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 H-modes are obtained for Hz in double-null plasmas and for 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.