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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.

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization., Previously documented record values of the fusion triple product in the stellarator Wendelstein 7-X are shown to be evidence for reduced neoclassical energy transport in this optimized device.

Reduced particle losses have been observed during the operation of an m/n = 6/2 resonant magnetic perturbation in the TEXTOR tokamak. The influence of the perturbation field on multi-scale turbulence and turbulent transport has been surveyed in detail across the perturbation-induced reduction of edge transport. The results indicate that with magnetic perturbation (MP), both the large-scale zonal flows and small-scale drift-wave turbulence are significantly reduced. At high MP currents, a reduction of edge transport can be realized due to primarily the decline of small-scale ambient turbulence and turbulent transport in the ergodic zone, where the turbulence eddy size is largely decreased. Investigation on the dynamic process of fluctuation quantities during the perturbation current ramp-up phase further shows that geodesic acoustic mode zonal flows and their nonlinear interaction with background turbulence decrease incessantly with increasing perturbation current. The transport reduction takes place only after small-scale turbulence starts to be strongly reduced by the MP when the MP strength reaches a certain threshold value.

Recent experiments on the ASDEX Upgrade tokamak aim at improving the physics base for ITER and DEMO to aid the machine design and prepare efficient operation. Type I edge localized mode (ELM) mitigation using resonant magnetic perturbations (RMPs) has been shown at low pedestal collisionality ( ν ∗ ped < 0 . 4 ) . In contrast to the previous high ν ∗ regime, suppression only occurs in a narrow RMP spectral window, indicating a resonant process, and a concomitant confinement drop is observed due to a reduction of pedestal top density and electron temperature. Strong evidence is found for the ion heat flux to be the decisive element for the L–H power threshold. A physics based scaling of the density at which the minimum P LH occurs indicates that ITER could take advantage of it to initiate H-mode at lower density than that of the final Q = 10 operational point. Core density fluctuation measurements resolved in radius and wave number show that an increase of R/L T e introduced by off-axis electron cyclotron resonance heating (ECRH) mainly increases the large scale fluctuations. The radial variation of the fluctuation level is in agreement with simulations using the GENE code. Fast particles are shown to undergo classical slowing down in the absence of large scale magnetohydrodynamic (MHD) events and for low heating power, but show signs of anomalous radial redistribution at large heating power, consistent with a broadened off-axis neutral beam current drive current profile under these conditions. Neoclassical tearing mode (NTM) suppression experiments using electron cyclotron current drive (ECCD) with feedback controlled deposition have allowed to test several control strategies for ITER, including automated control of (3,2) and (2,1) NTMs during a single discharge. Disruption mitigation studies using massive gas injection (MGI) can show an increased fuelling efficiency with high field side injection, but a saturation of the fuelling efficiency is observed at high injected mass as needed for runaway electron suppression. Large locked modes can significantly decrease the fuelling efficiency and increase the asymmetry of radiated power during MGI mitigation. Concerning power exhaust, the partially detached ITER divertor scenario has been demonstrated at P sep /R = 10 MW m − 1 in ASDEX Upgrade, with a peak time averaged target load around 5MWm − 2 , well consistent with the component limits for ITER. Developing this towards DEMO, full detachment was achieved at P sep /R = 7MWm − 1 and stationary discharges with core radiation fraction of the order of DEMO requirements (70% instead of the 30% needed for ITER) were demonstrated. Finally, it remains difficult to establish the standard ITER Q = 10 scenario at low q 95 = 3 in the all-tungsten (all-W) ASDEX Upgrade due to the observed poor confinement at low β N . This is mainly due to a degraded pedestal performance and hence investigations at shifting the operational point to higher β N by lowering the current have been started. At higher q 95 , pedestal performance can be recovered by seeding N 2 as well as CD 4 , which is interpreted as improved pedestal stability due to the decrease of bootstrap current with increasing Z eff . Concerning advanced scenarios, the upgrade of ECRH power has allowed experiments with central ctr-ECCD to modify the q -profile in improved H-mode scenarios, showing an increase in confinement at still good MHD stability with flat elevated q -profiles at values between 1.5 and 2. European Commission (EUROfusion 633053)

