Edge localized modes in the TCV tokamak

The TCV tokamak has produced a large number of diverted SN and DN plasmas, as well as limited plasmas. Following boronization in 1994 virtually all discharges with the ion ∇B drift towards an X-point and several elongated limited plasmas have produced Ohmic H-modes. The typical SN discharge demonstrates most of the features observed in H-modes. Transitions can be 'dithering', with the Dα level switching between L-mode and H-mode levels. They can also feature ELMs with pulses of Dα well above the L-mode level. Following the transition there generally is an initial ELM-free period. In the example presented there is a short period of irregular 'mossy' ELMs causing low amplitude fluctuations on the Dα signal, followed by several Large ELMs which significantly reduce the energy confinement time and interrupt the density rise and a final period of mossy ELMs. The plasma density rises rapidly in the absence of Large ELMs. Mossy ELMs have their largest amplitude in the divertor and do not appear to affect confinement. In the absence of Large ELMs H-modes frequently terminate in high density disruption, sometimes at the Greenwald limit GL=0.27 Ip/a2. (author) 4 figs., 6 refs