Activity Report

Transcription

1 CONSIGLIO NAZIONALE DELLE RICERCHE Dipartimento Energia e Trasporti Activity Report Istituto di Fisica del Plasma Piero Caldirola Associazione EURATOM-ENEA-CNR Via R. Cozzi Milano Italy

2 Edited by Augusta Airoldi 2 IFP Activity Report

3 Foreword / Prefazione Negli ultimi due anni l importanza su scala mondiale della ricerca in Fisica del Plasma ha ripreso a crescere significativamente, in larga misura come conseguenza della storica decisione di avviare il Progetto ITER per un prototipo di reattore a fusione termonucleare. La complessità dell impegno organizzativo di questa impresa, centrata sull esperimento da realizzarsi in Provenza nei prossimi decenni, pone nuove sfide e richiede precisi orientamenti programmatici degli istituti di ricerca nazionali. Solo questo infatti potrà permettere loro di avere un ruolo scientifico importante e fornire un sostegno di alte competenze al sistema produttivo nazionale. Allo stesso tempo va salvaguardata l indipendenza di giudizio e di scelta scientifica degli Istituti, essendo questa la sola garanzia di progresso e di capacità innovativa della ricerca, che non può essere ridotta ad un servizio finalizzato. In questo contesto l IFP ha sviluppato al suo interno risorse umane e scientifiche che lo mettono in grado di competere e collaborare con importanti Enti di ricerca nel settore della fisica del plasma e della fusione nucleare in Europa, mantenendo sia alte competenze disciplinari che la capacità di eseguire ricerca finalizzata sotto contratti attivi. Questa capacità di sostenere gli impegni richiede soprattutto un periodo di certezze organizzative all interno del CNR e nell ambito del contratto Euratom afferente al VII programma Quadro della Comunità europea. Il settore di elezione della ricerca svolta da IFP è da molti anni quello della fisica dell interazione di onde elettromagnetiche con i plasmi di laboratorio, con particolare riguardo alle sue applicazioni al confinamento magnetico. Attualmente IFP/CNR partecipa come Unità di Ricerca Euratom al programma di ricerca sulla Fusione Termonucleare previsto nel VII Programma Quadro della Unione Europea, nell ambito del Contratto FUAI-I di Associazione Euratom-ENEA e del 14.mo Atto Aggiuntivo del Contratto ENEA- CNR. Ancora per il 2007 questo contratto coprirà forfettariamente il costo pieno del personale dell Unità di Ricerca ed i costi di investimento e funzionamento al 20% relativi al programma approvato dal Comitato di Gestione Euratom. Il più rilevante impegno assunto dall IFP continua ad essere lo sviluppo e conduzione degli impianti e degli esperimenti di riscaldamento del plasma nel tokamak FTU del CRE-ENEA mediante assorbimento risonante di onde millimetriche (ECRH), per una potenza complessiva di 1,6 MW. Gli obiettivi del biennio ed i risultati raggiunti hanno permesso di acquisire visibilità internazionale sull importante tema del controllo automatico mediante ECRH delle instabilità magnetoidrodinamiche (MHD), che costituiscono uno dei più seri problemi dei tokamak, e sull altrettanto importante problema della realizzazione di avanzate diagnostiche elettromagnetiche per misure di temperatura elettronica e ionica. Sul tokamak FTU si sono ottenuti anche importanti risultati sulla tecnica di diagnosi della temperatura ionica basata sullo scattering collettivo. Nel biennio l IFP ha operato nell ambito di numerosi contratti finalizzati alla progettazione di componenti di ITER ed ha partecipato attivamente alla gestione scientifica dell esperimento comunitario JET inviando personale con ruolo chiave per la gestione sia di Task Forces che di esperimenti. Associati a questi temi di fisica sperimentale di alto livello, l IFP ha sviluppato ed approfondito ricerche teoriche sulla propagazione e l assorbimento risonante di fasci Gaussiani di onde ciclotroniche elettroniche, e sull associata generazione non induttiva di corrente, producendo anche un codice numerico che attualmente è riconosciuto internazionalmente come il più avanzato. Grazie al lavoro di pianificazione e di conduzione di esperimenti al JET da parte del personale IFP e all analisi ed interpretazione dei risultati relativi, si è costruito un quadro coerente dei fenomeni di trasporto di energia e di momento nei principali scenari di operazione del tokamak. Si sono sviluppati modelli fluidi generali di plasmi multispecie collisionali ed esplorati gli effetti della rotazione non uniforme del plasma sulle instabilità reattive non collisionali che sono considerate tra le cause più importanti del regime turbolento di trasporto di energia in un burning plasma. Un significativo lavoro teorico è stato svolto anche nell ambito dei modelli fisici nonlineari delle instabilità resistive neoclassiche e dei meccanismi di stabilizzazione. Nel biennio considerato IFP Activity Report