International audience; TCV is acquiring a new 1 MW neutral beam and 2 MW additional third-harmonic electron cyclotron resonance heating (ECRH) to expand its operational range. Its existing shaping and ECRH launching versatility was amply exploited in an eclectic 2013 campaign. A new sub-ms real-time equilibrium reconstruction code was used in ECRH control of NTMs and in a prototype shape controller. The detection of visible light from the plasma boundary was also successfully used in a position-control algorithm. A new bang-bang controller improved stability against vertical displacements. The RAPTOR real-time transport simulator was employed to control the current density profile using electron cyclotron current drive. Shot-by-shot internal inductance optimization was demonstrated by iterative learning control of the current reference trace. Systematic studies of suprathermal electrons and ions in the presence of ECRH were performed. The L?H threshold power was measured to be ?50?75% higher in both H and He than D, to increase with the length of the outer separatrix, and to be independent of the current ramp rate. Core turbulence was found to decrease from positive to negative edge triangularity deep into the core. The geodesic acoustic mode was studied with multiple diagnostics, and its axisymmetry was confirmed by a full toroidal mapping of its magnetic component. A new theory predicting a toroidal rotation component at the plasma edge, driven by inhomogeneous transport and geodesic curvature, was tested successfully. A new high-confinement mode (IN-mode) was found with an edge barrier in density but not in temperature. The edge gradients were found to govern the scaling of confinement with current, power, density and triangularity. The dynamical interplay of confinement and magnetohydrodynamic modes leading to the density limit in TCV was documented. The heat flux profile decay lengths and heat load profile on the wall were documented in limited plasmas. In the snowflake (SF) divertor configuration the heat flux profiles were documented on all four strike points. SF simulations with the EMC3-EIRENE code, including the physics of the secondary separatrix, underestimate the flux to the secondary strike points, possibly resulting from steady-state E × B drifts. With neon injection, radiation in a SF was 15% higher than in a conventional divertor. The novel triple-null and X-divertor configurations were also achieved in TCV.

Both the plasma poloidal curvature and the curvature of the probing wave-front play an important role for the spectral resolution of microwave reflectometry. The propagation of electromagnetic waves in a cylindrical plasma with a refractive index monotonically decreasing towards its center leads to elongation of the wave-front (i.e. an increase of the radius of curvature near the axis of radiation). Such a phase-front alteration can result in a major modification of the reflectometry response regarding on high k θ fluctuations. At the same time, the curvature of the cutoff surface in cylindrical plasma is always less than the curvature of the cutoff density for an O-mode wave. For specific geometry of the TEXTOR tokamak we perform numerical two-dimensional full-wave reflectometry synthesizing. The response from poloidaly rotating coherent density perturbation was analyzed on the variety of the plasma cutoff curvature and density scale length, which affects the incident wave-front curvature. The simulations are shown that the increase of the incident wave front curvature extends the spectral resolution of conventional reflectometer with gain antennas. On the other hand, the effect from the cutoff curvature is found to be much weaker as compare to the wave-front effect.

At TEXTOR extensive collaborations with foreign institutes have been established. For a successful collaboration, the participating scientists must be able to plan the experiment in advance. Therefore they need tools to submit and track their experiment proposals and pulse plans. During and after the experiment access to raw and evaluated data of a TEXTOR discharge is needed to analyze TEXTOR data remotely from the home labs. A goal-oriented data analysis needs in addition a various items of data not recorded with conventional data logging. This discharge specific comments and settings are stored in a logbook. At TEXTOR a logbook with different views is implemented for the scientific experiment and the machine operation, respectively. In a further development towards a TEXTOR experiment management system the information of auxiliary heating devices is stored. The electronic logbook is realized by a client/server approach which makes the access independent from the hard- and software on the client side and allows to access the data and data supplements (metadata) by any common web browsers. All information from TEXTOR is accessible for everybody via a uniform resource locator (URL). The concept of the electronic logbook, the first experience and its possibilities for data analysis will be discussed in this paper.

