Soft x-ray Tomographic Imaging in the RFX Reversed Field Pinch

This paper describes the soft X-ray (SXR) cross-sectional distributions measured in the European Reversed Field Pinch (RFP) experiment RFX by means of a tomographic diagnostic. Due to the non-invasive character of this diagnostic, the plasma radiation emissivity pattern can be reconstructed. The emissivity depends on various crucial parameters and can be used to study several plasma properties. To reconstruct SXR emissivity in RFX, three different inversion schemes have been compared: the Cormack technique, based on the Bessel-Fourier series expansion, a maximum entropy algorithm and a hybrid method where the previous techniques have been merged together. In this paper the first survey of the SXR emissivity properties in the RFP configuration have been presented. Measurements have been taken in a variety of plasma conditions. The measured SXR data has been compared with the results of a one dimensional impurity diffusion model, which predicts the absolute SXR emissivity, with good agreement. In particular the effect of mode locking on plasma equilibrium in the standard Multiple Helicity (MH) states has been studied. This has given a significant contribute to the description of the plasma MHD structure, both in MH regime and in the Quasi Single Helicity (QSH) state, whose signature is the growth of a sizable m=1 structure in the emissivity distributions, and which has been experimentally discovered thanks to the tomography. A good correlation between SXR tomographic reconstruction and magnetic measurements has been found.