4 in questo rapporto l IFP ha anche realizzato, installato e provato un nuovo apparato di misura della radiazione ECE al JET, corredandolo dei codici numerici di interpretazione degli spettri misurati. E proseguito lo sviluppo di ricadute tecnologiche nel settore di apparecchiature ausiliarie per misure di propagazione e assorbimento di microonde e di processi al plasma per il trattamento superficiale di materiali, inorganici ed organici, con registrazione di alcuni brevetti e coinvolgimento di piccole industrie specializzate. In particolare recentemente si sono sviluppate applicazioni avanzate di tecnologie al plasma nella realizzazione di polimeri semiconduttori. Per esprimere tutte le sue positive potenzialità, sviluppando le competenze di fisica del plasma orientate al progetto fusione, e cooperare utilmente con gli altri Istituti ed Enti Italiani e stranieri, nel 2006 l IFP ha iniziato il rinnovamento ed adeguamento del proprio laboratorio nella sede di Milano, basandosi interamente su un impiego razionalizzato di risorse proprie e di una concessione in comodato di materiale del CRPP di Losanna. L esperimento basato su un plasma multispecie confinato in cuspide magnetica è in fase di conclusione. Esso sarà sostituito da un nuovo apparato strumentale (GyM) consistente in una macchina lineare, attualmente in corso di allestimento, progettata per studiare in scala ridotta, secondo rigorosi criteri di similarità fisica, diversi problemi di interesse fusionistico ed applicazioni tecnologiche. Un elemento qualificante dell impianto è la sorgente a radiofrequenza costituita da un Gyrotron GYCOM da 28 GHz a 15 kw, e la possibilità di avere plasmi completamente ionizzati in regime non collisionale (a bassa temperatura e densità) facilmente diagnosticabili con sistemi di acquisizione dati moderni e automatizzati. Interessanti collaborazioni internazionali, con scambi di strumenti e personale scientifico sosterranno la realizzazione e sperimentazione. Diverse attività su contratti EFDA sono prevedibili, a partire dal 2008, su problemi delle instabilità nel divertore di un tokamak e della turbolenza ITG. Con una strumentazione adeguata e moderna con la giusta flessibilità per condurre studi di fisica del plasma che sarebbe troppo oneroso fare in grandi laboratori, l IFP, pur rimanendo un centro di ricerca di piccole dimensioni, può presentarsi nel contesto internazionale in modo attraente per collaborazioni scientifiche di valore per parecchi anni. L IFP svolge una costante azione formativa al livello di lauree e dottorati di ricerca in Fisica ed Ingegneria, con studenti e candidati di università italiane e straniere e sarà presto sede di una Borsa di Addestramento Europea EFTS, contribuendo in modo rilevante allo sviluppo delle competenze italiane nel settore fusione. Stabili rapporti culturali e operativi sono definiti con le Università degli Studi di Milano, l Università di Milano-Bicocca, il Politecnico di Milano, le Università di Padova, Pisa, Torino, Napoli Federico II, IST-CFN di Lisbona, Ecole Polytechnique Federale de Lausanne, Chalmers University of Technology di Goeteborg, Università di Uppsala, IAP dell Accademia delle Scienze Russa di Nizhny Novgorod, IPP Max-Planck, favorendo l integrazione dei gruppi universitari nei grossi progetti della fusione europea. La pluriennale esperienza e tradizione di competenze dell'ifp nella teoria della fisica del plasma e in particolare dell'interazione onde-plasma è uno dei punti di forza dell'istituto, riconosciuto internazionalmente. Queste competenze permettono di affrontare problemi di avanguardia e di acquisire numerosi contratti di ricerca e studio, mantenendo alto il livello di addestramento anche del personale più giovane e qualificando professionisti in grado di inserirsi con molto successo nei centri di ricerca internazionali. I piani di sviluppo scientifico e tecnico dell attività IFP proseguiranno, nell ambito delle iniziative europee e nazionali, in un quadro organizzativo per obiettivi, che portino a risultati verificabili, con uno sforzo naturalmente commisurato alla risorse umane e materiali disponibili. Il Direttore Enzo Lazzaro 4 IFP Activity Report

