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The identification of turbulence sources would drive to a deeper understanding of confinement dynamics in tokamak plasmas. Turbulence results from a mixture of instabilities corresponding to sources at different timescales and spatial scales. Using poloidal correlation reflectometry and multi-pin Langmuir probe, it was shown in the T-10 and the Tokamak Experiment for Technology Oriented Research (TEXTOR) tokamaks that the reflectometry frequency spectrum is the superposition of several components: broadband component, quasi-coherent (QC) modes and low-frequency components. The relevance of QC modes is associated with their link with the trapped electron mode instability. This link was exhibited in the transition from the linear ohmic confinement (LOC) to the saturated ohmic confinement (SOC) regime. A method is presented in this paper to extract the QC mode component from the reflectometry data, enabling its separation from the broadband component and the study of its time evolution. It is a first step toward the discrimination of turbulence sources. The central idea explores a way to combine the approach of signal processing and machine learning. The continuous wavelet transform on the basis of complex Morlet wavelet has proved to be efficient in providing a decomposition of a signal at different scales over time for fluctuation tackling; clustering techniques, such as the mini-batch K-means, are able to tackle clusters at different scales. The method was applied to Tore Supra and TEXTOR reflectometry data. In Tore Supra, the amplitude of the extracted QC mode component decreases during the LOC–SOC transition. In TEXTOR, the amplitude of the coherent spectra of the extracted QC mode component is similar to the experimental coherent spectra obtained through correlation reflectometry. The developed method permits the extraction of components, preserving their physical and statistical properties.

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International audience; In Tore Supra plasmas, the perpendicular velocity measured by Doppler reflectometry was observed to reverse in a localized zone close to a normalized radius???0.5?0.6, changing from a negative value (corresponding to a negative radial electric field E r ) to a positive value ( ##IMG## [http://ej.iop.org/images/0029-5515/57/4/046021/nfaa59bbieqn001.gif] E_\textr>0 ). This occurs in L-mode, ohmic plasmas with a negligible external momentum input, a non-circular limited cross-section, and an edge safety factor close to 3. This reversal is favoured by a decrease in the magnetic field, or an increase in density. It is accompanied by a characteristic behaviour of the MHD activity signal, whose amplitude decrease during a ramp-down of the edge safety factor as it approaches ##IMG## [http://ej.iop.org/images/0029-5515/57/4/046021/nfaa59bbieqn002.gif] q_a∼ 3.1 ?3.2. A m / n ??=??2/1 mode is involved in the mechanism causing these observations.

International audience; The sustainment of steady-state plasmas in tokamaks requires efficient current drive systems. Lower Hybrid Current Drive (LHCD) is currently the most efficient method to generate a continuous additional off-axis toroidal plasma current as well as reduce the poloidal flux consumption during the plasma current ramp-up phase. The operation of the Tore Supra ITER-like LH launcher has demonstrated the capability to couple LH power at ITER-like power densities with very low reflected power during long pulses. In addition, the installation of eight 700kW/CW klystrons at the LH transmitter has allowed increasing the total LH power in long pulse scenarios. However, in order to achieve pure stationary LH sustained plasmas, some R&D are needed to increase the reliability of all the systems and codes, from the RF sources to the plasma scenario prediction. The CEA/IRFM is addressing some of these issues by leading a R&D program towards an ITER LH system and by the validation of an integrated LH modeling suite of codes. In 2011, the RF design of a mode converter has been validated at low power. A 500 kW/5 s RF window is currently under manufacturing and will be tested at high power in 2012 in collaboration with NFRI. All of this work aims to reduce the operational risks associated with the ITER steady-state operations.