Long period sawteeth have been observed to result in low-beta triggering of neo-classical tearing modes, which can significantly degrade plasma confinement. Consequently, a detailed physical understanding of sawtooth behaviour is critical, especially for ITER where fusion-born a particles are likely to lead to very long sawtooth periods. Many techniques have been developed to control, and in particular to destabilize the sawteeth. The application of counter-current neutral beam injection (NBI) in JET has resulted in shorter sawtooth periods than in Ohmic plasmas. This result has been explained because, firstly, the counter-passing fast ions give a destabilizing contribution to the n=1 internal kink mode-which is accepted to be related to sawtooth oscillations-and secondly, the flow shear strongly influences the stabilizing trapped particles. A similar experimental result has been observed in counter-NBI heated plasmas in MAST. However, the strong toroidal flows in spherical tokamaks mean that the sawtooth behaviour is determined by the gyroscopic flow stabilization of the kink mode rather than kinetic effects. In NBI heated plasmas in smaller conventional aspect-ratio tokamaks, such as TEXTOR, the flow and kinetic effects compete to give different sawtooth behaviour. Other techniques applied to destabilize sawteeth are the application of electron cyclotron current drive (ECCD) or ion cyclotron resonance heating (ICRH). In JET, it has been observed that localized ICRH is able to destabilize sawteeth which were otherwise stabilized by a co-existing population of energetic trapped ions in the core. This is explained through the dual role of the ICRH in reducing the critical magnetic shear required to trigger a sawtooth crash, and the increase in the local magnetic shear which results from driving current near the q=1 rational surface. Sawtooth control in ITER could be provided by a combination of ECCD and co-passing off-axis negative-NBI fast ions.

Collapse events at theq= 2 surface occurring during the mode locking process of anm/n= 2/1 tearing mode have been observed on TEXTOR. The plasma confinement within theq= 2 surface collapses without much influence on the width of the island (O-point). With an external rotating resonant magnetic perturbation (RMP) field induced by the dynamic ergodic divertor, secondary structures moving near the separatrix of the primary large 2/1 island with the same frequency as the RMP have been observed after the collapse events. The plasma confinement recovers when the secondary structures vanish.

Quasi-coherent (QC) modes have been reported for more than 10 years in reflectometry fluctuations spectra in the core region of fusion plasmas. They have characteristics in-between coherent and broadband fluctuations as they oscillate at a marked frequency but have a wide spectrum. This work presents further evidences of the link recently established between QC modes and the trapped electron modes (TEM) instabilities (Arnichand et al 2014 Nucl. Fusion 54 123017). In electron cyclotron resonance heated discharges of Tore Supra, an enhancement of QC modes amplitude is observed in a region where TEM cause impurity transport and turbulence. In JET Ohmic plasmas, QC modes disappear during density ramp-up and current ramp-down. This is reminiscent of Tore Supra and TEXTOR observations during transitions from the linear Ohmic confinement (LOC) to the saturated Ohmic confinement (SOC) regimes. Evidencing TEM activity then becomes experimentally possible via analysis of fluctuation spectra. EURATOM 633053

The dynamic ergodic divertor (DED) on the TEXTOR tokamak allows for the reproducible destabilization of the m/n = 2/1 tearing mode which is phase locked to the external static or rotating perturbation field. In combination with its flexible heating systems (co- and counter-neutral beam injection, ion cyclotron resonance heating, electron cyclotron resonance heating (ECRH) with steerable launcher) dedicated experiments to study the mode onset, properties of large islands and mode stabilization can be performed. The dependence of the mode excitation threshold (field penetration) on the plasma rotation shows a resonance character, with minimum threshold when the external perturbation frequency matches the MHD frequency of the 2/1 mode. Mode stabilization by ECRH heating shows that for the TEXTOR plasma heating is more effective than the current drive in O-point. Extrapolation to ITER yields a significant contribution to the mode suppression originating from the temperature increase within the island. Alfven-like modes, which have been previously identified in the vicinity of large islands on FTU (Buratti et al 2005 Nuclear Fusion 45 1446), are found to be created already before island formation above a certain threshold of the externally applied perturbation field.

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.

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.