5 Contents I General information 7 II Collaborations 9 1. Experiments and modeling in toroidal fusion plasmas 1.1 ECH-LHCD synergy and disruption mitigation in FTU Collective Thomson Scattering on FTU Core transport studies in JET Transport studies in ASDEX Upgrade Control system of magnetic islands Development and operation of ECE oblique diagnostics on JET Neutron spectrometery instruments and measurements on JET Gamma spectroscopy diagnostics for JET Theoretical research in plasma physics 2.1 Gaussian beam propagation, absorption and current drive generation Interpretative ECE emission code Magnetohydrodynamic equations for multicomponent plasmas Critical island width for NTM modes Effects of sheared rotation on ITG modes stability Microislands and transport in tokamaks Electron holes in a non-uniform collisionless plasma Ultraintense electromagnetic radiation in plasmas ITER relevant studies 3.1 Capabilities of ECRH/ECCD system in ITER Studies for the ITER ECRH/ECCD system Development of innovative microwave Diplexer/Combiner Contribution to ITER reflectometry diagnostics Plasma microtorch for ITER wall diagnostic Microwave applications and instrumental facilities for wave plasma physics 4.1 Testing of the Flight Model-Flight Spares components for Planck-LFI instrument A cusp plasma device for plasma studies and technological applications Measurements of fluctuations spectra in a Carbon dusty plasma Methane cracking and hydrogen production in Ar plasma at atmospheric pressure Planning and objectives of a flexible plasma facility for scaled plasma experiments (GyM) Plasma-aided material processing 5.1 Plasma production of semiconducting and conducting polymers Plasma treatment of biodeteriorated ancient papers Plasma treatment of Polymeric Composite Materials for Industrial application (patent No.: MI2006A002482) Surface analysis of plasma exposed tokamak tiles (ERCAR task) Highly absorptive ceramic coatings for high power microwaves in the mm-waves range (patent No.: PCT/EP2006/050605) 49 IFP Activity Report

6 6. Patents European Contracts Industrial Contracts Publications Refereed Publications 55 Conference Proceedings 61 Conference Presentations 66 Scientific and technical reports 69 Books Acronym List 71 6 IFP Activity Report

7 General information In the frame of the organization of Italian Consiglio Nazionale delle Ricerche (CNR) the Istituto di Fisica del Plasma Piero Caldirola (IFP) is part of the Dipartimento Energia e Trasporti of CNR, and has an internal matrix structure along three functional lines ( commesse ) coordinated within the department project: Participation to international thermonuclear Fusion research. The mission of the Institute, grown in thirty years of activity performed in close contact with international institutions and within the Euratom research framework, is scientific and technological research in the field of laboratory plasma physics, with particular focus on the interaction of electromagnetic waves with plasmas. The institute is also a member of the Euratom-ENEA CNR Association and the Director of IFP and Head of the EURATOM Research Group in Milano is Dr. Enzo Lazzaro. The IFP professional staff amounts to 19 Permanent CNR members, 9 temporary contractors and research fellows, 5 external collaborators. The support staff includes 9 technicians, 5 administrative and secretarial personnel. Detailed information on the Institute may be obtained via web at: IFP Activity Report