Images of wide-angle visible standard CCD cameras contain information on Dust Creation Events (DCEs) that occur during plasma operations. Database on the DCEs can be built by analyzing the straight line-like dust trajectories in scrape-off layer caused by plasma–dust interaction along the vacuum vessel. The database provides short/long term temporal evolution and spatial distribution of origins of DCEs in fusion devices. We have studied DCEs of 2011 KSTAR campaign and compared with that of 2006 Tore Supra (TS) and 2007 ASDEX Upgrade (AUG) campaign. An analysis software, with which the location of dust trajectories in 3D position in the KSTAR vacuum vessel is identified, is developed and the dust velocity distribution in 2011 campaign is measured. © 2001 Elsevier Science. All rights reserved.

Measurements of electron temperature fluctuations by means of correlation electron cyclotron emission (ECE) diagnostics aid in understanding the nature of the turbulent transport infusion plasmas. On Tore Supra tokamak, a 32-channel heterodyne ECE radiometer has been upgraded to include two channels for temperature fluctuation measurements. The central frequency of the yttrium iron garnet filter on each channel is remotely monitored by a driver, allowing one to shift the observation volume in the plasma radially. To resolve the fluctuation amplitude of 0.1%, integration times of similar to 3 s at 1 MHz sampling rate are required for the statistical error level of 0.05%. Together with the 32-channel profile radiometer, correlation ECE is a powerful and unique tool for simultaneous observation of the magnetohydrodynamic phenomena and studies of the plasma transport properties in various heating regimes during long-shot multimegawatt operation. First measurements of temperature fluctuations, of various origins, on Tore Supra (including the observation of the toroidicity-induced Alfven eigenmodes), as well as a description of the analysis methods used in the data evaluation, are reported in this paper.

Backscattering of a microwave beam close to the cut-off allows for measurement of density fluctuations at a specified wave-number, selected by the scattering geometry , where ki is the beam wave-number at the reflection layer. On the Doppler reflectometry system installed on Tore Supra, both the scattering wave-number k⊥ and the scattering localization (r/a) can be changed during the shot owing to the steppable probing frequency and the motorized antenna. Operating in O mode, the spatial and wave-number ranges depend essentially on density profile, typically probing 0.5 < r/a < 0.95 and 2 < k < 15 cm−1. Wave number spectra are similar to those obtained with conventional scattering systems. The perpendicular fluctuation velocity in the laboratory frame is obtained from the Doppler shift of the frequency spectrum Δω = k⊥v⊥. It is dominated by the plasma Er × B velocity. In the core, the latter is mainly due to the projection of the toroidal velocity, as this is shown by comparison with measurements by charge exchange recombination spectroscopy. In the set of analysed Tore Supra ohmic and ICRH plasmas, the observed rotation is consistent with a poloidal velocity in the electron diamagnetic direction and/or a toroidal velocity in the counter current direction. The detailed structure of the velocity profile, at the edge and in different plasma regimes, allows us then to get information on the radial electric field distribution. The dynamics of the fluctuation velocity can be studied from the time frequency analysis of the signal, for investigating intermittent behaviour and transient regimes.

High frequency (> 100 kHz) modes have been observed for the first time in combined LHCD and ICR-heated minority H (D) plasmas on Tore Supra by means of a correlation ECE diagnostic and by fluctuation reflectometers. These modes have been identified as the toroidicity-induced Alfven eigenmodes (TAEs) that are destabilized by fast minority hydrogen ions with energies above 250 keV. In reversed central shear plasmas with non-linear oscillations of the central temperature, evolution of the TAE frequency is linked to the development of the local safety factor. This confirms that the central temperature and the q-profile evolve as a non-linearly coupled system in the so-called O-regime in Tore Supra.

