Search

The scrape off layer (SOL) plasma in a Tokamak device and its coupling with the edge dictate the performance of a discharge to a high degree - especially as all plasma has to go through the SOL, which is the main exhaust channel for the hot plasma. This contribution provides an introduction to the modelling efforts of the plasma dynamics in the SOL and the edge regions. We employ a fully dynamical fluid model, the HESEL code. HESEL is equipped with synthetic diagnostic tools as probe arrays, Li-beam spectroscopy, and Gas Puff Imaging. Using the synthetic probe arrays to measure the electron and ion heat advection and conduction we obtain the upstream power fall-off length for a broad range of plasma parameters and by applying non-linear fitting procedures we derive the scaling of the fall-off length. The obtained results are in agreement with recent experimental observations from L-mode ASDEX Upgrade data. A workflow for generating synthetic Lithium beam data, where the fluctuation data from HESEL are passed to the RENATE code are discussed using experimental results from ASDEX Upgrade.

The nature of plasma edge turbulence has been studied in the Reversed Field Pinch (RFP) RFX-mod, making use of a Gas Puffing Imaging (GPI) diagnostic, with high time (2 MHz) and space (5 mm) resolution. The edge turbulence has been characterized in terms of toroidal phase velocity of fluctuations, v(phi), of the. width of emission structures, 2 sigma, and of their linear density in the toroidal direction. The packing fraction f(p) of intermittent structures has been estimated as the fraction of occupied area in a plane perpendicular to the magnetic field. The behaviors of v(phi),f(p), and 2 sigma are reported as functions of the density normalized to its Greenwald value, n/n(G). A comparison at different plasma regimes of an effective diffusion coefficient D(p) due to the plasma trapped in coherent structures has been discussed. A decreasing trend of D(p)with n/n(G) values has been guessed. (C) 2009 Elsevier B.V. All rights reserved.

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'.

The Probability Distribution Function (PDF) Pn(n) of the particle density at the edge of several magnetic fusion devices, including tokamaks, stellarators, and linear devices, is known to be strongly non-gaussian. In this paper we present experimental results from RFX Reversed Field Pinch (RFP) [Rostagni G 1995 Fusion Eng. Design 25 301], confirming the universal shape of Pn also for RFP's. An explanation for the form of Pn is attempted, on the basis of simple conservation equations. The result of the model is shown to fit fairly well empirical data. The PDF for the particle flux P(I) driven by electrostatic fluctuations is then computed using Pn(n) together with an ansatz for the PDF of electrostatic potential. The result is checked against experimental data and compared with previous formulations [Carreras B A et al 1996 Phys. Plasmas 3 2664].

A feedforward scheme is applied for drift waves control in a magnetized magnetron sputtering plasma. A system of driven electrodes collecting electron current in a limited region of the explored plasma is used to interact with unstable drift waves. Drift waves actually appear as electrostatic modes characterized by discrete wavelengths of the order of few centimeters and frequencies of about 100 kHz. The effect of external quasi-periodic, both in time and space, travelling perturbations is studied. Particular emphasis is given to the role played by the phase relation between the natural and the imposed fluctuations. It is observed that it is possible by means of localized electrodes, collecting currents which are negligible with respect to those flowing in the plasma, to transfer energy to one single mode and to reduce that associated to the others. Due to the weakness of the external action, only partial control has been achieved., Comment: Submitted to Physics of Plasmas

The EXTRAP-T2 reversed field pinch has undergone a significant reconstruction into the new T2R device. This paper reports the first measurements performed with Langmuir probes in the edge region of EXTRAP-T2R. The radial profiles of plasma parameters like electron density and temperature, plasma potential, electrical fields and electrostatic turbulence-driven particle flux are presented. These profiles are interpreted in a momentum balance model where finite Larmor radius losses occur over a distance of about two Larmor radii from the limiter position. The double shear layer of the E×B drift velocity is discussed in terms of the Biglari-Diamond-Terry theory of turbulence decorrelation.

Experimental evidences of short wavelength electromagnetic modes are found in the reversed-field-pinch configuration device RFX-mod by means of in-vessel magnetic probes. The modes are revealed during the helical states of the plasma. Their amplitude is well correlated to the electron temperature gradient strength in the core. On the basis of linear gyrokinetic calculations we interpret these instabilities as microtearing modes.

