Search

A simple analytic model for the repartition of the Scrape-Off Layer (SOL) exhaust power between the inner and outer divertors in a diverted low-density tokamak plasma is introduced. Electron heat conduction is assumed to dominate the heat transport, from the outboard mid-plane to the divertor targets, with no heat sinks or sources in the SOL. Both divertor channels are in the attached, high-recycling regime. The model is in reasonable qualitative agreement with recent TCV experimental data and EMC3-Eirene simulations. For the Single Null divertor, it reproduces the experimentally observed increase in the power ratio between the inner and outer divertor plates of TCV, Pin/Pout, with increasing the outer divertor leg length or the outer target flux expansion. For the Snowflake Minus configuration, it reproduces the observed variation of Pin/Pout with X-point separation, although only for the reversed magnetic field direction. Within the model limitations, it provides a basic understanding of the power sharing in alternative divertor geometries. Keywords: Heat exhaust, SOL transport, Long-legged Single Null Divertor, Snowflake Divertor, In-out power sharing, TCV

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'.

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

The interplay between sheared E x B flows and turbulence has been experimentally investigated in the edge region of the Extrap-T2R reversed field pinch experiment. Electrostatic fluctuations are found to rule the momentum balance equation representing the main driving term for sheared flows which counterbalances anomalous viscous damping. The driving role of electrostatic fluctuations is proved by the spatial structure of the Reynolds stress and by the time behaviour of the mean energy production term which supports the existence of an energy exchange from the small scales of turbulence to the larger scales of the mean flow.

The results leading to the identification of coherent structures emerging from the background turbulence in the edge region of the reversed field pinch experiments EXTRAP-T2R and RFX are reviewed. These structures have traits of vortices in velocity field and blobs in density, and the reconstruction of their spatial structure and of their time evolution is discussed focusing on the analysis tools applied. The role of these structures in the particle anomalous transport is addressed, showing that their collisions can contribute up to 50% the total particle losses.This process is shown to be responsible for bursts in particle flux and it is found to set a characteristic collision time, which is in agreement with the statistical properties of laminar times for particle flux bursts. © 2006 The Royal Swedish Academy of Sciences.

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].

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.

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.

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.

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.

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)

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 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.

This paper reports the highlights of the RFX-mod fusion science programme since the last 2010 IAEA Fusion Energy Conference. The RFX-mod fusion science programme focused on two main goals: exploring the fusion potential of the reversed field pinch (RFP) magnetic configuration and contributing to the solution of key science and technology problems in the roadmap to ITER. Active control of several plasma parameters has been a key tool in this endeavour. New upgrades on the system for active control of magnetohydrodynamic (MHD) stability are underway and will be presented in this paper. Unique among the existing fusion devices, RFX-mod has been operated both as an RFP and as a tokamak. The latter operation has allowed the exploration of edge safety factor q edge < 2 with active control of MHD stability and studies concerning basic energy and flow transport mechanisms. Strong interaction has continued with the stellarator community in particular on the physics of helical states and on three-dimensional codes. © 2013 IAEA, Vienna.

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.