Within the last year of TEXTOR operation a major part of the experiments were dedicated to the interaction of the ergodized plasma edge, induced by the dynamic ergodic divertor (DED), with the core plasma. Systematic experiments are performed in the 3/1 configuration of the DED. Depending on β, the DED generates a locked 2/1 mode in the plasma at a critical current in the DED coils. Different diagnostics were used to measure the plasma response in the core and edge. The investigations in this paper are focused on the measurement of density fluctuation and the analysis of the coherent-mode structure outside the q = 2 surface using O-mode poloidal correlation reflectometry. A reduction of the quasi-coherent mode amplitude and frequency is observed, caused by the 2/1 mode together with a decrease of the density scale length. Moreover, the deduced poloidal rotation velocity at the q = 3 surface changes from the electron to the ion diamagnetic drift direction with the onset of the 2/1 island. Also a reduction of the density fluctuations is observed as well as a decrease in the poloidal correlation length. From Ar-injection a delayed inward propagation of the emission maximum is observed for different Ar ionization stages when the 2/1 mode is generated. Similar experiments with a slowly rotating DED show that the steepened density gradients are spatially localized and confirm the hypothesis that the 2/1 mode is responsible for the observations.

A programmable digital signal processing system designed for online measurement and control of fusion devices has been built and operated on TEXTOR–94 tokamak. It meets the following requirements: to give a wide variety of choice in the kind of functions to be performed on the measured signals, stability for a time duration of up to 1 h, at the same time measurement of MHD fluctuations with frequencies in the range of several kHz and isolate the data acquisition from high voltage spikes.The system is being used on TEXTOR e.g. to measure currents and fluxes and to retrieve control parameters from them, to calculate correlations between different signals, or to detect special events like sawtooth crashes.

Disruptions investigated on TEXTOR-94 are initiated by unstably growing modes. The island width of these modes corresponds to 5–10 cm resulting in a perturbation field amplitude of the order of 100 Gauss. The energy stored in the plasma is released in short and intense power fluxes towards the limiter; the power decay length is practically unchanged with respect to the normal power e -folding length. These observations can be interpreted in the following way: The perturbation fields associated with the modes create an ergodization of the full plasma cross-section creating a laminar zone with convective fluxes at the plasma edge. The energy transport inside the plasma is determined by the ergodic transport while the power flux to the limiter is convective. The assumption of the ergodization has important consequences for runaway electron level created during disruptions.

The radiative improved (RI) mode is a tokamak regime offering many attractive reactor features. In the article, the RI mode of TEXTOR-94 is shown to follow the same scaling as the linear ohmic confinement regime and is thus identified as one of the most fundamental tokamak operational regimes. The current understanding derived from experiments and modelling of the conditions necessary for sustaining the mode is reviewed, as are the mechanisms leading to L-RI mode transition. The article discusses the compatibility of high impurity seeding with the low central power density of a burning reactor, as well as RI mode properties at and beyond the Greenwald density.

An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of die 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high beta (up to a normalized beta, beta(n) = 2) with low edge q values (down to q(a) = 2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges an described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power remains. No deleterious effects of the neon seeding on fusion reactivity and plasma stability have been observed. (C) 1997 American Institute of Physics.

Localized magnetohydrodynamic modes occur frequently as precursors to dangerous plasma disruptions in tokamaks. Fast electron cyclotron emission (ECE) radiometry permits a space-resolved study of the dynamic behavior of these instabilities. Two types of multichannel, heterodyne ECE systems working on TEXTOR are briefly described. Temperature profile perturbations caused by tearing modes are shown. Typical Te fluctuations generated by rotating magnetic islands are presented. An early disruption warning system was built and tested on TEXTOR discharges. A real-time cross correlation calculation of two ECE signals is used to identify an m=2 mode. An analog alarm signal is generated then early during the instability evolution. An experimental test result is presented from a beam-heated tokamak plasma.

Time delay estimation (TDE) is a well-known technique to investigate poloidal flows in fusion plasmas. The present work is an extension of the earlier works of A. Bencze and S. Zoletnik 2005 and B. T\'al et al. 2011. From the prospective of the comparison of theory and experiment it seem to be important to estimate the statistical properties of the TDE based on solid mathematical groundings. This paper provides analytic derivation of the variance of the TDE using a two-dimensional model for coherent turbulent structures in the plasma edge and also gives an explicit method for determination of the tilt angle of structures. As a demonstration this method is then applied to the results of a quasi-2D Beam Emission Spectroscopy (BES) measurement performed at the TEXTOR tokamak., Comment: 8 pages, 10 figures

The mechanism governing the impact of the mass isotope on plasma confinement is still one of the main scientific conundrums facing the magnetic fusion community after more than thirty years of intense research. We have investigated the properties of local turbulence and long-range correlations in hydrogen and deuterium plasmas in the TEXTOR tokamak. Experimental findings have shown a systematic increasing in the amplitude of long-range correlations during the transition from hydrogen to deuterium dominated plasmas. These results provide the first direct experimental evidence of the importance of multiscale physics for unraveling the physics of the isotope effect in fusion plasmas.

