Heat pulse propagation studies around magnetic islands induced by the Dynamic Ergodic Divertor in TEXTOR

Since the efficiency of the tearing mode suppression by heating depends on the electron heat diffusivity it is important to know if the electron heat transport coefficients inside the island are reduced compared with the ambient plasma. With that aim, modulated ECRH has been employed for heat pulse propagation studies in and around magnetic islands at the TEXTOR tokamak. The combination of its special hardware tools of the Dynamic Ergodic Divertor to generate tearing modes, the ECRH system for producing heat pulses and the electron cyclotron emission imaging (ECE-Imaging) diagnostic for its analysis offered a direct view of the perturbed two-dimensional heat flow in around the magnetic island. Inside m/n = 2/1 and m/n = 3/1 islands with a flattened temperature profile, the electron heat transport is shown to be strongly reduced with respect to the surrounding plasma. Inside the islands a heat pulse diffusion coefficients chi(e) similar to 0.4 m(2) s(-1) was derived, while outside the island it is an order of magnitude larger chi(e) > 3 m(2) s(-1). In contrast, power balance calculations of strongly heated islands show that the electron transport is similar to the surrounding plasma. These results suggest that the heat transport inside a magnetic island is also governed by a critical gradient-like behaviour, similar to the bulk plasma.