8th Workshop on the Electric Field, Structures and Relaxation in Edge Plasmas, 3-4 de Julho de 2005, Tarragona, Espanha

	Study of Pellet Triggered ELMs using High Spatial and Temporal Resolution FM-CW Reflectometry at ASDEX Upgrade
	L. Fattorini, P. T. Lang, M. E. Manso, S. Kalvin, G. Kocsis e a Equipa ASDEX Upgrade
	Resumo
	In ASDEX Upgrade, injecting small size cryogenic deuterium pellets, it has been demonstrated an option for ELM pace making in order to achieve type-I ELMy H-mode plasmas combining high performance and acceptable ELM size[1]. Moreover, triggering ELMs by cryogenic pellets has been shown a promising technique to investigate ELM physics. Reflectometry with its high temporal and spatial resolution is a diagnostic specially suited for this type of experiments. The ASDEX Upgrade FM-CW reflectometer can probe the low field side (LFS) and the high field side (HFS) of the plasma simultaneously. By operating in swept frequency mode we obtain the plasma response in a large probed plasma region, with a minimum repetition rate of 35 μs. The extracted group delay due to wave propagation to the density cutoff layers is the relevant data for density profile reconstruction. A new method has been developed that gives not only the direct evolution of density profile but also the localized fluctuations caused by the ELMs. It is based on the time evolution of the Fast Fourier Transform of the swept frequency signal at selected plasma layers. Complementary operation in fixed frequency mode resolves the turbulence spectrum at selected layers with higher temporal resolution (1 μs sampling). This diagnostic technique gives information on the characteristics of natural and pellet triggered ELMs. Preliminary results confirm earlier findings[1] that the spatial and temporal dynamics of triggered and intrinsic ELMs are entirely identical. Especially it is shown that triggered ELMs start – like their intrinsic counterpart - to evolve at the LFS despite the fact that pellets impose their massive initial perturbation close to their entrance point into the plasma at the HFS resulting in the peaking of the density profile at the HFS. Combining observations from the reflectometry with data from the fast Mirnov coil system and a fast framing camera system allows investigating with high precision the correlation between the imposed perturbation and its impact on the ELM release. [1] P. T. Lang et al., Nuclear Fusion 44 (2004) 665-677.