Effect on confinement of edge localized modes in TCV

The effect of ELMs on confinement in ohmic H-modes has been investigated by analysing the rates of change of global parameters such as the electron content and the stored energy as a function of ELM frequency. In Double Null H-modes ELMs are found to expel on average 2% of the electron content and 2.5% of the stored energy. In Single Null discharges which are separated by more than 2.2 cm from the inner wall, larger ELMs are observed which expel on average 3-7% of electron content and 3-12% of the stored energy. When the plasma-wall distance is reduced ELM frequencies increase and ELM amplitudes decrease. Quasi-stationary H-modes are obtained for fELM≅130 Hz in Double Null plasmas and fELM in the range 50-300 Hz in Single Null, depending on plasma to wall distance and ELM amplitude. The reduction in particle confinement due to the ELMs is sufficient to bring the ratio of particle to energy confinement times below 8 to 12 depending on ELM size and frequency. (author) 3 figs., 5 refs