Parametrization of reflectometry fluctuation frequency spectra for systematic study of tokamak fusion plasma turbulence

We describe a way to parameterize power spectra extracted from fixed-frequency reflectometry data, with a view to systematic studies of turbulence properties in tokamak plasmas. Analysis of typical frequency spectra obtained from a new database suggests decomposition in a set of four key components: the direct current component, low-frequency fluctuations, broadband (BB) turbulence, and the noise level. For the decomposition in the identified components, different kinds of functions are tested and their fitting performance is analyzed to determine the optimal spectrum parametrization. In particular, for the BB turbulence, three models are compared qualitatively based on a number of representative spectrum test cases, notably the generalized Gaussian, the Voigt, and the Taylor model. In addition, quantitative performance testing is accomplished using the weighted residual sum of squares and the Bayesian information criterion in a large database including 350 000 spectra obtained in Tore Supra. Next, parametrization by the Taylor model is applied to Ohmically heated plasmas, and a BB energy basin is systematically observed in the core plasma region, which shrinks with decreasing radial position of the q = 1 surface. This basin might be explained by a drop of the density fluctuation level inside the q = 1 surface

Effective collecting area of a cylindrical Langmuir probe in magnetized plasma

International audienceLangmuir probe diagnostic on magnetic plasma devices often encounters more challenges in data processing than in non-magnetized plasmas, the latest itself being far from simple. In this paper, a theory of particle collection by a probe at the plasma potential in collisionless weakly ionized plasmas is constructed, accounting for velocities distributed according to the Maxwell equation and different mechanisms of particle collection depending on their speed. Experimental validation of the presented theory has been done with 2 cylindrical probes (rpr = 75 mum and Lpr = 1 cm and rpr = 0.5 mm and Lpr = 1 cm) parallel to B --> on a linear plasma device Aline, with magnetic fields of 0.0024-0.1 T and plasma densities of 1015-1017 m-3 in helium. Cylindrical probe measurements are compared to data from a planar probe perpendicular to the magnetic field, and the results for electron density, temperature, and plasma potential are presented. The introduced theory is initially constructed for a cylindrical probe but is applicable to various probe sizes, shapes, and orientations. Alongside the main subject, a number of associated issues are addressed with different details: a probe design issue relative to the magnetized environment, the "intersection" method of plasma potential evaluation, and the robustness of the conventional "1st derivative" method, a current bump near the plasma potential, lower limit for electron temperature estimation, and self-consistent calculation of electron temperature and density

IShTAR: a test facility to study the interaction between RF wave and edge plasmas

International audienceExistence of high electric fields near an RF antenna launcher causes a number of parasitic phenomena, such as arcing and impurity release, which seriously deteriorate the performance of an ICRF heating scheme in fusion devices. Limited accessibility of the near antenna region in large-scale fusion experiments significantly complicates the associated experimental studies. The IShTAR (Ion Sheath Test Arrangement) test facility has been developed with the requirement to provide a better accessibility and diagnosability of plasmas in the direct vicinity of an ICRF antenna. The purpose of this work is to give a detailed description on the experimental setup and the available diagnostics. Furthermore the paper will demonstrate the capability of the experiment to study phenomena near an ICRF antenna launcher which are relevant for large-scale fusion ICRH systems

SOL RF physics modelling in Europe, in support of ICRF experiments

A European project was undertaken to improve the available SOL ICRF physics simulation tools and confront them with measurements. This paper first reviews code upgrades within the project. Using the multi-physics finite element solver COMSOL, the SSWICH code couples RF full-wave propagation with DC plasma biasing over “antenna-scale” 2D (toroidal/radial) domains, via non-linear RF and DC sheath boundary conditions (SBCs) applied at shaped plasma-facing boundaries. For the different modules and associated SBCs, more elaborate basic research in RF-sheath physics, SOL turbulent transport and applied mathematics, generally over smaller spatial scales, guides code improvement. The available simulation tools were applied to interpret experimental observations on various tokamaks. We focus on robust qualitative results common to several devices: the spatial distribution of RF-induced DC bias; left-right asymmetries over strap power unbalance; parametric dependence and antenna electrical tuning; DC SOL biasing far from the antennas, and RF-induced density modifications. From these results we try to identify the relevant physical ingredients necessary to reproduce the measurements, e.g. accurate radiated field maps from 3D antenna codes, spatial proximity effects from wave evanescence in the near RF field, or DC current transport. Pending issues towards quantitative predictions are also outlined