Stationary high energy confinement is observed on TEXTOR-94 for times limited only by the flux swing of the transformer using strong edge radiation cooling. Necessary tools are the feedback control of the radiated power and of the plasma energy content. At the highest densities obtained (up to 1.2 times the Greenwald limit), energy confinement exceeds the edge-localized-mode-free H-mode scaling ITERH93-P by more than 20%. beta limits of TEXTOR-94 are reached with f(H89)/q(a) approximate to 0.6 No detrimental effect of the seeded impurity is seen. These high confinement discharges meet many conditions necessary for a fusion reactor regime.

Improved confinement is achieved on TEXTOR under high power conditions (up to 4 MW of additional heating with NBI-co+ICRH, NBI-co+counter or NBI-co+counter+ICRH) with edge radiative cooling employing silicon or neon as the radiating impurities. It is shown that in quasi-stationary conditions up to 85% of the input power can be radiated. Such high power fractions offer the possibility of utilizing these techniques to facilitate the power exhaust problem for a Tokamak reactor. Discharges with edge radiative cooling exhibit enhanced confinement properties at high densities, e.g. at a central line averaged electron density of 7.5*1013 cm-3, an enhancement factor of 1.7 over ITER L89-P confinement scaling is obtained with an edge q value as low as 2.7. Stable discharges have been obtained even with the q=2 surface located inside the radiating zone. Furthermore, for radiatively cooled discharges heated with balanced NBI-co+counter with or without ICRH, supershot-like peaked electron density profiles, with central density values above 1.0*1014 cm-3 are observed. The present results show that there is no impurity accumulation in the centre and the Ne and/or Si concentration is so low that the reactivity of the plasma remains unaffected

The heterodyne hopping reflectometer system based on wide-tuned low noise yttrium iron garnet sources was developed for TEXTOR experiment. Being installed in 1998 it successfully operates more than 10 years providing the measurements of plasma density fluctuations. Owing to the advance multihorn antennae systems installed at three different positions around the tokamak, the correlation properties as well as the propagation measurements of plasma density fluctuations are realized. The reflectometer operates in ordinary polarization mode providing the access mostly to plasma gradient and pedestal region. The capabilities of the diagnostic are illustrated with the examples of measured fluctuation characteristics in the variety of TEXTOR plasmas.

Experimental investigations of sawteeth interaction with fast ions measured by collective Thomson scattering on TEXTOR are presented. Time-resolved measurements of localized 1D fast-ion distribution functions allow us to study fast-ion dynamics during several sawtooth cycles. Sawtooth oscillations interact strongly with the fast-ion population in a wide range of plasma parameters. Part of the ion phase space density oscillates out of phase with the sawtooth oscillation during hydrogen neutral beam injection (NBI). These oscillations most likely originate from fast hydrogen ions with energies close to the full injection energy. At lower energies passing fast ions in the plasma centre are strongly redistributed at the time of sawtooth collapse but no redistribution of trapped fast ions is observed. The redistribution of fast ions from deuterium NBI in the plasma centre is found to vary throughout velocity space. The reduction is most pronounced for passing ions. We find no evidence of inverted sawteeth outside the sawtooth inversion surface in the fast-ion distribution function.

The electrostatic potential and density fluctuations have been measured at the edge of the TEXTOR tokamak by two toroidally distant Langmuir probe arrays. The geodesic acoustic mode (GAM) zonal flows (ZFs) are observed in potential fluctuations with a toroidal and poloidal symmetric structure. The GAM frequency, fGAM, changes monotonically with the local temperature and is close to the frequency-dispersion predicted by theories. Bispectral analysis shows clear nonlinear coupling between the GAM and broadband ambient turbulence. The GAM packet has a narrow radial extent with kr 0.5–0.7 cm−1 and exhibits explicitly a radially outward propagation. Furthermore, the radial correlation structure of GAMs and their radial propagation have been investigated in a wide range of parameters by varying plasma density and edge safety factor (5.0 ≤ q(a) ≤ 5.9). It is found that the magnitude of the GAM correlations reduces remarkably with the increase in the plasma density approaching the density limit, while the radial wavelength of GAMs only decreases slightly in higher density and larger q(a) discharges. With increasing plasma density, the radial propagating phase speed of GAMs is strongly reduced along with the drop in the local temperature. The results provide new evidence on the propagation properties of GAM ZFs.