We present direct experimental observations in a laboratory fusion plasma of microtearing modes, obtained in the RFX-mod reversed-field pinch (RFP) plasma. The so-called Quasi Single Helicity magnetic equilibrium features in RFX-mod a central plasma volume with good magnetic surfaces and transport barriers with reduced heat transport. Recently, a theoretical stability analysis performed with the gyrokinetic GS2 code revealed the QSH states to be prone to microtearing modes. Measurements are carried out with a system of in-vessel probes located at the wall, capable of detecting magnetic fluctuations with high time and space resolution. Small-scale electromagnetic modes are revealed during the helical states of the plasma: their amplitude is correlated to the electron temperature gradient strength in the core. The identification in terms of microtearing modes derives from the comparison of experimental data with linear gyrokinetic simulations. The relation between the spectral properties of microtearing modes and the magnetic topology, mainly in terms of the helical safety factor profile, is investigated. The effects on the time evolution of the temperature profiles and on heat transport in RFP plasma are also discussed.

The high current regimes of the RFX-mod (R = 2 m, a = 0.46 m) reversed field pinch experiment are characterized by the presence of a dominant m/n = 1/−7 kink-type mode that moulds the plasma in a 3D topology. This paper describes the strong influence of such global helical state on the scrape-off layer, which becomes helically modulated itself. In particular, the link between the edge magnetic topology and the temporal variation of locally measured quantities (like particle influx, electron density and temperature, toroidal flow and floating potential) is analyzed for different plasma density. The edge radial electric field Er exhibits a periodicity similar to the dominant m/n = 1/−7 mode, being more negative in correspondence to the outward magnetic shift. Indications are obtained about the ambipolar nature of the edge radial electric field. This suggests a link with the mechanism of density limit in the RFP, where Er is determined by different electron mobility modulated by m/n = 0/1 islands.

In simple magnetized toroidal plasmas, field-aligned blobs originate from ideal interchange waves and propagate radially outward under the effect of $\grad B$ and curvature induced $E \times B$ drifts. We report on the first experimental two-dimensional measurements of the field-aligned current associated with blobs, whose ends terminate on a conducting limiter. A dipolar structure of the current density is measured, which originates from $\grad B$ and curvature induced polarization of the blob and is consistent with sheath boundary conditions. The dipole is strongly asymmetric due to the nonlinear dependence of the current density at the sheath edge upon the floating potential. Furthermore, we directly demonstrate the existence of two regimes, in which parallel currents to the sheath do or do not significantly damp charge separation and thus blob radial velocity.

Recently, a European transport project has been carried out among several fusion devices for studying the possible link between the mean radial electric field (E r ), long-range correlation (LRC) and edge bifurcations in fusion plasmas. The main results reported in this paper include: (i) the discovery of low-frequency LRCs in potential fluctuations which are amplified during the development of edge mean E r using electrode biasing and during the spontaneous development of edge sheared flows in stellarators and tokamaks. Evidence of nonlocal energy transfer and the geodesic acoustic mode modulation on local turbulent transport have also been observed. The observed LRCs are consistent with the theory of zonal flows described by a ‘predator–prey’ model. The results point to a significant link between the LRC and transport bifurcation. (ii) Comparative studies in tokamaks, stellarators and reversed field pinches have revealed significant differences in the level of the LRC. Whereas the LRCs are clearly observed in tokamaks and stellarators, no clear signature of LRCs was seen in the RFX-mod reversed field pinch experiments. These results suggest the possible influence of magnetic perturbations on the LRC, in agreement with recent observations in the resonant magnetic perturbation experiments at the TEXTOR tokamak. (iii) The degree of the LRCs is strongly reduced on approaching the plasma density-limit in tokamaks and stellarators, suggesting the possible role of collisionality or/and the impact of mean E r × B flow shear on zonal flows.