To shed some light on the development of the fast m/n = 1/1 precursor to the sawtooth crash and its influence on plasma transport properties in the vicinity of the q = 1 surface, series of dedicated experiments have been conducted on the Tore Supra and TEXTOR tokamaks. It has been concluded that, before a crash, the hot core gets displaced with respect to the magnetic axis, drifts outwards by as much as 8–10 cm and may change its shape. Observation of the magnetic reconnection process has been made by means of electron cyclotron emission diagnostics. The heat pulse is seen far outside the inversion radius. The colder plasma develops a magnetic island on the former magnetic axis, after the hot core expulsion. Different kinds of behaviour of the m = 1 precursor before the crash, with respect to the displacement of the hot core and the duration of the oscillating phase, have been observed. An ideal kink model alone cannot be used for explanation; therefore, resistive effects play an important role in the mode development. Possible mechanisms that lead an m = 1 mode to such behaviour, and their links to the change in the central q-profile, are discussed. Results have been discussed in the light of various theoretical models of the sawtooth.

The experimental programme of Tore Supra, the largest superconducting tokamak in the world (a = 0.72?m, R = 2.4?m, Ip < 1.7?MA, BT < 4.5?T) was devoted in 2003 to studying the heat removal capability and particle exhaust in steady-state fully non-inductive current drive discharges simultaneously. This required both advanced technology integration and steady-state real-time plasma control. In particular, an improvement of the plasma position to within a range of few millimetres, and new real-time controls of radio frequency power and various actuators built around a shared memory network, have allowed Tore Supra to access a powerful steady-state regime with an improved safety level for the actively cooled plasma facing components. Feedback controlled fully non-inductive plasma discharges have been sustained in a steady-state regime for up to 6?min with a new world record for injected?extracted energy exceeding 1?GJ. Experimental results and an analysis of the physics involved in these discharges are reported and discussed.

The Tore Supra gigajoule (GJ) scenario was developed during the 2002 experimental campaign, with the goal of realizing long pulses in a stationary state, at a loop voltage therefore close to zero. This challenge has led to very successful results, with a record injected energy of 0.75 GJ and perfect control of the plasma density over 4 min 25 s. The performance is however limited by the lower hybrid system reliability and MHD limits. This paper focuses on the three kinds of MHD activity that were observed in the development of this scenario. The first one, probably a manifestation of the feedback control on the plasma position, is benign and, once triggered, stays at a low level. The second one is the triggering of a fast growing instability, which leads to a strong degradation of the confinement in the core region and to a redistribution of the plasma current. After such events, a regime with a persistent MHD activity sets in and represents the third kind of MHD activity in the GJ experiments. This paper reports on the analysis of this MHD activity. Perspectives for the future development of the GJ scenario in Tore Supra are drawn.

Wall conditioning techniques applicable in the presence of permanent toroidal magnetic field will be required for the operation of ITER, in particular for recovery from disruptions, vent and air leak, isotopic ratio control, recycling control and mitigation of the tritium inventory build-up. Ion Cyclotron Wall Conditioning (ICWC) is one of the most promising options and has been the subject of considerable recent study on current tokamaks. This paper reports on the findings of such studies performed on European tokamaks, covering a range of plasma-facing materials: TORE SUPRA, TEXTOR, ASDEX Upgrade and JET. (C) 2010 Elsevier B.V. All rights reserved.

Measurements of ion energies in the boundary of tokamak plasmas in L-mode discharges and during ELMs are reviewed. A profile of the ion-to-electron temperature ratio T i / T e from the edge of the confined plasma into the scrape-off layer (SOL) is produced by compiling the available T i measurements. The picture that emerges is that in the SOL, as well as in the edge, T i is systematically higher than T e (ratios up to 10 just outside the last closed flux surface) for most plasma parameter regimes. Far SOL ELM ion energies measured in JET, and more recently in MAST and AUG, agree with the models of the ELM transients, providing strong evidence that ELM ions can reach the first wall with significant fraction of the pedestal energies.

International audience; In a dedicated collisionality scan in Tore Supra, the geodesic acoustic mode (GAM) is detected and identified with the Doppler backscattering technique. Observations are compared to the results of a simulation with the gyrokinetic code GYSELA. We found that the GAM frequency in experiments is lower than predicted by simulation and theory. Moreover, the disagreement is higher in the low collisionality scenario. Bursts of non harmonic GAM oscillations have been characterized with filtering techniques, such as the Hilbert-Huang transform. When comparing this dynamical behaviour between experiments and simulation, the probability density function of GAM amplitude and the burst autocorrelation time are found to be remarkably similar. In the simulation, where the radial profile of GAM frequency is continuous, we observed a phenomenon of radial phase mixing of the GAM oscillations, which could influence the burst autocorrelation time.