A new regime of enhanced confinement (I-mode) is found in plasmas with circular cross-sections in the pump limiter tokamak TEXTOR with boronized walls. This regime is obtained with three types of auxiliary heating, namely NBI co-injection, NBI counter-injection+ICRH and NBI co-injection+NBI counter-injection, and has many similarities with the H-mode regime obtained in divertor tokamaks. The energy confinement times obtained in these discharges scale as favourably as those in stationary H-mode discharges with edge localized modes. A detailed analysis of the scaling of the confinement time with plasma current, heating power and plasma density is presented. Characteristic electron density and temperature profiles are observed, with large central values and well developed edge pedestals. They are compared with those found in H-mode discharges and supershots. A poloidal beta limit of 1.6 is found in the I-mode discharges of TEXTOR. The maximum toroidal beta values obtained reach nearly 1%, i.e. 0.7 times the Troyon limit in TEXTOR. I-mode confinement is always linked with low recycling and absence of MHD activity. If these conditions are not met. L-mode scaling is retrieved. MHD activity, which is more likely to occur at low plasma densities and currents, can cause a sudden drop to L-mode scaling. So far, no transition from the L-mode scaling to the I-mode scaling has been observed

In this paper the edge diagnostic equipment at the TEXTOR tokamak is described and its importance for detailed transport studies in the helical ergodic divertor imposed by the dynamic ergodic divertor (DED) is demonstrated. The helical ergodic divertor topology is controlled by adapting the perturbation field base mode number, the perturbation field strength and the plasma equilibrium. As unique feature the DED perturbation field can be rotated with frequencies of up to 10kHz. The impact of the helical divertor field structure and the fast rotation of the external perturbation field on local transport properties is analyzed here on selected examples from the various edge diagnostics with high time and/or high spatial resolution.

In this contribution we report on spectroscopic measurements of plasma rotation and the radial electric field in the plasma boundary under the operation of the dynamic ergodic divertor in TEXTOR. Under the influence of the stochastic magnetic field we observe an increase of both poloidal and toroidal rotation of carbon nuclei into ion-diamagnetic drift/co-current direction. With static magnetic perturbation the electric field in the ergodic region increases by 10 kV/m and becomes positive. Application of dynamic perturbation leads to a comparable change of the rotation, independent of the direction of the rotation of the DED field and only depending on the degree of ergodization. We introduce a model to explain the experimental findings by the onset of transverse currents in the ergodic region and a subsequent torque onto the plasma. (c) 2007 Published by Elsevier B.V.

Controlled experiments on the suppression of the m/n = 2/1 tearing mode with electron cyclotron heating and current drive in TEXTOR are reported. The mode was produced reproducibly by an externally applied rotating perturbation field, allowing a systematic study of its suppression. Heating inside the island of the mode is shown to be the dominant suppression mechanism in these experiments. An extrapolation of these findings to ITER indicates that the projected system for suppression of the tearing mode could be significantly more effective than present estimates indicate, which only consider the effect of the current drive but not of the heating inside the island.

The dynamic ergodic divertor (DED) in TEXTOR is designed to provide a perturbed volume in the plasma edge in order to control heat and particle exhaust. The stochastic boundary layer is generated in the outermost region of the plasma, which can be divided into ergodic and laminar regions. The topology of the magnetic field is substantial for the transport properties and plasma parameters. It is expected that the formation of the proper laminar zone allows decoupling the plasma edge from the core. Due to enhancement of the radial electron heat transport in the ergodic region the electron temperature in the plasma boundary is reduced. Therefore one needs to find the proper ratio of the ergodic and laminar zone, which gives optimal performance of the divertor. The structure of the perturbed volume strongly depends on the safety factor profile and the plasma pressure. At the higher level of ergodization (i.e. at higher plasma current and lower beta poloidal) the laminar zone is dominant, while at the lower level of ergodization the ergodic region is more important. The topology of the perturbed volume is modeled with the ATLAS‐code and basing on the results of modeling the experiments are performed and compared to the calculated structures.

