[en] In a Reversed Field Pinch (RFP) device impurities may degrade the plasma performance through direct energy losses and also affecting the relaxation phenomena by the change of the plasma resistivity profile, especially at the edge where the highly radiative ionization stages locate, ultimately causing an enhancement in the loop voltage. This paper summarizes the analysis of the radiation measurements performed on RFX in 0.5 MA discharges in terms of impurity content, total radiated power and effective charge. (author) 6 refs., 7 figs

[en] In the RFX experiment, as in other reversed field pinches, the sustainment of the configuration in standard conditions entails the presence of a relatively wide spectrum of internally resonant tearing modes with saturated amplitude. Such modes cause a non-axisymmetric perturbation of the magnetic configuration. In this paper a detailed comparison of the perturbed magnetic surface with the footprint of enhanced plasma wall interaction evidenced by CCD camera images of the RFX graphite first wall is presented. The comparison confirms the accuracy of the magnetic reconstruction algorithm, which computes peak-to peak perturbation of the plasma surface as large as 5 cm in some cases and a contribution from m=0 modes which can be comparable to that of m=1 modes, particularly at low currents

[en] Plasma wall interactions become a crucial issue in the Reversed Field Pinch RFX at high current (>0.7 MA). Wall-Mode Locking (WML) leads to carbon bloom, enhanced recycling and makes the density control very difficult to achieve. Several wall conditioning techniques have improved the capability of controlling recycling, especially boronisation with diborane, but at 1 MA of plasma current removal of the WML becomes mandatory. Encouraging results have been achieved by rotating an externally induced perturbation that can unlock the WML. The strong impurity screening mechanism found at intermediate current does not degrade significantly at 1 MA. Modification of the tiles geometry could further reduce the power density dissipation and mitigate the PWI. (author)

[en] Full text: The turbulent transport at the edge of the RFX-mod Reversed Field Pinch is described with regard to its evolution as the Greenwald density limit is approached. RFX features in fact an upper density limit remarkably well defined by the Greenwald's law found in Tokamaks despite the many differences between the two magnetic configurations. The outer regions of RFP's and Tokamaks share several analogies: edge particle transport is mainly driven by electrostatic turbulence and is characterized by a highly sheared ν_ExB flow. The Reynolds' stress, which is known to drive the E x B flow against anomalous viscosity, exhibits a strong gradient in the layer of high velocity shear, almost entirely due to the electrostatic component. Coherent structures emerge from the turbulent background as intermittent bursts, co-rotating with the local mean velocity shear. In RFX such structures contributed up to 50% of the total radial diffusivity. The novelty is represented by new measurements of the RFX-mod edge plasma from a Gas Puffing Imaging diagnostic (GPI) and a sophisticated array of Langmuir and electromagnetic probes. As in Tokamaks and Stellarators the probability distribution function of the edge fluctuations data is typically non Gaussian, where the deviation decreases moving from the far scrape off layer into the plasma. The paper concentrates on the results obtained in the high density regimes close to the Greenwald limit. As the density is increased - at least up to n/n_G = 0.8, where n_G is the Greenwald density - the frequency of the highly non Gaussian intermittent events increases. The toroidal velocity of such intermittent events at the edge decreases with the density but saturates beyond n/n_G > 0.35. Further experiments are planned to explore the parameters space beyond n/n_G = 0.8 in order to verify whether the Greenwald limit is defined by a degradation of the edge transport. The analysis of the edge data is complemented by a one-dimensional modeling of the RFP transport based on the RITM code which helps understanding the relative role of transport and neutral gas influx in the determination of the density limit. The discussion includes the comparison with helium discharges, which have been seen to slightly overcome the Greenwald limit. (author)

[en] A new fast visible spectrometer built for the charge exchange diagnostic system of the RFX-mod Reversed Field pinch experiment is described. The optical mounting is of the Littrow type. The spectral resolution is defined by a 3000 grooves/mm, 180x143 mm² wide reflection grating, and a focal length of 400 mm. The collimating optics is based on a commercial high quality f/2.8 telephoto lens, so that the input fibers can be vertically stacked without space in between. The detector is a two-dimensional charge coupled device back-illuminated sensor to ensure high quantum efficiency. Thus the spectrometer combines high speed, high spectral resolution, and excellent imaging quality.

