ELM CONTROL DURING DOUBLE-NULL OHMIC H MODES IN TCV

Precise control of the shape and position of double-null divertor conf igurations in the TCV tokamak has been used to trigger reliably the tr ansition from ELM-free to ELMy ohmic H mode and back. Preprogrammed mo dulation of the magnetic axis height (+/- 1.25 cm), with fixed shaping field, caused the plasma to evolve, during a single discharge, betwee n double-null-upper, double-null and double-null-lower configurations, with the secondary X point remaining close to the separatrix. The tra nsition from ELM-free to ELMy and back again was observed to be synchr onous 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 s. A novel method of plasma densit y control (which depends on ELM control) was demonstrated during a lon g duration double-null H mode by coupling the density error signal int o the vertical position controller, rather than to the gas inlet valve .