The deuterium Dα line emitted in front of an equatorial neutraliser plate in Ergodic Divertor Tore Supra plasmas is a common diagnostic of recycling particles in present Tokamaks. First, the possible perturbation of the Dα spectral lineshape by the nearby HeII line Brβ has been studied. Although its effect on the Da line profile shape is evaluated to be non negligible in plasmas with large Helium contents, it is concluded that it should be of no influence in usual fusion plasmas. Second, a detailed study of the Da spectral lineshape has been performed to obtain temperatures and densities of the different neutral deuterium species. A synthetic profile method taking into account Zeeman and Doppler effects has been used. This shows that two deuterium populations with specific behaviours have to be considered: the first one ('cold atoms') is composed of atomic deuterium coming from D 2 desorbed from the neutraliser plate, while the second one ('warm atoms') is composed of charge exchange (CX) and reflected deuterium. The Doppler temperature of cold atoms increases with the edge electron density, n edge e (1.4 eV 10 18 m -3 ) is found for the whole range of edge electron temperature (10 eV < T e < 40 eV) while the ratio of the atomic to the molecular densities is around 1.8. The density of atoms coming from molecular dissociation is ∼ 0.8 times the density of atoms created by charge exchange or reflection of deuterons on the neutraliser.

Upgrade of the Tore Supra ergodic divertor (ED) has led to significant progress in ED physics. Pulse durations of 30?s with LHCD have been achieved demonstrating the heat exhaust capability of both the actively cooled technology at hand and of this specific divertor concept. The disruptive limit governed by the stochastization of the outer magnetic surfaces is found to occur for a value of the Chirikov parameter reaching two on the magnetic surface q = 2+(3/12). This experimentally observed robustness allows one to operate at very low safety factor on the?separatrix (q~2). Numerical analysis of ballooning turbulence in a stochastic layer indicates that the decay of the density fluctuations is associated with an increase of the fluctuating electric drift velocity. This results in an enhanced cross-field transport in the vicinity of the target plates. This lowering of confinement appears to be compensated by an intrinsic transport barrier on the electron temperature. The three-dimensional response of the temperature field is computed with a fluid code. The code can recover the intrinsic transport barrier at the separatrix, reported experimentally, together with small amplitude temperature modulations in the divertor volume. Experimental evidence for the three density regimes (linear, high recycling and detachment) is reported. The low critical density values for transitions between these regimes indicate that similar parallel physics governs the axisymmetric and ED, despite the open configuration of the latter. Measurement and understanding of these density regimes provide a means for feedback control of plasma density and an improvement in ion cyclotron radiofrequency heating coupling scenarios. Experimental data also indicated that particle control with the vented target plates is effective. Increase of both impurity control and radiation efficiency are reviewed. Global power balance has been analysed in order to account for non-axisymmetric radiation. These results, taken together, confirm the large radiation capability of the ED.