[en] Full text: In the Reversed Field Pinch RFX the density operating space exhibits an upper boundary that is well described by the Greenwald limit. The basic findings are described and similarities and differences with the Tokamak case are analysed. The high-density limit occurs as a non-disruptive limit. Fast terminations have been observed but in a wide area of the operating space and therefore are unlikely to be related to the density limit. Radiation induced thermal collapse can also be ruled out being radiation losses always a relatively small fraction of the ohmic input power. The occurrence of plasma detachment while approaching the density limit can be excluded. The absence of additional heating makes it difficult to distinguish with certainty between a power balance effect and an intrinsic physical limit but the overlapping of the Greenwald limit with the upper boundary of the density operating space is however remarkable, especially in the case of He plasmas . While in Tokamaks one of the plausible causes for the limit is a transport induced edge thermal instability, in RFX, as density increases the density profile becomes hollow, inside the last closed magnetic surface particle diffusion decreases and the global energy confinement time improves. The normalised density fluctuations measured by Langmuir probes and by the outermost chords of an interferometer do not increase. Simulations of the hydrogen discharges with the RITM code confirm the importance of recycling in determining the edge density gradient and the minor role of radiation losses. Differences between H and He cases are analysed in terms of particle penetration capability and in terms of edge ExB shear. A statistical analysis of the edge density fluctuations is presented, looking for differences arising as density is increased. Finally, the possible role of pressure driven modes is discussed. (author)

No abstract available

[en] Full text: RFX-mod embodies the characteristics of flexibility of an experiment where cross-configuration studies can be carried out. Such condition has aroused lively interest and has led to important collaborations with laboratories worldwide (JT60-SA, DIII-D, AUG, PPFL, PPPL, Auburn University, ORNL). As a Reversed Field Pinch (RFP), RFX-mod addresses many basic physics issues that are common to both Tokamak and Stellarators, just in a different region of the parameters space of a hot plasma, characterized by low magnetic fields. In addition RFX-mod can be run directly as a low current, ohmic, circular Tokamak and apply to it its state-of-the-art system for active MHD feedback control and investigate, for instance, Resistive Wall Modes and Resonant Field Amplification processes. The control of the (2, 1) mode has allowed exploring equilibria down to q(a) = 1.6, showing the importance of correctly treating the aliasing of the sidebands generated by the correction coils. A high triangularity plasma with double x-point has also been produced. Entering an ohmic H mode would open the way to further important studies such as ELM's control. The bridge with the Stellarator community has been established because the RFP helical states can provide a good test-bed for numerical codes conceived to deal with 3D effects. The equilibrium codes VMEC and V3FIT developed for the Stellarator have been successfully adapted to reconstruct RFX-mod equilibria with diagnostics. Such equilibria show a good agreement with the results of the RFP equilibrium reconstruction code SHEq, providing the additional information on the role of pressure. The resulting q profiles show a non monotonic radial shape and the presence of a maximum where usually a strong thermal barrier develops. The contribution of coherent structures to the transport of particles and energy at the plasma edge has been studied on RFX-mod with direct observations of current density filaments in the edge region both in the RFP configuration - where drift kinetic Alfven structures have been identified - and in the Tokamak configuration, where small scales current filaments have been found. These results are compared with the findings in the TORPEX experiment, those obtained in the AUG experiments during type one ELM's, and with those in the TJ-II Stellarator device where similar investigations are in progress. (author)

[en] As in Tokamaks RFX-mod discharges are subject to the Greenwald density limit. The similarity between Reversed Field Pinches (RFP) and Tokamaks extends to many properties of the edge transport physics, including the electrostatic nature of turbulence, the presence of highly sheared ExB flows - sustained by the Reynolds' stress - and the presence of coherent structures emerging out of the turbulent background. The origin of the density limit in RFX-mod is sought along the legitimate idea that it should be similar to the Tokamak case. It is proposed that the poloidally symmetric and toroidally asymmetric strong radiation emission that develops where the plasma interacts with the wall, at high densities triggers a thermal instability that cools the plasma core. The process resembles the evolution of a MARFE in a Tokamak, but in the RFP the effect on the plasma core occurs well before the total radiated power approaches the (ohmic) power input and the result is a soft landing of the discharge. (author)

[en] RFX is a new reversed field pinch experiment under construction at Padova. The main impurities in it are carbon, oxygen and nickel. All ions of the impurities emit essentially in the XUV range (λ≤300A), which is therefore the crucial range for RFX spectroscopic diagnostic. Two spectrographs cover the XUV region of the spectrum: the first one is a low resolution flat-field grazing incidence (α=71^o) and has the main objective of providing for each discharge a survey of the impurity emission over the whole range 100-1100A. The second is a 2 m extreme grazing incidence spectrograph (α=89^o) for high resolution spectroscopy in the range 10-340 A. This second spectrograph can be absolutely calibrated. (author) 9 refs., 3 figs., 2 tabs