Integrated exhaust scenarios with actively controlled ELMs

Abstract

An integrated radiative high performance scenario has been established at ASDEX Upgrade based on simultaneous feedback control of the average divertor neutral particle and power flux in combination with a high, pellet induced frequency of edge localized modes (ELMs). This approach is fully compatible with the present tungsten wall coating covering about 65% of the plasma facing components and is intended for application in the envisaged full-tungsten experiment. In these experiments, divertor recycling and effective divertor temperature (derived from thermoelectric currents) were tuned by acting on fuel gas puff and argon injection rates. The ELM frequency (fELM) was kept high by repetitive injection of small cryogenic deuterium pellets to avoid the radiative instabilities seen at low fELM and high radiated power, and to control the ELM energy. No confinement loss is observed in this radiative type-I ELMy scenario with relatively flat density profiles. In contrast, similar type-III ELM scenarios achieved in hydrogen show a confinement loss of 25% as compared to the type-I phase. In parallel to pellets, alternative ELM trigger techniques have been investigated as well. Fast vertical plasma oscillations are able to synchronize the ELM frequency to values higher and lower than the intrinsic fELM, but remain to be tested in the integrated scenario. Supersonic gas injection showed better fuelling efficiencies than usual gas puffing but instantaneous ELM release has not been achieved. A particular experimental challenge for AUG conditions is to obtain a high pace making frequency, to establish scalings of confinement and energy loss as a function of controlled ELM frequency.