2D Tomographic Imaging of the Edge Turbulence in RFX-Mod

Abstract

In the reversed Field Pinch Experiment RFX-mod a Gas Puffing Imaging Diagnostic (GPID) is used to investigate the turbulence of the edge plasma. The system consists of a gas puffing nozzle and 32 optical channels to measure the HeI (668 nm) line emission. The lines of sight are arranged into three fans intersecting each other in an area normal to the main magnetic field. The diagnostic system provides an analogue bandwidth of 2 MHz and all channels are simultaneously sampled at 10 MSamples/s for the whole discharge duration (350 ms). Different inversion techniques have been applied to the data in order to obtain a 2D tomographic reconstruction of the light emission pattern from the line integrals. Comparison shows that the most suitable method is based on 2D spatial Fourier expansion, applying the Singular Value Decomposition technique with regularisation. The high sampling time allows to obtain a 2D image every 100 ns. It is found that emission structures ('blobs') emerge from the background turbulence; a characterisation is given by computing the energy of the Fourier modes.