International audience

A review of the LH current drive experiments carried out on the Tore Supra tokamak is presented. The issues for an effective application of the LH wave at high power in reactor relevant conditions are highlighted. A promising performance has been obtained with the new launcher that is designed to couple up to 4 MW during 1000 s at a power density of 25 MW m-2. The heat load on the guard limiter of the antenna and the fast electron acceleration in the near electric field of the grill mouth remain at a low level, while the mean reflection coefficient never exceeds 10%. The powerful diagnosis capabilities of hard X ray (HXR) fast electron bremsstrahlung tomography has led to significant progress in the understanding of LH wave dynamics. The role of the fastest electrons driven by the LH wave is clearly identified. From HXR measurements, an increase of LH current drive efficiency with plasma current is predicted and confirmed by a direct determination at zero loop voltage. Lower hybrid power absorption is observed to be off-axis under almost all plasma conditions, and its radial width clearly depends on the antenna phasing conditions. A correlation between the HXR profiles and the onset of an improved core confinement is identified in fully non-inductive discharges. This regime ascribed to some vanishing of the magnetic shear is found to be transient and usually ends when the minimum of the safety factor becomes very close to 2, leading to a large MHD activity. Experimental observations and numerical simulations suggest that LH power is absorbed after a few transits across the plasma. However, besides toroidal mode coupling, additional mechanisms probably contribute to a spectral broadening of the LH wave.

With a new lower hybrid (LH) antenna that will allow us to couple up to 4 MW for 1000 s at a power density of less than 25 MW m-2, and improved diagnosis capabilities provided by the hard x-ray (HXR) fast electron bremsstrahlung tomography, significant progress has been made towards the achievement of fully non-inductive current driven plasmas in TORE SUPRA. Very promising performances have been obtained with the new launcher successfully tested up to 3 MW. At equivalent power levels, coupling properties behave like those of the former antennas (mean reflection coefficient ≤10%), but the heat load on the guard limiter as well as the fast electron acceleration in the near electric field of the grill mouth are considerably reduced. Using the two launchers, a full non-inductive discharge has been sustained without MHD activity for 9.5 s by 4.7 MW of LH power, at a plasma current Ip of 0.8 MA. The mean LH current drive efficiency at zero-loop voltage and for plasma currents lower than 0.8 MA is found to be 0.65×10+19 A W-1 m-2. HXR measurements suggest a significant improvement of the current drive efficiency with Ip, as a consequence of a widening of the quasilinear plateau towards higher velocities, a tendency which is well confirmed experimentally. Profiles of the HXR emission indicate that LH power is absorbed in a very few number of passes at Bt = 3.9 T, the plasma equilibrium playing in this case an important role in the wave dynamics, in agreement with ray-tracing and Fokker-Planck calculations. However, besides toroidal mode coupling, additional mechanisms are likely to contribute to a spectral broadening of the LH wave. When LH power absorption is broad and off-axis, an improved electron core confinement is observed in fully non-inductive discharges. This regime, which is ascribed to some vanishing of the magnetic shear, is found to be transient and usually ends when the safety factor becomes very close to 2, leading to the onset of large MHD activity.

Recently, reversed magnetic shear operation was performed using only ion-cyclotron-resonance frequency minority heating (ICRH) during current ramp-up. A wide region of reversed magnetic shear has been obtained. For the first time, an electron internal transport barrier sustained by ICRH is observed, with a dramatical drop of density fluctuations. This barrier was maintained, on the current flat top, for about 2 s.

Although ergodic divertors are primarily designed to control particle and heat fluxes at the plasma edge, they also happen to affect the MHD stability of tokamak discharges. On Tore Supra, the ergodic divertor has long been known to stabilize the m/n = 2/1 tearing mode induced, for instance, by edge radiation and detachment processes, thus allowing safe high-current and high-density operations. More recently, though, in discharges where ergodic divertor operations were optimized relative to the control of the edge-plasma (i.e. with large divertor perturbation), a detrimental increase in the disruptivity has been observed. The action that the ergodic divertor has on the MHD activity is interpreted in terms of a redistribution of the current profile. The latter results from a large increase in the edge resistivity, primarily induced by the degradation of the electron energy confinement in the ergodic layer. The possibility that a transport barrier develops in the vicinity of the separatrix strongly affects the considered modelling.