Experimental study of the interaction between RF antennas and the edge plasma of a tokamak

Les antennes en opération dans la gamme de fréquence cyclotron ionique représentent un moyen utile pour chauffer du plasma dans les tokamaks et autres plasmas de fusion. Ces systèmes de chauffage sont amenés à jouer un rôle important dans le projet ITER. Conjointement avec le chauffage souhaité, les interactions parasites avec le bord du plasma et de la limite des matériaux apparaissent. Plusieurs de ces effets délétères sont causés par la formation de la radio-fréquence des gaines. L'objectif de cette thèse est d'étudier, principalement de façon expérimentale, les modifications du plasma de bord scrape-off layer causées par les effets non-linéaires des gaines RF. Cela se fait en utilisant les sondes électrostatiques (de Langmuir, Retarded Field Analyser, tunnel) pour différentes configurations du plasma et des antennes: avec des études paramétriques en fonction du déséquilibre entre les émetteurs de l'antenne, de la puissance injectée et de la densité SOL. De plus, l'influence des gaines RF sur les mesures du potentiel de la gaine avec le RFA sont analysées. Cette étude s'effectue à l'aide d'un code 1D basé sur le modèle cinétique particle-in-cell . Ces simulations ont montré que la RFA est capable de mesurer de manière fiable le potentiel gaine, toutefois cela reste limité pour les fréquences de plasma ionique pi proche de la fréquence injectée rf. Par contre, pour des conditions réelles du SOL ( pi> rf), quand RFA est magnétiquement connectée à la structure de l'antenne RF, il est fortement sous-estimé. Enfin, les mesures de RFA dans Tore Supra indiquent que les potentiels RF se propagent au moins jusqu'à de 12m de l'antenne le long de lignes de champ magnétiquesAntennas operating in the ion cyclotron range of frequency (ICRF) provide a useful tool for plasma heating in many tokamaks and are foreseen to play an important role in ITER. However, in addition to the desired heating in the core plasma, spurious interactions with the plasma edge and material boundary are known to occur. Many of these deleterious effects are caused by the formation of radio-frequency (RF) sheaths. The aim of this thesis is to study, mainly experimentally, scrape-off layer (SOL) modifications caused by RF sheaths effects by means of Langmuir probes that are magnetically connected to a powered ICRH antenna. Effects of the two types of Faraday screens' operation on RF-induced SOL modifications are studied for different plasma and antenna configurations - scans of strap power ratio imbalance, injected power and SOL density. In addition to experimental work, the influence of RF sheaths on retarding field analyzer (RFA) measurements of sheath potential is investigated with one-dimensional particle-in-cell code. One-dimensional particle-in-cell simulations show that the RFA is able to measure reliably the sheath potential only for ion plasma frequencies pi similar to RF cyclotron frequency rf, while for the real SOL conditions ( pi > rf), when the RFA is magnetically connected to RF region, it is strongly underestimated. An alternative method to investigate RF sheaths effects is proposed by using broadening of the ion distribution function as an evidence of the RF electric fields in the sheath. RFA measurements in Tore Supra indicate that RF potentials do indeed propagate from the antenna 12m along magnetic field linesNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Modélisations de phénomènes de polarisation par gaines RF ou-et faisceau de particules dans un plasma magnétisé