This paper summarizes the main achievements of the RFX fusion science program in the period between the 2008 and 2010 IAEA Fusion Energy Conferences. RFX-mod is the largest reversed field pinch in the world, equipped with a system of 192 coils for active control of MHD stability. The discovery and understanding of helical states with electron internal transport barriers and core electron temperature >1.5 keV significantly advances the perspectives of the configuration. Optimized experiments with plasma current up to 1.8 MA have been realized, confirming positive scaling. The first evidence of edge transport barriers is presented. Progress has been made also in the control of first-wall properties and of density profiles, with initial first-wall lithization experiments. Micro-turbulence mechanisms such as ion temperature gradient and micro-tearing are discussed in the framework of understanding gradient-driven transport in low magnetic chaos helical regimes. Both tearing mode and resistive wall mode active control have been optimized and experimental data have been used to benchmark numerical codes. The RFX programme also provides important results for the fusion community and in particular for tokamaks and stellarators on feedback control of MHD stability and on three-dimensional physics. On the latter topic, the result of the application of stellarator codes to describe three-dimensional reversed field pinch physics will be presented. © 2011 IAEA, Vienna.

The relationship between edge flow features and local magnetic topology has been studied by means of ad hoc transient experiments, so that different plasma equilibria are compared within the same discharge, and of the edge flow monitor of helical magnetic islands in the RFX-mod Reversed Field Pinch experiment. The different components of the plasma flow characterizing the edge region and their shear are obtained by combining and comparing information provided by a noticeable set of edge diagnostics including insertable probes, toroidally distributed arrays of sensors and optical diagnostics. In particular a Gundestrup probe head is applied which allows obtaining information on both parallel and perpendicular Mach number. This probe head has been used with all the pins collecting floating potential signals and a model has been developed in order to reconstruct the flow.

Edge turbulent structures are commonly observed in fusion devices and are generally believed to be responsible for confinement degradation. Among their origin Drift-Alfven turbulence is one of the most commonly suggested. Drift-Alfven paradigm allows the existence of localized vortex-like structures observed also in various systems. Here we present the evidence of the presence of drift-Alfv\'en vortices in the edge region of RFX-Mod RFP device, showing how these structures are responsible for electromagnetic turbulence at the edge and its intermittent nature., Comment: 4 pages, 5 figures, Revised version submitted for publication to Physical Review Letters

In all major confinement devices (tokamaks, stellarators, spheromaks and reversed-field pinches-RFPs), a density limit has been found. Results summarized in a recent work by Puiatti et al (2009 Nucl. Fusion 49 045012) show that in the RFP high density does not cause a disruption, but a sequence of increasingly critical phenomena. First, at intermediate density there is the disappearance of the high-confinement quasisingle helicity/single helical axis regimes. Then, at densities close to the Greenwald limit, toroidally and radially localized density accumulation and radiation condensation are observed, together with a fast resistive decay of the plasma current, which constitutes the real operative limit of the device. In this paper we discuss the effect of the magnetic ripple on test particle motion, showing that the accumulation of electrons in the X-points of the edge m = 0 islands is responsible for a modulation of the radial electric field E(r) which is at the core of the density limit mechanism. These results can be also relevant for the explanation of X-point multifaceted asymmetric radiation from the edge formation, observed in L-mode density limit discharges of JET.

In this contribution a detailed study of the edge flow properties performed on the outer region of the RFX-mod experiment using a Gundestrup probe is presented. In particular, both parallel and perpendicular components (with respect to the magnetic field) have been evaluated in several low current discharges providing a complete reconstruction of the edge flow profiles. Different methods based on both magnetized and unmagnetized models have been used to evaluate the averaged plasma flow components and their time variations. Furthermore, configurations involving either ion saturation current signals or floating potential ones have been used to evaluate both parallel and perpendicular flow components and a good agreement has been found between different methods. Finally, the dynamic behaviour of edge flow fluctuations induced by gradient modifications following pellet injection and by varying the edge magnetic topology (the m = 0 islands radial position) have been studied (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Coherent structures emerging from turbulence background have been detected in the edge region of the RFX-mod Reversed Field Pinch fusion device by using a new and original probe system measuring simultaneously and on the same location magnetic and electrostatic fluctuations. In agreement with theory it has been experimentally found that pressure structures detected in the cross-field plane, often referred in literature as 'blobs', correspond to E × B vorticity structures and are associated to current density filaments, oriented along the main magnetic field. The results are compared with those provided by the measured relationship between magnetic fluctuations and coherent structures on HeI emission observed by the Gas Puffing Imaging system, confirming the presence of such structures also at high current regimes. © 2009.