The lower hybrid current drive (LHCD) system of Tore Supra has been upgraded for long pulse operation at higher power (7-8MW). The two launchers have coupled on plasma 3.8MW and 2.7MW separately. This new power capability allows extending the operational domain of Tore Supra for long pulses at higher current and density. 38 long (20s-155s) discharges with very low loop voltage (VL=30-60mV) were performed with combined LHCD (5-5.7MW) and ICRH (1-3MW) powers, with up to 1GJ of injected energy. Higher LHCD efficiency, with respect to the previous long discharges, is reported. MHD stability of these discharges is very sensitive to the LHCD power and parallel wave index, in particular in the preforming phase. The ICRH power also has a beneficial effect on the MHD stability.

On many tokamaks (C-Mod, EAST, FTU, JET, HT-7, TS), a decrease in current drive efficiency of the Lower Hybrid (LH) waves is observed in high electron density plasmas. The cause of this behaviour is believed to be: Parametric Instabilities (PI) and Scattering from Density Fluctuations (SDF). For the ITER LH system, our knowledge must be improved to avoid such effects and to maintain the LH current drive efficiency at high density. The ITPA IOS group coordinates this effort [1] and all experimental data are essential to validate the numerical codes in progress. Usually the broadening of the LH wave frequency spectrum is measured by a probe located in the plasma edge. For this study, the frequency spectrum of a reflected power signal from the LH antenna was used. In addition, the spectrum measurements are compared with the density fluctuations observed on RF probes located at the antenna mouth. Several plasma currents (0.6 to 1.4 MA) and densities up to 5.2 × 1019 m−3 have been realised on Tore Supra (TS) long pulses and with high injected RF power, up to 5.4 MW-30s. This allowed using a spectrum analyser to make several measurements during the plasma pulse. The side lobe amplitude, shifted by 20-30MHz with respect to the main peak, grows with increasing density. Furthermore, for an increase of plasma current at the same density, the spectra broaden and become asymmetric. Some parametric dependencies are shown in this paper.

International audience

The behavior of energetic particle driven modes in magnetically confined plasmas is an important issue for ITER, once these modes can affect the transport of the fast particles. This paper presents some data analysis methods suitable for dealing with these modes. These methods are exemplified through the characterization of electron fishbones and Beta-induced Alfven Eigenmodes instabilities in the tokamaks Tore Supra and FTU.

International audience; Ripple-induced thermal loss effect on plasma rotation is investigated in a set of Ohmic L-mode plasmas performed in Tore Supra, and comparisons with neoclassical predictions including ripple are performed. Adjusting the size of the plasma, the ripple amplitude has been varied from 0.5% to 5.5% at the plasma boundary, keeping the edge safety factor constant. The toroidal flow dynamics is understood as being likely dominated by turbulence transport driven processes at low ripple amplitude, while the ripple-induced toroidal friction becomes dominant at high ripple. In the latter case, the velocity tends remarkably towards the neoclassical prediction (counter-current rotation). The radial electric field is not affected by the ripple variation and remains well described by its neoclassical prediction. Finally, the poloidal velocity is of the order of the neoclassical prediction at high ripple amplitude, but significantly departs from it at low ripple.

A collisionality scaling experiment associating a confinement analysis, turbulence measurements across the whole plasma and gyrokinetic modelling is reported. In Tore Supra L-mode plasmas, mid-radius dimensionless collisionality ν* has been varied performing a four-points scan from ≃0.1 to ≃0.7. The normalized confinement time exhibits a dependence with respect to collisionality: Bτ E ∝ ν* −0.3±0.3 which is strongly modified when accounting for the confinement dependence on the normalized Larmor radius, ρ*, and normalized pressure, β, since one obtains Bτ E ∝ ν* 0.0±0.7. This weak dependence is consistent with ITER L mode scaling laws and dedicated experiments elsewhere (Luce 2008 Plasma Phys. Control. Fusion 50 043001). The global analysis is confirmed by normalized effective heat transport coefficients which do not vary outside their error bars in a limited radial range of reliability. The analysis is completed by density fluctuation δn e/n e measurements across the whole plasmas. For normalized radius r/a < 0.7, δn e/n e does not depart from its error bars and the radial wave-vector spectra are not modified. These observations are well reproduced by non-linear gyrokinetic simulations, where, despite high ν* values, no zonal flow damping mechanism is at play. At the plasma edge (r/a > 0.7), the lowest poloidal wave-vector measured by the Doppler reflectometer exhibits a decrease in δn e/n e with increasing ν*, while the other turbulence measurements remain unaffected.

