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In this paper we present the result of a study carried out at the Frascati Tokamak Upgrade (FTU), on a set of full non-inductive current driven, electron heated, L-mode discharges aimed at investigating the parametric dependence of the electron density profile on the electron temperature and safety factor gradients as predicted by quasi-linear drift-turbulence transport theory. Experiments in FTU allow the extension of similar studies carried out on other tokamaks to plasmas with higher density and higher magnetic field. Magnetic shear and electron temperature gradients are found to drive opposite turbulent particle flows in the gradient region (0.3 < r/a ? 0.5), while inward thermo-diffusion alone is found in the plasma core (r/a ? 0.3). Density profiles at midradius appear to be controlled by a convective term proportional to the density and independent of the gradient of temperature and magnetic shear. A linear increase in density peaking versus effective collisionality is found, differing from the scaling observed in other FTU plasma regimes.

Electron transport in fusion plasmas is intensively studied in coordinated experiments and great progress in physics understanding has been achieved during the past years. A threshold in normalized gradient explains most of the observations, both in steady-state and transient conditions. The results convincingly suggest that trapped electron modes (TEM) dominate electron transport at low and moderate collisionality, with electron heating. The stabilization of these modes at high collisionality predicted by theory is found in the experiments. Electron transport is then driven by the ion temperature gradient modes. At low collisionality, if TEM are stabilized by negative shear and Shafranov shift effects, electron internal transport barriers may develop.

The Frascati Tokamak Upgrade (FTU) is a high field and high density device which allows one to study confinement and transport at plasma parameters generally not accessible to other tokamaks and is relevant to next generation experiments (ITER). In this paper we study the effect of different density, temperature and q profiles on confinement and transport in various types of plasmas including ohmic, externally heated and pellet-fuelled discharges. Peaking of the electron density profile in pellet-fuelled discharges has been found to enhance the energy confinement time on FTU to values as high as 120 ms at densities of the order of those expected in ITER. Experiments designed to further improve confinement by injecting pellets at higher densities show the existence of a second threshold for saturation of confinement when the local electron heat conductivity corresponds to the same order of the local ion neoclassical conductivity in the region of maximum temperature gradient. Reflectometry measurements show turbulence suppression in pellet-injected discharges (as predicted by microstability analysis) and give insights to the change in turbulence in electron internal transport barrier plasmas. Finally, particle transport has been studied in experiments with full LH current drive.

A tokamak plasma with internal transport barriers (ITBs) is the best candidate for a steady ITER operation, since the high energy confinement allows working at plasma currents (Ip) lower than the reference scenario. To build and sustain an ITB at the ITER high density (>=1020 m-3) and largely dominant electron (e-) heating is not trivial in most existing tokamaks. FTU can instead meet both requests, thanks to its radiofrequency heating systems, lower hybrid (LH, up to 1.9 MW) and electron cyclotron (EC up to 1.2 MW). By the combined use of them, ITBs are obtained up to peak densities ne0 > 1.3 × 1020 m-3, with central e- temperatures Te0 ? 5.5 keV, and are sustained for as long as the heating pulse is applied (>35 confinement times, ?E). At ne0 ? 0.8 × 1020 m-3 Te0 can be larger than 11 keV. Almost full current drive (CD) and an overall good steadiness is attained within about one ?E, 20 times faster than the ohmic current relaxation time. The ITB extends over a central region with an almost flat or slightly reversed q profile and qmin ? 1.3 that is fully sustained by off-axis lower hybrid current drive. Consequent to this is the beneficial good alignment of the bootstrap current, generated by the ITB large pressure gradients, with the LH driven current. Reflectometry shows a clear change in the turbulence close to the ITB radius, consistent with the reduced e- transport. Ions (i+) are significantly heated via collisions, but thermal equilibrium with electrons cannot be attained since the e--i+ equipartition time is always 4-5 times longer than ?E. No degradation of the overall ion transport, rather a reduction of the i+ heat diffusivity, is observed inside the ITB. The global confinement has been improved up to 1.6 times over the scaling predictions. The ITB radius can be controlled by adjusting the LH power deposition profile that is affected mostly by the q value of the discharge, while the ITB strength can be varied through central EC heating. FTU experiments have shown that ITER-like e-ITBs are achievable.