It has recently been shown that a Yaglom law for electrostatic turbulence, that is, a relation for the third-order mixed moment involving the particle number density as a passive scalar and the E x B drift velocity, can be deduced from a simple model of electrostatic fluctuations, which describes bursty turbulence in plasmas. In this letter, the existence of the Yaglom law for electrostatic turbulence in laboratory magnetized plasmas is reported for the first time. Using measurements of intermittent transport at the edge of the RFX-mod reversed field pinch plasma device, we found that the above scaling relation is nicely verified at intermediate scales of few centimeters. In this range of scales, that unambiguously represents the inertial range of electrostatic turbulence, we also analyze the intermittency properties of electrostatic turbulence by measuring anomalous scaling exponents of density and velocity structure functions. Copyright (C) EPLA, 2009

Optimization of machine operation, including plasma position control, density control and especially feedback control on multiple magnetohydrodynamic modes, has led RFX-mod to operate reliably at 1.5 MA, the highest current ever achieved on a reversed field pinch (RFP). At high current and low density the magnetic topology spontaneously self-organizes in an Ohmical helical symmetry, with the new magnetic axis helically twisting around the geometrical axis of the torus. The separatrix of the island disappears leaving a wide and symmetric thermal structure with large gradients in the electron temperature profile. The new topology still displays an intermittent nature but its overall presence has reached 85% of the current flat-top period. The large gradients in the electron temperature profile appear to be marginal for the destabilization of ion temperature gradient modes on the assumption that ions and electrons have the same gradients. There are indications that higher currents could provide the conditions under which to prove the existence of a true helical equilibrium as the standard RFP configuration. © 2008 IOP Publishing Ltd.

In the Reversed Field Pinch eXperiment RFX-mod a Gas Puffing Imaging Diagnostic (GPID) is used to investigate the dynamical structure of plasma edge turbulence. The GPID measures the radiation emitted from He gas puffed in H discharges and allows the investigation of edge plasma properties with high time and space resolution at high plasma current during the entire discharge. The characterization of edge turbulence has been carried out in terms of power spectrum, cross-correlation and toroidal velocity of structures. The scaling of such velocity with the density corresponds to different values of the dimensionless ratio nu(ci)/nu(ei) (ion cyclotron frequency/electron-ion collision frequency) ranging from below to above unity. The toroidal characteristic length of intermittent structures at different time scales has been determined by applying the conditional average technique on the structures identified by the continuous wavelet transform. These lengths are in the range 15-45 mm, while the number of structures increases with the electron density, the propagation velocity, opposite to the plasma current, decreases with the density and saturates at -20 km/s. (c) 2007 Elsevier B.V. All rights reserved.

A common feature of magnetized plasmas is the presence of fluctuations, which can lead to fully developed turbulence. Often large events - called bursts - emerge from the remaining low-level turbulence, giving an intermittent character to fluctuations; namely the statistical properties of fluctuations are found to depend on the temporal scale over which the investigation is conducted. In magnetized plasmas, the bursts are generally believed to be due to the presence of magnetic-field-aligned structures. Moreover, it has been experimentally shown that the intermittent events detected in the signals are associated with a relevant contribution to the loss of particles from the plasma. All these observations are common to plasmas spanning a wide range of temperature and density and magnetically confined both in linear and toroidal devices. In particular, in high-temperature plasmas for thermonuclear fusion, research aims at devising suitable ways to control transport by acting on the plasma structures, for instance by biasing the plasma edge using electrodes. This paper gives a characterization of the structures found in magnetized plasmas and of their contribution to the particle transport; moreover, the effect of the velocity shear on structures is addressed. Emphasis will be laid on to the most advanced diagnostics allowing the reconstruction of turbulent structures by optical and electrostatic techniques. © 2007 IOP Publishing Ltd.

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.

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 edge region of the rebuilt Extrap-T2R reversed field pinch experiment has been investigated using Langmuir probes. Radial profiles of main plasma parameters are obtained and compared with those of the previous device Extrap-T2. The spontaneous setting up of a double shear layer of E x B toroidal velocity is confirmed. The partic le flux induced by electrostatic fluctuations is calculated and the resulting effective diffusion coefficient is consistent with the Bohm estimate. A close relationship between electrostatic fluctuations at the edge and non-linear coupling of MHD modes in the core is found.