International audience; A better understanding of turbulent transport in a tokamak plasma requires precise comparisons between experimental observation and theoretical prediction of micro-turbulence characteristics. The repartition of fluctuation energy over different spatial scales, which contains detailed information about the character of underlying instabilities and the mechanisms involved in energy transfer between different scales, is one of the few quantities allowing a high detail comparison. The present article reports the investigation performed on the Tore Supra tokamak on the wavenumber spectrum of micro-turbulence using Doppler backscattering. The theoretical approach consists of the derivation of spectral models that include interactions between fluctuations and large scale flow structures. Une meilleure compréhension du transport turbulent dans les plasmas de tokamak exige des comparaisons précises entre les observations expérimentales et les prédictions théoriques des caractéristiques de la micro-turbulence. La répartition de lʼénergie des fluctuations sur les différentes échelles spatiales, qui contient des informations sur le type instabilités sous-jacentes et sur les mécanismes de transfert dʼénergie entre échelles spatiales, est lʼune des rares quantités permettant une comparaison de niveau élevé. Cet article présente le travail mené sur le tokamak Tore Supra sur lʼétude du spectre en nombre dʼonde de la micro-turbulence mesuré par rétro-diffusion Doppler. Lʼapproche théorique consiste en la dérivation de modéles spectraux qui inclut les interactions entre les fluctuations et les structures dʼécoulement de grandes échelles.

International audience

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International audience

In the framework of the ion cyclotron resonance frequency heating development at CEA Cadarache, a prototype antenna based on the load-resilient electrical layout foreseen for ITER has been built. This prototype was recently tested in Tore Supra. The ITER-like electrical scheme has been validated during fast perturbations at the edge plasma. Clear load resilience properties are reported. The main conclusions and consequences learned from the development of the ITER antenna are discussed.

International audience; The Tore Supra tokamak is the largest superconducting magnetic fusion facility, has been devoted to long-duration high-performance discharge research. With a steady-state magnetic field and water cooled plasma facing components, discharges up to 6 minutes 24 seconds duration with injected / extracted energy up to 1 GJ have been performed. The Tore Supra real time measurements and control (RTMC) system has been upgraded to address schemes dedicated to long pulse operation with simultaneous control of an increasing number of plasma parameters. This includes plasma equilibrium control with possible self calibration during the discharge, plasma density control with possible pellet injection, current profile control to avoid magneto-hydrodynamic (MHD) instabilities and infrared monitoring of plasma facing components preventing overheating. Most of these improvements are relevant to the tokamaks operation in a fully steady state regime.

Long pulse operation on the Tore Supra tokamak has entered a new phase, characterized by the use of heating power level in excess of 10?MW, during pulses lasting several tens of resistive times. This has been made possible by the use of ion cyclotron range of frequency (ICRF) heating (9?MW coupled to the plasma at 57?MHz), combined with lower hybrid current drive (LHCD: 3?MW at 3.7?GHz) and efficient fuelling techniques (supersonic gas injection, pellets). This paper addresses key technological, operational and physics issues related to the long pulse operation of the Tore Supra ICRF system and required for a reactor: R&D on the ICRF plant, real-time control and safety procedures, integration with other tokamak subsystems, experimental investigation and theoretical modelling of the edge ICRF physics (wave coupling, heat loads on antenna front faces). As far as possible lessons are drawn from the experience gained on Tore Supra for the design and operation of a next-step device.

