Observation of Nitrogen Seeding Effects on Density Fluctuations at the Edge Pedestal by Radial Correlation Reflectometry in JET

In JET, nitrogen (N) seeding has been in use to decrease the power loads on the divertor by increasing its radiation and thereby lowering its temperature. Furthermore, in recent experiments with the ITER-like wall (ILW) it was observed that the injection of the extrinsic impurity N in high-triangularity ELMy H-mode plasmas led to a recovery of the edge pedestal pressure, which had dropped by 40% with the change from the carbon wall to the ILW [Giroud 2013, Giroud 2014]. Existing pedestal models did not predict this effect [Leyland 2014], and a better understanding of the underlying physics is necessary. The recovery of pedestal pressure with N can arguably be related to changes in the properties of turbulence within the pedestal. It is therefore important to have diagnostics dedicated to the measurement of such properties. Radial correlation reflectometry (RCR) is capable of making localized measurements of density fluctuations, and the recently upgraded RCR diagnostic in JET can be programmed in a flexible way to adapt to different experimental conditions [Meneses 2011]. This paper describes RCR measurements inside the edge pedestal during N seeding experiments in high triangularity plasmas with 2.5MA and 2.7T, using the same divertor configuration reported in [Giroud 2013].
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