In the simple magnetized torus TORPEX, field-aligned blobs originate from ideal interchange waves and propagate radially outward due to del B and curvature induced drifts. Time-resolved two-dimensional measurements of the field-aligned current density J(parallel to) associated with blobs are obtained from conditionally sampled data from a single-sided Langmuir probe and a specially designed current probe. The profile of J(parallel to) exhibits an asymmetric dipolar structure, which originates from the polarization of the blob and is consistent with sheath boundary conditions. The asymmetry results from the non-linear dependence of J(parallel to) at the sheath edge upon the floating potential. Using internal measurements, we directly confirm the existence of two regimes, in which parallel currents to the sheath do or do not significantly damp charge separation and thus blob radial velocity. To investigate the effect of the observed asymmetry of J(parallel to) on the blob motion, we carried out numerical simulations of seeded blobs, using a two-field fluid model, which evolves electron density and vorticity. Simulations are performed spanning a wide range of blob sizes covering both regimes. We use either the complete or a linearized form for the sheath dissipation term in the vorticity equation. The structure of the parallel current density and plasma potential is found to be different in the two cases. Asymmetric profiles are observed in simulations with the complete form, while symmetric profiles are obtained when a linearized form is used. Negligible effects are, however, observed in terms of blob radial velocity. The relevance of the present results for fusion devices is also discussed.

First direct experimental evidence of high frequency, high toroidal mode number (n>20), magnetic fluctuations due to unstable resistive interchange modes (g-modes) resonant in the edge region of a reversed field pinch (RFP) plasma is presented. Experimental characterization of time and space periodicities of the modes is provided by means of highly resolved in-vessel edge and insertable magnetic diagnostics. It is found that the spectral mode properties are in good agreement with the predictions of the theoretical linear resistive magnetohydrodynamic stability analysis. A simple model is proposed for the observed saturation levels of the modes., Submitted to Physical Review Letters

Progress in basic understanding of turbulence and its influence on the transport both of the plasma bulk and of suprathermal components is achieved in the TORPEX simple magnetized torus. This configuration combines a microwave plasma production scheme with a quasi equilibrium generated by a toroidal magnetic field onto which a small vertical component is superimposed simulating a simplified form of tokamak scrape off layers. After having clarified the formation of blobs in ideal interchange turbulence TORPEX experiments elucidated the mechanisms behind the blob motion with a general scaling law relating their size and speed. The parallel currents associated with the blobs responsible for the damping of the charge separation that develops inside them hence determining their cross field velocity have been measured. The blob dynamics is influenced by creating convective cells with biased electrodes arranged in an array on a metal limiter. Depending on the biasing scheme radial and vertical blob velocities can be varied. Suprathermal ion transport in small scale turbulence is also investigated on TORPEX. Suprathermal ions are generated by a miniaturized lithium source and are detected using a movable double gridded energy analyser. We characterize vertical and radial spreading of the ion beam associated with the ideal interchange dominated plasma turbulence as a function of the suprathermal ion energy and the plasma temperature. Experimental results are in good agreement with global fluid simulations including in cases of non diffusive behaviour. To investigate the interaction of plasma and suprathermal particles with instabilities and turbulence in magnetic configurations of increasing complexity a closed field line configuration has recently been implemented on TORPEX based on a current carrying wire suspended in the vacuum chamber. First measurements indicate the creation of circular symmetric profiles centred on the magnetic axis and instabilities driven in the region of strong gradients with a strong ballooning character. © 2013 IAEA Vienna.

High-frequency magnetic activity has been detected at the edge region of the RFX-mod reversed-field pinch (RFP) device. The analysis, performed by means of an insertable probe measuring the magnetic field fluctuations, shows coherent modes clearly depending on the Alfvén velocity, at a frequency in the range 0.1–1.5 MHz. Two main distinct modes are observed, characterized by low (|n| ⩽ 1) toroidal periodicity, with the features of discrete Alfvén eigenmodes. A splitting of the mode frequency is detected, a sign of the relevance of non-linearity in the physical process under study. The experimental observations could suggest an interpretation of the modes in terms of global Alfvén eigenmodes. Some hints about the possible drive mechanisms are also given.