Edge density characterization on RFX-mod using the ultrafast microwave reflectometer

The edge properties of the RFX-mod device are strongly affected by its magnetic configuration, characterized, in particular, by a strong toroidal flow (up to ten times larger than in a tokamak) and a wide spectrum of density fluctuations. In this scenario the conventional and widely used reflectometric schemes to determine the radial plasma position can be seriously disturbed. The original scheme of the RFX microwave reflectometer has been revisited accordingly to the numerical analysis proposed in [1]. The results of this model drove the design of the ultrafast microwave reflectometer whose first Ka-band (with a triangular FM modulation between 27-30 GHz in 1 ?s) has been operational on the RFX-mod device since the mid-2009. After the work of setting up, calibration and validation, the system allowed to evaluate the distance dc of the so called cut-off density layer (with ne in the range 1-1.1 x 1019 m-3 ) from the first wall, with a high time resolution (t = 2?s). A first analysis of these data showed a peculiar correlation of the time evolution of dc with the the toroidal and radial components of the edge magnetic field . This suggested a relation between the density and the local magnetic topology. Some more accurate studies [2] highlighted the different roles played by the m=0 MHD modes (creating at the edge a chain of poloidally symmetric magnetic islands) and the (m=1, n=7) tearing mode (leading to the main helical plasma deformation). From the edge density point of view a local interplay between these two global perturbations seems to define the local edge plasma wall interaction. Moreover, an extensive analysis of a large discharge database has been performed to investigate the average behaviour of dc during the QSH states [3]. This work highlighted the global parameters affecting the edge density such as the plasma current and the different magnetic equilibria. References [1] R. Cavazzana and M. Moresco, Rev. Sci. Instrum., 79 10F105 (2008) [2] G. De Masi et al., Nuclear Fusion, to be published [3] R. Lorenzini et al., Nature Physics, 5, 570 (2009)