MHD spectroscopy in FTU has revealed high frequency (HF) oscillations between 30 and 70 kHz that accompany the development of large m0 = −2, n0 = −1 islands in ohmic plasmas. This frequency range is one order of magnitude above the island rotation frequency, one order of magnitude below the first toroidal gap in the Alfven continuum and of the same order of the low frequency gap introduced by finite beta effects. The HF spectrum is organized in pairs of modes with opposite toroidal numbers, n = 1 and n = −1 for the stronger ones, which form standing waves in the island rest frame. The poloidal structure of HF modes is |m| = 2 for the main lines. HF lines appear when the level of poloidal field perturbation produced by the island at the plasma edge exceeds 0.2%. The absence of energetic ions in FTU ohmic plasmas, the strict correlation between HF and tearing modes and the existence of a threshold indicate that HF modes tap their energy from the tearing mode by a non-linear coupling mechanism.

Coherent structures emerging from the background turbulence have been detected by electrostatic measurements in the edge region of two Reversed Field Pinch experiments, RFX (Padua) and Extrap-T2R (Stockholm). Measurements have been performed by arrays of Langmuir probes which allowed simultaneous measurements of temperature, potential and density to be carried out. These structures have been interpreted as a dynamic balance of dipolar and monopolar vortices, whose relative population are found to depend on the local mean E × B flow shear. The contribution to the anomalous transport of these structures has been investigated and it has been found that the corresponding diffusion coeffcient accounts up to 50% of the total diffusivity. The experimental findings indicate that the diffusion coeffcient associated to the coherent structures depends on the relative population of the two types of vortices and is minimum when the two populations are equal. An interpretative model is proposed to explain this feature.

In order to test the self-organized criticality (SOC) paradigm in transport processes, a novel technique has been applied for the first time to plasmas confined in reversed field pinch configuration. This technique consists of an analysis of the probability distribution function of the times between bursts in density fluctuations measured by microwave reflectometry and electrostatic probes. The same analysis has also been applied to intermittent events sorted out from the Gaussian background. In both cases, the experimental results disagree with the predictions for a SOC system.

Electrostatic turbulence in the edge region of the RFX device has been shown to be intermittent, i.e. not self-similar. In fact the probability distribution function of fluctuations at large scales is Gaussian in shape, whereas it displays strong tails for small scale fluctuations. The events responsible for these heavy tails are associated with turbulent structures with well defined density and floating potential spatial profiles, which have been characterised using arrays of probes. These structures tend to occur preferentially in correspondence to large scale magnetic relaxation events driven by core-resonant tearing modes. They also display an inclination related to the average velocity shear.

We consider the effect of small-scale fading on thedetection of weak signals in cognitive radio systems.We formulatea model for the detection problem taking fading into account,and give the associated likelihood ratio tests. Additionally we givean expression for the asymptotic detection performance and usethis to consider the tradeoff between the detection performanceand the coherence time and bandwidth of the channel. ©2009 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.Erik G. Larsson and Giorgio Regnoli, Primary System Detection for Cognitive Radio: Does Small-Scale Fading Help?, 2007, IEEE Communications Letters, (11), 10, 799-801.http://dx.doi.org/10.1109/LCOMM.2007.070923

High frequency peaks in the spectra of magnetic field signals have been detected at the edge of Extrap-T2R [P. R. Brunsell, H. Bergsåker, M. Cecconello, J. R. Drake, R. M. Gravestijn, A. Hedqvist, and J.-A. Malmberg, Plasma Phys. Controlled Fusion, 43, 1457 (2001)]. The measured fluctuation is found to be mainly polarized along the toroidal direction, with high toroidal periodicity n and Alfv́nic scaling (f∝B mi ni). Calculations for a reversed field pinch plasma predict the existence of an edge resonant, high frequency, high- n number toroidicity-induced Alfv́n eigenmode with the observed frequency scaling. In addition, gas puffing experiments show that edge density fluctuations are responsible for the rapid changes of mode frequency. Finally a coupling with the electron drift turbulence is proposed as drive mechanism for the eigenmode. © 2005 American Institute of Physics.