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9 II Collaborations International Institutions EFDA-ITER, Garching (D) EFDA-JET, Abingdon (UK) Max-Planck Institut für Plasmaphysik, Garching (D) Max-Planck Institut für Plasmaphysik, Greifswald (D) University of Technology Darmstadt, Darmstadt (D) DRFC CEA-Association EURATOM-CEA sur la Fusion, Cadarache (F) Conseil National de Recherche Scientifique, CNRS (F) CELIA-CNRS, Université Bordeaux 1 (F) FOM Instituut voor Plasmaphysica, Rijnhuizen (NL) CFN-Instituto Superior Tecnico, Lisboa (P) CRPP-EPFL, Losanna (CH) Uppsala University (S) Chalmers University of Technology (S) INTAS Project, Brussels (B) Institute of Plasma Physics, Czech Academy of Sciences, Prague (CZ) Istituto di Fisica Generale, Russian Academy of Sciences, Mosca(CSI) Istituto di Fisica Applicata (IAP), Russian Academy of Sciences, Nizhny Novgorod (CSI) Istituto di Fisica Nucleare "Budker", Russian Academy of Sciences, Novosibirsk (CSI) Istituto di Fisica, Georgian Academy of Sciences, Tbilisi (Georgia) MIT, Cambridge (USA) LLNL, Livermore (USA) General Atomics, Livermore (USA) University of Saskatchewan, Saskatoon, Canada National Institute for Fusion Science (J) JAEA, Advanced Photon Research Center, Kizu, Kyoto, (J) National Institutions ASI - Roma CINECA-Bologna CRE-ENEA, Frascati CRE-ENEA, Bologna CNR-IASF Bologna e Milano CNR-IENI, Milano Consorzio RFX, Padova Università di Napoli "Federico II" Dip. Scienze Fisiche, Monte Sant'Angelo Università di Milano Università di Milano-Bicocca Politecnico di Milano Università di Pisa IFP Activity Report

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11 1. Experiments and modeling in toroidal fusion plasmas 1.1 ECH-LHCD synergy and disruption mitigation in FTU ECH-LHCD synergy One of the motivations of the ECRH project on FTU was to study suprathermal absorption of EC waves by the fast electrons sustained by LHCD, this being a promising mechanism for a quantitative non-inductive current drive in tokamak. Suprathermal absorption is based on an up/down shift of the EC frequency due to a combination of a Doppler effect, in case of toroidal injection of the wave, and a relativistic effect shift due to the speed of the fast electron tail. The wave is absorbed directly by the fast electrons increasing their parallel momentum and creating a net term of current drive with an efficiency larger than in the usual bulk ECCD. On FTU the down-shift (B T >B res ) absorption regimes has been widely studied and exploited in synergy experiments with LHCD carried out with a central B T =7T, plasma current in the kA range and average electron density in the m -3 range. Up to 1.2MW of EC power in O-mode have been injected in a plasma where partial current drive was achieved injecting 1MW (or more) of LH power. The EC toroidal angle was varied from 30 to +20, while the LH power was injected at n // =1.58. The cold resonance was outside the plasma column. The overall absorbed power fraction varied in the range 50-80% depending on the LHCD power and the electron density. In order to compare the synergetic effects with the results obtained by LH alone, we used the CD efficiency: η cd = I R n cd e (P LH + P EC ) (Z eff + 5) 6 with R the major radius, n e the average line density, P LH and P EC the injected LH and EC power, respectively. The last term at the r.h.s of the formula above renormalizes the data at the ideal case with Z eff =1. The synergy efficiency, calculated considering the whole injected power, is comparable with that of LH alone and two order of magnitude higher than the one obtained with bulk ECCD, as shown in Fig η cd (10 20 A W -1 m -2 ) LHCD LHCD+EC ECCD measured ECCD calculated Fig Comparison of the CD efficiency for LHCD (squares), synergetic LHCD+EC (circles) and ECCD (diamonds from calculation and triangles from measurements). The abscissa is the volume averaged electron temperature <Te> Disruption mitigation A most important issue for ITER operation is the avoidance of disruptions. Besides a fast system acting on the currents of the coils controlling the plasma position, independent IFP Activity Report

