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 the 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 I-I-modes are obtained for f(ELM) approximate to 130 Hz in double-null plasmas and for f(ELM) in the range 50-300 Hz in single-null plasmas, depending upon the plasma-wall distance and the ELM amplitude. The reduction in particle confinement due to the ELMs is sufficient to bring the ratio of particle to energy confinement times below 8-12 depending upon the ELM size and frequency.