Frequency control of type-I ELMs by magnetic triggering in ASDEX Upgrade
Magnetic triggering of edge localized modes (ELMs) was reported first from TCV in ohmic plasmas showing type-III ELMs. This method, showing successful locking of the ELM frequency to an imposed vertical plasma oscillation, has now also been demonstrated in the ITER-relevant type-I ELM regime in ASDEX Upgrade. Our experiments showed the ELM frequency becoming identical to the driving frequency in steady state for an applied motion of only about twice the value caused by an intrinsic ELM event. Triggered ELMs still showing clear type-I features were found when the plasma down-shift velocity reached its maximum, corresponding to the lowest edge current value. This is the opposite of the behaviour expected from the peeling-ballooning nature attributed to the ELM boundary and to TCV observations. The reason for this behaviour is not yet clear.