In ASDEX Upgrade, experimental efforts aim to establish pace making and mitigation
of type-I edge localized modes (ELMs) in high confinement mode (H-mode) discharges.
Injection of small size cryogenic deuterium pellets (similar to(1.4 mm)(2) x 0.2 mm
approximate to 2.5 x 10(19) D) at rates up to 83 Hz imposed persisting ELM control
without significant fuelling, enabling for investigations well inside the type-I ELM
regime. The approach turned out to meet all required operational features. ELM pace
making was realized with the driving frequency ranging from I to 2.8 times the intrinsic
ELM frequency, the upper boundary set by hardware limits. ELM frequency enhancement
by pellet pace making causes much less confinement reduction than by engineering means
like heating, gas bleeding or plasma shaping. Confinement reduction similar tof -0.16/(ELM)
is observed in contrast to the similar tof-0.6/(ELM) typical for engineering parameters.
Matched discharges showed triggered ELMs ameliorated with respect to intrinsic counterparts
while their frequency was increased. No significant differences were found in the
ELM dynamics with the available spatial and temporal resolution. By breaking the close
correlation of ELM frequency and plasma parameters, pace making allows the establishment
of f(ELM) as a free parameter giving enhanced operational headroom for tailoring H-mode
scenarios with acceptable ELMs. Use was made of the pellet pace making tool in several
successful applications in different scenarios. It seems that further reduction of
the pellet mass could be possible, eventually resulting in less confinement reduction
as well.
