OHMIC H-MODES IN THE TCV TOKAMAK

The TCV tokamak has obtained ohmic H-modes in virtually all diverted p lasmas with the ion del B drift directed towards an X-point and in sev eral elongated limiter plasmas. Troyon factors (beta(tor)aB/l(p)) up t o 2 and line average densities up to 2.2 x 10(20) m(-3), corresponding to the Greenwald limit, have been obtained in diverted ELM-free H-mod es. Quasi-stationary H-modes lasting for the entire current flat top ( 1.5 s) have been obtained in the presence of regular ELMs. The occurre nce 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 dependencies have permitted active control of ELM be haviour in TCV. A continuous spectrum of ELM amplitudes and frequencie s has been observed, ranging from clearly identifiable type III ELMs t o 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 EL Ms. A previously unknown, benign kind of ELM, with a maximum amplitude in the divertor region, has also been observed.