In this paper the results of the experiments of combined injection of LH waves and EC waves performed in the FTU tokamak are reported. Such experiments were mainly devoted to study and to control the access to the advanced tokamak scenarios in plasma conditions close to ITER parameters (Bt≈5T or higher, ne,line≈10 20 m -3 ). Two different absorption mechanisms were used for the EC waves. In the first one the cold resonance absorption of EC waves launched with a toroidal angle was used to induce small modification of the current profile mainly maintained by LH waves. In the second one the absorption through Doppler shift due to the fast electron tails generated by LHCD was used.

The momentum balance has been applied to the ExB flow in the edge region of a reversed field pinch (RFP) configuration. All terms, including those involving fluctuations, have been measured in stationary condition in the edge region of the Extrap-T2R RFP experiment. It is found that the component of the Reynolds stress driven by electrostatic fluctuations is the term playing the major role in driving the shear of the ExB flow to a value marginal for turbulent suppression, so that the results are in favor of a turbulence self-regulating mechanism underlying the momentum balance at the edge. Balancing the sheared flow driving and damping terms, the plasma viscosity is found anomalous and consistent with the diffusivity due to electrostatic turbulence.

EX/P4-13 · The Greenwald density limit in the Reverse Field Pinch M. Valisa1), F. Auriemma, A. Canton, L. Carraro R. Lorenzini, S. Ortolani, R. Paccagnella, M.E. Puiatti, F. Sattin, P. Scarin, G. Spizzo, M. Spolaore, N. Vianello, A. Alfier, V. Antoni, L. Apolloni, M. Bagatin, W. Baker, O. Barana, F. Basso, D. Bettella, P. Bettini, T. Bolzonella, D. Bonfiglio, A. Buffa, S. Cappello, R. Cavazzana, M. Cavinato, G. Chitarin, A. Cravotta, F. D'Angelo, S. Dal Bello, M. Dalla Palma, A. De Lorenzi, L. De Pasqual, D. Desideri, D.F. Escande, P. Fiorentin, P. Franz, L. Frassinetti, G. Gadani, E. Gaio, L. Garzotti, E. Gazza, L. Giudicotti, F. Gnesotto, L. Grando, S.C. Guo, P. Innocente, A. Luchetta, G. Malesani, G. Manduchi, G. Marchiori, D. Marcuzzi, L. Marrelli, P. Martin, E. Martines, S. Martini, A. Masiello, F. Milani, M. Moresco, A. Murari, P. Nielsen, R. Pasqualotto, S. Peruzzo, R. Piovan, P. Piovesan, N. Pomaro, G. Regnoli, G. Rostagni, V. Schmidt, G. Serianni, P. Sonato, E. Spada, C. Taliercio, D. Terranova, V. Toigo, P. Zaccaria, P. Zanca, B. Zaniol, L. Zanotto, E. Zilli, G. Zollino, M. Zuin 1) Consorzio RFX - Associazione EURATOM-ENEA sulla Fusione, Padova, Italy Abstract: 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.