Ces travaux abordent la problématique des points chauds induits par des flux de particules accélérées dans les tokamaks. Il est montré que la polarisation due aux gaines du plasma de bord soumis à un faisceau électronique à haute énergie peut atteindre plusieurs centaines de volts et ainsi allonger la zone de dépôt. La notion de gaine entravée est introduite et explique l'accélération de ce dépôt par réduction du potentiel de gaine. Ensuite, une modélisation fluide 2D des tubes de flux devant les antennes ICRF a permis de calculer les potentiels rectifiés en tenant compte des courants de polarisation transverses aux lignes de champ magnétique. Le code fluide 2D mis au point valide les résultats analytiques qui montrent que ces courants de polarisation peuvent augmenter de 50% la valeur DC du potentiel rectifié par rapport aux modèles classiques sans courant. Enfin, l'application simultanée d'un faisceau et d'un potentiel RF révèle que les potentiels induits, propres à chaque phénomène, s'additionnent pour des potentiels RF bien plus grands que la polarisation due uniquement au faisceau. La déplétion de densité des tubes de flux polarisés dans les simulations PIC 2D est caractérisée mais non expliquée.This work investigates the problematic of hot spots induced by accelerated particle fluxes in tokamaks. It is shown that the polarization due to sheaths in the edge plasma in which an electron beam at a high level of energy is injected, can reach several hundreds volts and thus extend the deposition area. The notion of obstructed sheath is introduced and explains the acceleration of energy deposition by decreasing of the sheath potential. Then, a 2D fluid modeling of flux tubes in front of ICRF antennae allows us to calculate the rectified potentials taking into account RF polarization currents transverse to magnetic field lines. The 2D fluid code designed validates the analytical results which show that the DC rectified potential is 50% greater with polarization currents than without. Finally, the simultaneous application of an electron beam and a RF potential reveals that the potentials due to each phenomenon are additives when RF potential is much greater than beam polarization. The density depletion of polarized flux tubes in 2D PIC simulations is characterized but not yet explained.NANCY1-SCD Sciences & Techniques (545782101) / SudocSudocFranceF

Étude de la microturbulence par réflectométrie dans un plasma de fusion sur le tokamak Tore-Supra

La réflectométrie à balayage en fréquence en polarisation extraordinaire permet de mesurer des spectres locaux en nombre d onde radiaux, S[_n/n](kr, r), et ainsi les profils radiaux de fluctuations de densité du plasma, sur le tokamak Tore Supra. La décomposition sur une base d ondelettes outil d analyse position-fréquence a permis de prendre en compte la forte variation radiale de la turbulence mesurée. Une attention particulière a été prêtée à la validation des spectres et profils turbulents mesurés, avec d une part la confrontation avec des mesures expérimentales (réflectométries, sondes) et des simulations numériques. Cette mesure des fluctuations de densité a été appliquée à l analyse du transport local, par l étude de l impact de la collisionnalité normalisée, __, sur le confinement du plasma. Ces analyses adimensionnelles permettent de comparer directement les plasmas réalisés dans les différents tokamaks. Une nette diminution du temps de confinement normalisé a été mise en évidence : B_E _ __-0.5+-0.15. Point d innovation, les profils radiaux de fluctuation de densité mesurés ont montré une forte augmentation de la turbulence de bord (r/a>0.8) liée à l augmentation du __ augmentation aussi observée à l aide de la réflectométrie Doppler fournissant donc une base physique expliquant la diminution de B_E avec __. Les régions plus centrales n ont pas présenté de variations nettes (_n/n, _eff). Des simulations du plasma de coeur (code de stabilité linéaire KineZero et code gyrocinétique non-linéaire GYRO) ont été menées, afin d analyser le comportement expérimental du plasma.Fast-sweeping reflectometry in extraordinary mode allows direct measurement of radial wavenumber local spectra S(_n/n](kr, r), and radial profiles of density fluctuations, on Tore Supra tokamak. Wavelet-based approach a mathematical tool for position-frequency analysis made possible to consider the strong radial variation of the measured turbulence. Special consideration was given to the validation of spectra and turbulent profiles measurements, by comparing with experimental measurements (reflectometries, probes) and numerical non-linear gyrokinetic simulations. This density fluctuations measurement method has been used to analyse the local transport, by performing a dimensionless scaling on collisionnality, __. The scaling experiments allow direct comparisons of plasmas from different tokamaks. A clear decrease of the normalized confinement time of the plasma energy with the normalized collisionnality was observed : B_E _ __-0.5+-0.15. These new measurements of density fluctuations profiles have shown an intense rise of the edge turbulence (r/a > 0.8) when increasing also observed by Doppler reflectometry diagnostic providing a physical explanation of the loss of confinement with the normalized collisionnality. More central regions did not present apparent variations (_n/n, _eff). Core plasma simulations (linear stability code KineZero and non-linear gyrokinetic GYRO) were performed, in order to analyse the experimental behaviour of the plasma.NANCY1-Bib. numérique (543959902) / SudocSudocFranceF