Te from SXR in Reversed Field Pinches: 2D imaging of 3D structures

Reconstruction of the soft x - ray (SXR) emissivity map is an important tool in fusion devices due to the tight link with transport and Magnetohydrodynamic (MHD) equilibrium. A notable example is the occurrence of helical magnetic self - organized structures in the core of the reversed field pinch during Quasi Single Helicity (QSH) plasmas, when a single m=1 tearing mode dominates the MHD magnetic spectrum and all remaining modes have reduced amplitudes. More valuable information can be obtained taking into account the strong dependence of the continuum SXR radiation on electron temperature T e : the direct - brightness two - color technique can be applied to line integrated measurement s to obtain the T e along the designed lines of sight. A sophisticated multi - chord diagnostic has been developed on RFX - Mod for SXR emissivity and electron temperature profiles measurements, with good spatial and temporal resolution to identify helical stru ctures and track fast events [1] . An advanced instrument, similar to that of RFX - Mod, has been designed and installed also on MST [2] . This allows the study of QSH plasmas in both RFPs. To improve the SXR and T e data reconstruction in the presence of helical magne tic equilibria, mapping algorithms that reproduce the magnetic topology during the QSH regimes have been developed to map the measurements on flux surfaces. The analysis shows, in RFX - Mod, a clear difference in the temperature gradient (related to the pres ence of transport barriers) between the rising phase of QSH and the saturated (or flattop) phase. In the rising phase a correlation of temperature gradient with magnetic spectrum is found , showing a positive trend between dominant mode amplitude and therma l structure dimension, as well as a negative correlation of secondary modes amplitudes and temperature gradient. On the other hand, in the flattop phase the presence of a thermal structure is intermittent, with several crashes and flattening of the profile , and much less influenced by the magnetic dynamics. On the other hand, measurements at MST confirm the correlation between magnetic QSH and the appearance of a SXR structure in the emissivity distribution, but no clear indication of a thermal structure in the Te profile has been found, indicating a different origin of the SXR island. References [1] P. Franz, et al. , Nucl. Fusion, 53 053011 (2013) [2] M. B. McGarry, et al. , Rev. Sci. Instrum. 83 , 10E129 (2012)