The dynamic ergodic divertor (DED) on the TEXTOR tokamak allows for the creation of static and rotating helical magnetic perturbation fields. In the 3/1 configuration the strong m/n = 2/1 sideband excites a locked 2/1 tearing mode above a critical perturbation field strength. The mode onset threshold depends strongly on the plasma fluid rotation with respect to the mode. Rotation in plasma current direction destabilizes the mode in a certain range of rotation frequencies, whereas counter-rotation has a stabilizing influence. The threshold shows a minimum when the frequency of the external perturbation equals the MHD frequency of the mode.

Small-scale structures with high poloidal mode numbers ($m=10--20$) have been observed in the TEXTOR tokamak plasma with pulsed radar reflectometry and an electron cyclotron emission diagnostic, in conjunction with large $2/1$ and $1/1$ islands. The small islands have a peaked density profile, similar to that of the simultaneously observed large-scale $2/1$ islands. This together with the observation that high-frequency density and temperature fluctuations are very pronounced near the $X$ points of the large islands hints to a strongly perturbed magnetic topology around the $X$ points.

Disruptions remain a major concern for tokamak devices, particularly for large machines. The critical issues are the induced (halo) currents and the resulting forces, the excessive heating of exposed surfaces by the instantaneous power release, and the possible occurrence of highly energetic runaway electrons. The key topics of the investigations on TEXTOR in the recent years concerned (a) the power deposition pattern recorded by a fast infrared scanner, (b) the runaway generation measured by synchrotron radiation in the infrared spectral region, (c) method development for "healing" discharges that are going to disrupt, and (d) massive gas puffing for mitigating the adverse effects of disruptions.

The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.

A significant influence of the dynamic ergodic divertor (DED) on the density limit in TEXTOR has been found. In Ohmic discharges, where without DED detachment normally arises at the density limit, a MARFE (multifaceted asymmetric radiation from the edge) develops when the DED is operated in a static regime. The threshold of the MARFE onset in the neutral beam heated plasmas is increased by applying 1 kHz ac DED at the high-field side. The theoretical predictions based on the parallel energy balance taking poloidal asymmetries into account agree well with the experimental observation.

This letter reports on quasi-coherent (QC) modes observed in fluctuation spectra from Tore Supra and TEXTOR reflectometers. QC modes have characteristics in between coherent and broad-band fluctuations as they oscillate around a given frequency but have a wide spectrum. They are ballooned at the LFS midplane and appear usually on a frequency ranging from 30 to 120 kHz. In ohmic plasmas from both tokamaks, QC modes are detected only in linear ohmic confinement (LOC) regime and disappear in saturated ohmic confinement (SOC) regime. Linear simulations from Tore Supra predict that the LOC and SOC regimes are dominated by electron and ion modes respectively. Measurements of the perpendicular velocity of density fluctuations have been made from the top of TEXTOR by poloidal correlation reflectometry. They suggest that QC modes have a phase velocity similar to 400 ms(-1) higher in the electron diamagnetic direction than lower frequency fluctuations. Additionally, the onset of QC modes during electron cyclotron resonance heating has been observed in a Tore Supra region where turbulence is suspected to be driven by electron modes. These experimental results and instability calculations show a correlation between onsets of QC modes and predictions of trapped electron modes.

International audience; The geodesic acoustic mode (GAM) is a coherently oscillating zonal flow that may regulate turbulence in toroidal plasmas. Uniquely, the complete poloidal and toroidal structure of the magnetic component of the turbulence-driven GAM has been mapped in the TCV tokamak. Radially localized measurements of the fluctuating density, ECE radiative temperature and poloidal flow show that the GAM is a fully coherent, radially propagating wave. These observations are consistent with electrostatic, gyrokinetic simulations.

In order to mitigate the effect of disruptions in tokamaks, it is proposed to inject quickly a relatively large amount of helium; first experiments on this topic have been performed on TEXTOR. For this purpose, a fast valve has been developed which releases 10 mbar L of helium gas within 1 ms; the valve is located at a vessel flange such that a fast response is guaranteed even if it is triggered at the onset of the disruption. The amount of gas is sufficient to exceed the density limit even with low density discharges. The intention of the proposal is to shorten the plasma current decay phase, to reduce halo currents, to suppress runaway electrons and to provide good conditions for the start of the following discharge. In particular, for achieving the last goal, helium is the optimum choice of all the elements. The experiments performed on TEXTOR have proven various of these mitigation aspects: the current decay time is shortened, runaway electrons are expelled by the gas puff and the conditions for the start of the next discharge have neither deteriorated with respect to gas release from wall components nor with respect to excessive impurity production.

