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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.

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.

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.

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.

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.

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.