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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.

An investigation of the edge region in a large reversed-field pinch (RFP) device has been undertaken for the first time using a high time resolution gas-puff imaging (GPI) system, applied to the TPE-RX experiment. The GPI diagnostic is aimed at the investigation of turbulence in the edge region under the severe heat flux conditions of thermonuclear fusion-grade plasmas, where conventional electrostatic probes cannot be used. To compare the data obtained by the GPI during low power experiments, a retractable array of electrostatic probes has been added. The whole diagnostic system provided high quality measurements of fluctuations with fine spatial and temporal resolution, as well as radial profiles of relevant plasma parameters. The measurements confirm several important properties of plasma edge already found in other experiments, like the presence of a strong ExB shear layer and the 'intermittent' behaviour of the measured quantities, which display non-Gaussian fluctuation spikes at different time scales. Along with the conventional power spectra and correlation analyses, the application of continuous wavelet transform techniques to GPI data allowed a detailed description of the spatial structure and of the propagation of these localized 'intermittent events'.

Plasma density fluctuations in the edge plasma of the RFX-mod device are measured through the Gas Puffing Imaging Diagnostics. Statistical features of the signal are quantified in terms of the Probability Distribution Function (PDF), and computed for several kinds of discharges. The PDFs from discharges without particular control methods are found to be adequately described by a Gamma function, consistently with the recent results by Graves et al [J.P. Graves, et al, Plasma Phys. Control. Fusion 47, L1 (2005)]. On the other hand, pulses with external methods for plasma control feature modified PDFs. A first empirical analysis suggests that they may be interpolated through a linear combination of simple functions. An inspection of the literature shows that this kind of PDFs is common to other devices as well, and has been suggested to be due to the simultaneous presence of different mechanisms driving respectively coherent bursts and gaussian background turbulence. An attempt is made to relate differences in the PDFs to plasma conditions such as the local shift of the plasma column. A simple phenomenological model to interpret the nature of the PDF and assign a meaning to its parameters is also developed., Comment: 27 pages. Published in PPCF

An extensive statistical characterization of the electromagnetic turbulence in the edge region of RFX-mod, operated as a tokamak and as a Reversed Field Pinch, is presented. Striking similarity are observed on the intermittent character of the electrostatic turbulence between the two configurations whereas the difference between magnetic turbulence is likely to be imputed to the different beta value of the two configurations. Blobs are characterized in terms of their spatial structure, the associated enstrophy and energy and the mechanism of energy and enstrophy transfer between intermittent fluctuation at different scales is addressed.

Edge turbulence and blobs are studied in the three-dimensional magnetic topology of the RFX-mod reversed field pinch. The edge of the RFX-mod shows a three-dimensional structure dominated by a helical equilibrium with (1, -7) symmetry, which gives the same space-time modulation to all of the kinetic properties. The interaction between the edge turbulence and this magnetic topology is studied. It is shown that the edge blobs are current-carrying filaments aligned with the magnetic field, and in the perpendicular plane each blob is a positive peak of electron density and a valley of temperature. The inner nature of these blobs is not affected by the presence of the O and X points of the (1, -7) island; however, the statistical properties are sensitive to them, pointing to the influence of the magnetic topology on the edge fluctuations.

The observation of the periodic pressure profile time oscillations in the edge region (0.93 < r/a < 1) of a RFX-mod device allows one to investigate the role of pressure as a turbulence regulation mechanism. The proportionality between the radial correlation length of edge fluctuations and the radial characteristic pressure length L-p suggests a sort of regulation of the turbulent spatial scale, where L-p sets the radial dimension of the blobs. Moreover, the spatial scale regulation is confirmed by observing a linear scaling between the energy injection scale and the toroidal dimension of the edge coherent structures. On the other hand, the link between Delta P-e and turbulence level does not hold locally: the edge fluctuation level is higher in the toroidal region of lower edge pressure gradient. Hence if coherent structures are generated locally in the region of strongest Delta P-e, they flow and accumulate where Delta P-e is lower.

In the edge region of fusion devices coherent structures (blobs) are commonly observed. In order to investigate their evolution and the possible driving mechanisms, we studied the relationship between the size of the coherent structures and the local characteristic length L(p) of the edge electron pressure profile P(e)(r), together with the effect of magnetic fluctuations on del P(e). The results show an increase in the blob size with both Lp and magnetic fluctuations, suggesting an interplay between electron pressure gradient, magnetic fluctuations and edge turbulence generation. Measurements of the spatial scales of the edge turbulence structures and the corresponding toroidal phase velocity are obtained in the reversed field pinch RFX-mod by means of a gas puffing imaging diagnostic, while the local edge profiles of electron density n(e)(r) and temperature T(e)(r) and therefore P(e)(r) have been measured via the line intensity ratio technique of helium emission lines.

The high-frequency range of power spectra of turbulent fluctuating quantities measured at the edge of magnetized plasmas displays a variety of trends: from power laws with different spectral indices to exponential. We propose a model able to account for the whole phenomenology simply by tuning the distribution in the duration of the signal spikes. Comparisons with data from RFX-mod and Alcator C-Mod experiments are performed. An attempt to relate the statistics of the bursts with their generating mechanism is made.

Neutral density distribution in the divertor region of ASDEX Upgrade is estimated in different discharge conditions. The 2D divertor emission of D-alpha and D-gamma is obtained with a tomographic inversion of their brightness measured by two cameras. From these emissions, 2D maps of neutrals density n(0) can be obtained by a simple model that takes into account both excitation and recombination processes populating deuterium n = 3 and n = 5 levels. In attached discharges, the neutral density n(0) is larger in the inner divertor, and when increasing core n(e) its maximum moves toward the dome and the outer divertor region. In particular, the modification of the n(0) distribution is studied in plasmas where the mean electron density in the core increases, moving from an attached to a detached phase. In these discharges, the outboard midplane scrape off layer electron density increases and its profile flattens, with the formation of the so called density shoulder. The modification of the neutral density is followed during this evolution, showing that both the shape and the absolute value of the n(0) distribution change in time, suggesting that they play a role in the two phenomena of shoulder formation and detachment. Moreover, in the detached phase, also the recombination contributes to the neutral deuterium emission.