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Images of wide-angle visible standard CCD cameras contain information on dust creation events (DCEs) that occur during plasma operations. Analysing the straight line-like dust traces in the shallow volume of scrape-off layer along the vacuum vessel, caused by plasma–dust interaction, the database on the DCEs is built. The database provides short/long term temporal evolution and spatial distribution of origins of DCEs in fusion devices. We have studied the DCEs of CIMES (2006) and DITS (2007) Tore Supra (TS) campaigns, and the DCEs of the 2007 ASDEX Upgrade (AUG) campaign. The results from the TS CIMES campaign show different patterns of DCEs meaning different plasma–wall interaction depending on power coupling. The TS DITS campaign indicates that dusts may be an operational limit if a fixed plasma operation scenario is used repeatedly. Different behaviours of DCEs between the carbon limiter machine and the full tungsten divertor machine are found, which is important for next generation fusion machines like ITER.

International audience; Various indications on the weakly nonlocal character of turbulent plasma transport both from experimental fluctuation measurements from Tore Supra and observations from the full-f, flux-driven gyrokinetic code GYSELA are reported. A simple Fisher equation model of this weakly nonlocal dynamics can be formulated in terms of an evolution equation for the turbulent entropy density, which contains the basic phenomenon of radial turbulence spreading in addition to avalanche-like dynamics via coupling to profile modulations. A derivation of this model, which contains the so-called beach effect, a diffusive and convective flux components for the flux of turbulence intensity, in addition to linear group propagation is given, starting from the drift-kinetic equation. The proposed model has the form of a transport equation for turbulence intensity, and may be considered as an addition to transport modelling. The kinetic fluxes given, can be computed using model closures, or local gyrokinetics. The model is also used in a particular setup that represents the near edge region as a relatively stable zone between the core and edge region where the energy injection is locally more substantial. It is observed that with constant, physical coefficients, the model gives a convincing qualitative profile of fluctuation intensity when the turbulence is coming from the core region with either a group velocity or a convective flux.

The paper gives a picture of the present status and understanding of technology and physics of Lower Hybrid Current Drive for long pulse operation in tokamaks, including the development of continuous wave (CW) high power klystrons, and its evolutions towards ITER. 3.7 GH / 700 kW CW klystrons produced in series by Thales Electron Devices are now in operation on Tore Supra. First series of eight klystrons delivered more than 4 MW to sustain non-inductive plasmas during 50 s. Moreover, a prototype of 500 kW CW klystron operating at 5 GHz developed for KSTAR by Toshiba Electron Tubes and Devices, and foreseen for ITER, is able to produce RF output powers of 300 kW / 800 s and 450 kW / 20 s on matched load. The situation on wave coupling and antennas is reported, with the latest Tore Supra results of the new CW Passive-Active Multi-junction (PAM) launcher: the antenna concept foreseen for ITER. First experiments with the PAM antenna in Tore Supra have provided extremely encouraging results in terms of power handling and coupling. Relevant ITER power density of ∼25 MW/m 2 (2.7 MW of power injected into the plasma) has been maintained over ∼80 s. In addition, LH power of 2.7 MW has been coupled at a plasma-antenna distance of 10 cm.

International audience; Frequency swept reflectometry has reached the symbolic value of 1 µs sweeping time, thisperformance has been made possible due to an improved control of the ramp voltage drivingthe frequency source. In parallel, the memory depth of the 1 Gs/s acquisition system has beenupgraded and can provide up to 200 000 density profiles during a plasma discharge.Additionally, improvements regarding the trigger delay determination for the acquisition, whichneeds to be precisely set and the frequency sweep linearity required by this ultra-fast techniquealong with the stability of the ramp voltage driving the VCOs, have been made. While thisdiagnostic is traditionally dedicated to the density profile measurement, such a fast sweepingrate can provide the study of fast plasma events and turbulence with unprecedented time andradial resolution from the edge to the core and thus compete with the fixed frequency systems.Experimental results obtained on ASDEX Upgrade plasmas will be presented to demonstratethe performances of the diagnostic.