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 trigger reliably the transition from ELM-free to ELMy ohmic H mode and back. Preprogrammed modulation of the magnetic axis height (+/- 1.25 cm), 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 the secondary X point remaining close to the separatrix. 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 s. A novel method of plasma density control (which depends on ELM control) was demonstrated during a long duration double-null H mode by coupling the density error signal into the vertical position controller, rather than to the gas inlet valve.