The results of an extensive investigation of electrostatic and magnetic turbulence in the edge region of two Reversed Field Pinch (RFP) experiments EXTRAP-T2R and RFX are reported . In both experiments particle transport is mostly driven by electrostatic turbulence and a highly sheared ExB flow takes place. Recent results have shown that almost 50% of the particle losses is due to coherent structures. These structures have features reminiscent of monopolar or dipolar vortices and their relative population and preferred vorticity is determined by the local ExB shear. It has been demonstrated that the plasma diffusivity D can be separated in two comparable terms: one due to coherent structures and another one due to background turbulence. The ExB shear results to affect both terms, modifying the populations of vortices and the phase of background density and velocity fluctuations . This effect has been proved by several experiments of transport control based on modification of the ExB shear by insertion of active electrodes or by applying transient modifications of the magnetic field. In order to investigate the process responsible for the observed ExB flow profile, the momentum balance has been recently addressed and it has been found that also transport of momentum is anomalous as experimental kinematic viscosity results consistent with anomalous diffusion. Moreover the balance reveals that plasma flow profile is regulated by turbulence and in particular by the electrostatic component of the Reynolds Stress (RS). These results prove the existence of a dynamic interplay between turbulence properties, anomalous transport and mean profiles. All results are discussed highlighting the similarities with other magnetic configurations.

Coherent structures identified in two reversed field pinch experiments are interpreted as a dynamic balance of dipolar and monopolar vortices growing and evolving under the effect of the ExB flow shear. For the first time their contribution to the anomalous transport has been estimated in fusion related plasmas, showing that they can account for up to 50% of the total plasma diffusivity. The experimental findings indicate that the diffusion coefficient associated with the coherent structures depends on the relative population of the two types of vortices and is minimum when the two populations are equal. An interpretative model is proposed to explain this feature.

The properties of plasma turbulence in the outer region of the Reversed Field Pinch experiments RFX and EXTRAP-T2R are reviewed. The statistical properties of fluctuations in the range of scales relevant for transport are presented. The observation of coherent structures emerging from the background turbulence and their interpretation in terms of vortices is reported. The interplay between these structures and the mean E×B flow of the plasma is demonstrated with emphasis to the action on the preferential rotation direction. The effect on the particle transport induced by the background turbulence and by the structures is discussed. Finally the methods tested to control turbulence and to mitigate the related transport are illustrated and discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of the E × B velocity shear on the particle flux carried by electrostatic fluctuations at the edge is discussed comparing the results of the experiments T2R (R = 1.2 m, a = 0.18 m) and RFX (R = 2 m, a = 0.5 m). In both experiments electrostatic turbulence accounts for most of the particle flux. The shear of the E × B velocity is close or larger than the value required for turbulence suppression or reduction. Particle fluxes exhibit bursts which carry a large fraction of the total particle flux, and which tend to cluster during magnetic relaxation phase. On the average a Bohm estimate of the experimental plasma diffusivity accounts for the experimental data. Finally a discussion on the methods for turbulence control is given. © 2003 Elsevier Science B.V. All rights reserved.

In this paper, we analyse the main features of the pulsed poloidal current drive (PPCD) technique, used in the reversed field pinch configuration to achieve improved confinement conditions. In the RFX experiment, PPCD corresponds to a decrease of the magnetic fluctuations, to a peaking of the temperature profile, and to a reduced transport and plasma-wall interaction. A three-dimensional MHD nonlinear code and one-dimensional time-dependent transport models have been applied to study the effect of PPCD on the magnetic and plasma profiles. The three-dimensional MHD simulations show that the external inductive drive pinches and peaks the current profile driving the configuration through a transient phase, where the spontaneous turbulent dynamo action is quenched. The one-dimensional transport codes indicate that the experimental profile modifications associated with PPCD are consistent with a reduction of the stochastic transport.

In the Reversed Field Pinch experiment RFX a highly sheared E×B flow is observed in the edge region, with shear value close to the value required for turbulence suppression or reduction. Recent observations have shown that almost 50% of particle flux due to turbulence is due to intermittent events which tend to cluster during magnetic relaxation phase. These events have been associated in RFX to vortex like structures whose rotation direction depends on the local value of the mean E×B flow shear.