12 systems (e.g., fast gas injectors) are presently under investigation to reduce or mitigate disruptions. As ECRH power has been demonstrated to affect the evolution of the MHD activity, the driving mechanism of most disruptions, experiments have been carried out focused on controlling the evolution of disruptions by ECRH. Disruptions have been triggered in FTU by impurity injection using both the Laser Blow Off (LBO) technique and gas puffing to bring the plasma density above the Greenwald limit. The ECRH power has been triggered based on a threshold in the loop voltage, which always increases before a disruption. The experiments have been carried out using up to 3 gyrotrons (1.2 MW) and injecting the EC power, both on-axis or off-axis, in the O-mode and in perpendicular direction. Disruptions due to the density limit (n e = m -3, I p =360kA, B T =5.3T), were avoided injecting 0.8MW of ECRH power at r dep =0, after the start of MHD activity. In the case of disruptions triggered injecting metallic impurities (molybdenum on n e = m -3, I P =500kA, B T =5.3T plasma), instead, disruption avoidance, or retardation, was obtained with off-axis heating Δt D (s) Δt ECRH <100 ms Fig Disruption duration vs power deposition radius (from the ECWGB code) ρ dep Fig.1.1.2, where Δt D is plotted versus ρ dep, shows the difference between the time at which I p reaches a nearly zero level and the time at which the mode starts growing. The soft x-ray data and beam tracing calculations further show that disruption avoidance is obtained only when the power is absorbed near to the position of the m=2 island. 1.2 Collective Thomson Scattering on FTU Of special relevance among the several ITER-oriented experimental activities of FTU is the diagnostic experiment of mm-wave collective Thomson scattering (CTS) due to its being performed in the propagation window below the EC resonance, f gyr <f EC, presently proposed for ITER. A recent feasibility study clearly stated that the CTS diagnostic of the fast ions, including the fusion-born alphas, in ITER will require propagation in the X mode at f gyr =50-60GHz, to be compared with f EC0,ITER =151GHz (B T0,ITER =5.4T). Being related to the magnetic field, the possibility of performing CTS in these conditions is peculiar to a high field device as FTU. The probing radiation is provided by a gyrotron at f gyr =140GHz, shared with ECRH applications. The experiment has been run at 7T<B T0 <8T, corresponding to 196GHz<f EC <224GHz. Interesting results were obtained following the unambiguous interpretation of strongly anomalous non-thermal spectra systematically observed both in aligned and misaligned antenna conditions and finally explained at the conclusion of a dedicated experimental campaign carried out in The results being extensively reported in a dedicated paper, here we limit ourselves to summarize the main of them. 12 IFP Activity Report

13 Fig Schematic view of the geometry of one of the two multimirror quasi-optical antennas of CTS in FTU Fig schematically shows one of the two multi-mirror quasi-optical antennas we used in CTS on FTU. The gyrotron beam is injected from the top and the scattered beam is collected from the bottom of the same port. To avoid modulations at the fast magnetosonic frequency in the spectra, the scattering plane is tilted by about 10 with respect to the poloidal plane. The propagation conditions are such that the polarization of the two plasma modes is sensibly elliptical. In the experimentation the scattering volume was normally placed on the vessel axis. The several modifications implemented in the receiving system and in the operational procedures to profitably investigate the anomalous spectra, a typical example of which is shown in Fig , are described in a dedicated paper to be published S [kev] Δt=+85ms Fig Typical anomalous spectrum f [GHz] While significantly varying from shot to shot, the spectral power densities of the anomalous spectra are several orders-of-magnitude higher than those predicted for the ion-thermal feature (=0.5keV). Moreover, at its origin the spurious signal underlying these spectra is even stronger since it is collected after multiple reflections at the vessel wall and therefore subject to antenna decoupling, of the order of 50dB. Only a few spectral lines were present in a single spectrum. The line frequencies were verified to be recurrent statistically. Attempts of explaining the spectra in terms of plasmawave processes soon failed. Among the several tests performed to investigate the origin of these spectra, of special significance in definitely confirming their nature of perturbed gyrotron spectra were tests carried out with the signal picked-up directly from the high-power transmission line, hence before the probe beam entered the plasma. Indeed, these tests revealed that the spectra were produced even in the absence of plasma only, provided a toroidal magnetic field was applied. This evidence led us to investigate the possible effects of resonances and cut-offs in causing a back-reflection of the beam power. In the CTS configuration with f gyr <f EC an EC layer, an upper hybrid layer, and the right-handed cut-off for the extraordinary (X) mode are unavoidably crossed by the probe beam when propagating in the injection port. In these conditions, whenever a mode mixture is injected as it was our IFP Activity Report

