Application of 2D tomographic imaging techniques to edge turbulence in RFX-mod

In the reversed field pinch experiment RFX-mod a gas puffing imaging diagnostic 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 He I (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−1 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 precise method is based on the 2D spatial Fourier expansion, applying the singular value decomposition technique with a suitable regularization method to avoid artefacts. The high time resolution allows one to obtain a 2D image every 100 ns. Emission structures ('blobs') that move along the E × B flow emerge from the background turbulence and they are characterized by computing energy and phase of the Fourier modes.