ELM control during double-null Ohmic H-modes in TCV

Precise control of the shape and position of Double-Null divertor configurations in the TCV tokamak has been used to reliably trigger the transition from ELM-free to ELMy Ohmic H-Mode and back. Pre-programmed modulation of the magnetic axis height, with fixed shaping field, caused the plasma to evolve, during a single discharge, between Double-Null-Upper, Double-Null and Double-Null-Lower configurations, with both X-points remaining close to the last closed flux surface. The transition from ELM-free to ELMy and back again was observed to be synchronous with the configuration modifications. Using this method, several ELM-free and ELMy phases enabled reproducible disruption-free H-mode discharges in TCV lasting up to 1.5 seconds. The plasma density during a long-duration H-mode was directly controlled by feeding back onto the vertical height of the magnetic axis. (author) 4 figs., 18 refs