EFFECT ON CONFINEMENT OF EDGE-LOCALIZED MODES IN TCV

The effect of ELMs on confinement in ohmic H-modes has been investigat ed by analysing the rates of change of global parameters such as the e lectron 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 e lectron content and 2.5% of the stored energy. In single-null discharg es 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 redu ced, ELM frequencies increase and ELM amplitudes decrease. Quasi-stati onary I-I-modes are obtained for f(ELM) approximate to 130 Hz in doubl e-null plasmas and for f(ELM) in the range 50-300 Hz in single-null pl asmas, 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 de pending upon the ELM size and frequency.