2 x 1020 m-3, corresponding to the Greenwald limit, have been obtained in diverted ELM-free H-modes. Quasi-stationary H-modes lasting for the entire current flat top (1.5 s) have been obtained in the presence of regular ELMs. The occurrence and magnitude of ELMs have been found to depend on configurational parameters such as the position of the 'active' X-point in unbalanced double-null discharges and the plasma-wall separation in single-null discharges. These dependences have permitted active control of ELM behaviour in TCV. A continuous spectrum of ELM amplitudes and frequencies has been observed, ranging from clearly identifiable type III ELMs to large, low-frequency ELMs which expel up to 12% of the stored energy and up to 7% of the particle content and are reminiscent of type I ELMs. A previously unknown, benign kind of ELM, with a maximum amplitude in the divertor region, has also been observed. (author)