The paper highlights the progress made in the modelling and understanding of the mechanisms underlying particle and energy transport in the RFP. A transport analysis has been performed on the perturbed transport coefficients for pellets of different characteristics launched both in standard RFP plasmas and in combination with pulsed poloidal current drive, this latter scenario potentially allowing an efficient fuelling and a good energy confinement. The study has confirmed the importance of large-scale MHD modes in determining the transport properties in the RFP core. As far as confinement in the edge is concerned, the analysis has been focussed on the role played by burst events on the electrostatic particle flux and to the 'intermittent events'. The intermittent events cluster during relaxation processes and such behaviour is more evident close to the magnetic reversal surface. This result confirms an interplay between the small scale edge turbulence and the large scale relaxation events driven by core-resonant tearing modes.

Bursts of electrostatic turbulence measured in the edge region of two reversed field pinch experiments (RFX and T2R) are observed to be associated with electrostatic structures. The corresponding E×B velocity drifts have been reconstructed showing features reminiscent of monopolar vortices. These vortices have been grouped in two classes depending on their rotation direction. It is observed that the prevalent rotation direction depends on the shear of the surrounding mean E×B drift velocity, showing a clear analogy with ordinary fluids.

A study of transport processes in reversed-field-pinch plasmas was performed. Statistical properties of particle fluxes driven by electrostatic turbulence were also compared with those predicted by the self-organized-criticality (SOC) model. The statistical analysis revealed that laminar times between bursts carrying more than 50% of the particle transport at the edge exhibited a power law decay. The decomposition of particle flux fluctuations in the edge region of RFX showed that fluctuations were not self-similar in character.

The basic statistical features of the electrostatic fluctuations in the edge plasma of the RFX reversed field pinch fusion device and their relationship with magnetic relaxation events are reported. The analysis of time series of floating potential measurements obtained at low plasma current (< 400 kA) has shown a clear relation between low-frequency (f < 5 kHz) electrostatic fluctuations and reversal parameter F fluctuations related to relaxation events. The electrostatic turbulence measurements have been investigated through the wavelet decomposition technique, and intermittent structures have been identified. These structures are found to occur mainly during relaxation events. The probability distribution function of waiting times between intermittent structures shows a power law trend, which is a clear evidence of memory-characterized phenomena.

The linear stability of dissipative drift electrostatic modes in tokamak plasmas is studied numerically with an extended version of the gyrokinetic code Kinezero [C. Bourdelle et al., Nucl. Fusion 42, 892 (2002)] including a modified Krook collision operator to account for collisional effects on the trapped electron response. This new version of Kinezero has been successfully tested and benchmarked against the results of a more sophisticated and complete gyrokinetic solver, GS2 [M. Romanelli, C. Bourdelle, and W. Dorland, Phys. Plasmas 11, 3845 (2004)]. The critical density and temperature gradients for ion gradient driven modes (ITG) and trapped electron modes (TEM) have been computed for different values of collisionality. The threshold for TEM disappears as the collisionality increases. A detailed study of the dependence of the growth rate of the dissipative TEM on collisionality and density gradient is presented in the paper. The range of parameters where the dissipative trapped electron modes are destabilized by increasing collisionality corresponds to very steep density gradients that can hardly be achieved in today's large tokamaks.

The intermittency of the floating potential fluctuations as measured at the edge of a plasma of interest for controlled thermonuclear fusion research is investigated. The probability distribution functions of fluctuations are not scale invariant, that is the wings of these functions are more important at the smallest scales, a classical signature of intermittency. Self-similarity is recovered at scales greater than about 20 μs.

The new Plasma Kristall Experiment (PK-4) is scheduled to fly on the International Space Station in 2012 with one of the experiments designed to investigate the existence of two fluid phases and a critical point in complex plasmas. A crucial issue is the parameter regime where the critical point could be found and this requires, as a first step, knowledge of the parameter range where dust-dust attraction can exist. This problem is addressed in the present work, extending previous works on long-range screening and attraction of negatively charged dust particles in plasmas. The roles of nonlinearities, ion-neutral collisions, electron dynamics, and plasma source on the depth and long-range behavior of the attractive well are established and the impact of these results on the PK-4 experiments is discussed.