14 case, the power fraction in the X mode is partially reflected at the cut-off layer, provided a breakdown plasma converting the critical layers from latent to active is excited by the beam itself. A reconstruction of the isolines of the magnetic field in the beam injection port further showed that the EC layer was located just near the port mouth (see Fig.1.2.3). Evidence was achieved that the critical layers crossed the last two mirror surfaces. While slightly displaced outwards, the other two critical layers fall close to the EC layer. A final confirmation of the interpretation given above was obtained at the end of the experimental campaign, where antenna inspection showed that wakes were present on the surfaces of the last two mirrors at a location and with an inclination exactly corresponding to the resonant (B=fgyr/28=5T) isolines for the axial magnetic fields most frequently used in the experimentation (BT0=7.2T and 8T). Fig Isolines of the magnetic field in the FTU port. The position of the final antenna mirror is also shown. Peculiarities of the layout, e.g., the high power density consequent to the necessity of containing the beam size in the port due to spatial limitations typical of a cryogenic devices, may have contributed to strengthen the gyrotron perturbation in our specific case. Nevertheless the risk for this perturbation to occur in CTS experiments with f gyr<fec can be considered quite general. Transmitting antennas such as to make the critical layers remain latent independently of the propagation conditions and of the power levels involved are therefore better adopted in these experiments. Also considering the MW power level at which the future CTS diagnostic will be operated, the constraints on the transmitting antenna indicated by our results, and the positive indications on how to overcome them they suggest, quite straightforwardly extend to the design of mm-wave CTS for ITER. 1.3 Core transport studies in JET IFP-CNR has played an active role in the coordination of transport studies in JET through the leadership of the JET Task Force Transport (TFT) by P.Mantica. In particular, the following topics have been addressed within TFT during with direct involvement of IFP-CNR: Core heat transport: electron and ion stiffness in L-mode, H-mode and hybrid plasmas has been investigated by means of electron temperature and, for the first time, ion temperature modulation. Experimental findings are consistent with the picture of anomalous transport driven by electrostatic instabilities (ITG-TEM) with a threshold in inverse temperature gradient length. Experimental values of thresholds are in the right ball-park of theory predictions, but detailed quantitative comparison of parametric dependences is on-going. JET plasmas are generally very stiff in both electron and ion channels, with stiffness level increasing with temperature, which extrapolates to very stiff behaviour and temperature profiles strongly determined by thresholds in ITER. 14 IFP Activity Report

15 Internal transport barriers (ITBs): although toroidal rotation driven by NBI is an important component in sustaining a fully formed ITB, the ITB triggering mechanisms are still not completely clear, with two main factors possibly playing a role, i.e. the negative magnetic shear and alpha stabilization effect, and the ExB shear due to an anomalous poloidal velocity. Experimentally ITB at JET are strongly triggered by proximity of the minimum q to a low order rational value. In addition, new measurements have indicated the presence of very large values of poloidal velocity during ITB phases, much larger than predicted by neoclassical theory. From Alfven cascades measurements it is clear that ITBs are formed slightly before q min reaches the rational value. The cause-effect relation between the onset of poloidal rotation and ITB triggering is under study. Electron and ion temperature modulation experiments in ITBs have also been performed, showing that ITBs are layers where the plasma is below the critical threshold for turbulence onset (see Fig.1.3.1). The heat wave behaviour when approaching and crossing the ITB has peculiar signatures fully consistent with the critical gradient picture. Fig.1.3.1: Left side) Experimental profiles at t=5.5 s of T e, T i, n e and q for shot (3.25T/2.6 MA, 3 He~20%, ICRH f=37 MHz). The ITB region is highlighted. Right side) profiles of Fourier component of amplitudes A (red squares) and phases ϕ (blue circles) at the modulation frequency (20 Hz) during the time interval s. Estimated RF power deposition profiles are also plotted (dashed black line). The strong damping of amplitudes and increase of phases in the ITB layer indicates that ITBs are narrow regions below threshold. Particle transport: further investigation of the increase in density peaking at low collisionality in H-mode plasmas has led to merging of JET and AUG database and to the prediction of a density peaking n e (ρ pol =0.2)/<n e > vol ~1.5 for ITER. Core particle sources are important, but not dominant, the main player for density peaking being an anomalous inward pinch. The peaking does not seem to depend on magnetic shear nor on temperature peaking, which is at variance with expectations from common models of turbulence-driven particle pinches such as the curvature pinch and thermodiffusion. Impurity transport: impurity transport in JET plasmas at low collisionality is dominated by anomalous turbulence-driven transport. Impurity pinches arise due to thermodiffusion, curvature and parallel velocity compressibility. Detailed studies using the linear gyrokinetic code GS2 indicate that the impurity peaking is a weak function of the impurity charge, unlike in neoclassical transport. This insensitivity of peaking to the impurity charge has been confirmed experimentally by injecting He, Ar, Ne and Ni and determining the diffusion and convection terms for each species. It has also been found that the use of RF as electron heating source is effective in eliminating impurity accumulation with respect to plasmas with no RF or RF in the ion heating scheme. The IFP Activity Report

16 effect takes place through a reversal of the convective velocity, and is attributed to the parallel compressibility pinch, which switches from inward to outward when the dominant instability changes from Ion Temperature Gradient Modes to Trapped Electron Modes. Momentum transport: a vigorous effort has been devoted to the investigation of both toroidal and poloidal momentum transport in JET. It was found that the ratio of the toroidal momentum to the ion heat diffusivity χ ϕ /χ i is well below 1, which is the standard assumption used for ITER predictions. Models like Weiland or GLF23 seem to reproduce the JET results reasonably well. An anomalously high value of the poloidal velocity with respect to its neoclassical values has been measured in JET in plasmas with ITBs. Theoretical investigation has led to suggest that such poloidal flow is generated by turbulence via terms like the Reynolds and Maxwell stresses. Comparison with the experimental data is presently under way and seems quite promising. A crucial issue that remains to be clarified is that whether the anomalous poloidal velocity is either the cause or a consequence of ITB formation. 1.4 Transport studies in ASDEX Upgrade Experiments in H-mode plasmas have shown that both heat and particle transport are sensitive to the ratio of the electron and the ion temperatures, T e /T i. While decreasing T e /T i is beneficial for confinement, increasing the electron heating in these plasmas deteriorates the confinement. H-mode plasmas with low T e /T i are often accompanied by a high toroidal rotation velocity v φ. Its gradient can destabilize the ion temperature gradient mode (ITG) through its parallel component in the parallel-velocity shear. It also has stabilizing effects since it produces an ExB shearing rate (ω ExB ). The correlation on T e /T i and v φ has been studied and compared with calculations made with the gyro-landau-fluid model GLF23 and the gyrokinetc code GS2. Experimentally it is shown that the normalized-gradient length of the ions (R/L Ti ) is correlated with both T e /T i and v φ. Peaked ion temperature profiles are obtained only with low T e /T i and high v φ, and vice-versa. The changes in T e /T i act directly on the ITG threshold, while the ones in v φ modify the ω ExB shearing rate, leading to changes in the effective threshold. 1.5 Control system of magnetic islands A crucial problem of present research in thermonuclear plasma physics in tokamaks is the control of rotating helical magnetic perturbations, associated with local distortions of the current density profile. These unstable perturbations can seriously degrade plasma and energy confinement and hamper the reliability of operation of the device. Control or total suppression is possible by restoring the current profile by means of localized injection of RF power. However the control problem requires a real-time experimental identification of the radial location of the rotating magnetic islands growing on tokamak isobaric magnetic surfaces where the field line pitch is a rational number q=m/n. Magnetic islands in a tokamak cause localized partial flattening of the plasma temperature (and pressure) profile that can be detected by radiative signals in the microwave band as Electron Cyclotron Emission (ECE) using a multichannel polychromator or radiometer tuned to frequencies corresponding to different radii R in the tokamak meridian cross-section. The typical signal can be considered as the superposition of an average component related to the equilibrium electron temperature, that to leading order is a flux function T 0 = T e (Ψ 0,t), and a sum of noise and coherent fluctuations due for instance to the magnetic perturbations that produce rotating 16 IFP Activity Report