To aid the scientific research program on the Toroidal Experiment for Technology Oriented Research (TEXTOR)-94 tokamak, the existing electron cyclotron emission (ECE) diagnostics for the electron temperature studies are being extended with new systems that feature a good temporal and radial resolution. Using the possibility of obtaining the data on a fast time scale, ECE systems allow one to study fast phenomena like the dynamics of small-scale structures such as magnetic islands and transport barriers and microscopic temperature fluctuations throughout the plasma. The quantitative information about nonthermal electron populations can be obtained from the comparison of the second and third harmonic emission. A new 16-channel frequency tunable radiometer and a four-channel second harmonic system were recently installed at TEXTOR-94. The detailed description of the new TEXTOR-94 ECE systems and the recent results will be given and discussed in this article. (C) 2001 American Institute of Physics.

Three new electron cyclotron emission (ECE) systems will be installed at TEXTOR: a 16-channel ECE imaging diagnostic, a 16-channel radiometer, with frequency-tunable local oscillators in the second down-conversion step, and a 4-channel second harmonic radiometer that will be used in combination with an already available third harmonic system. The new diagnostics, in combination with the systems already present at TEXTOR offer unique measurement capabilities. In this paper, all the three systems along with their diagnostic potential will be described. (C) 2001 Published by Elsevier Science B.V.

The beta limit in radiative improved (RI mode) discharges where energy confinement is enhanced due to an increased radiation level from the edge by impurity seeding of neon has been investigated. Under these discharge conditions spontaneous confinement transitions from higher to lower energy confinement can be observed when the beta limit is approached. The empirically determined limit of the normalized toroidal beta, PN, is 2.2, pp reaches values up to 1.5 in the circular limiter tokamak TEXTOR-94. The sawtooth activity is often stabilized and replaced by central mode oscillations around q = 1 when high values (greater than or equal to 1) of the confinement enhancement factor f(H93) With respect to the ELM-free ITER-93H mode scaling are reached. Measurements of the plasma current distribution show that already before the confinement transition the central current density decreases, i.e. the current profile broadens and magnetic shear is decreased. The deterioration of energy confinement has been found to correlate with the onset of MHD activity either at the q = 1.5 and/or the q = 2 surface. The enhanced transport after the beta drop is caused by the MHD mode activity in the plasma; the favourable stabilization of the ITG mode under RI mode conditions is not altered. The possible role of pressure driven contributions to the destabilization of the observed MHD modes will be discussed.

The characteristics of the toroidal ion temperature gradient (ITG) instability, considered as the main source of anomalous transport in the low (L) confinement mode of tokamaks, are analysed for the conditions of the radiatively improved (RI) mode triggered by seeding of impurities. Based on experimental profiles from TEXTOR-94 we deduce that the ITG-induced turbulence is quenched in the RI-mode in a large part of the plasma radius. Under those conditions the dissipative trapped electron (DTE) instability dominates the transport. The peaking of the electron density, one of the most remarkable features of the L-RI transition, is explained by the difference in contributions of ITG and DTE unstable modes to the diffusive and convective components of the electron flow. The shearing of the plasma toroidal velocity observed in the RI-modes with unbalanced neutral injection is successfully modelled by taking into account both the anomalous plasma viscosity due to ITG-turbulence and predictions of the revisited neoclassical theory.

In a tokamak plasma the maximum achievable density is limited. A too high density will result in a violent end of a discharge. Two types of density limit disruption can be distinguished: (a) impure and moderately heated discharges, if the radiative power exceeds the input power, (b) clean, auxiliary heated discharges, where the Greenwald limit is encountered. It has been found that in TEXTOR-SI these two density limits differ by the radiative instability in the plasma boundary, which preceeds the disruption. A symmetric radiative mantle and a detachment are observed prior to the first type, while the Greenwald limit has a MARFE precursor. Control of the impurity content, edge and recycling properties prevents the growth of the MARFE and makes it possible to exceed the Greenwald limit in TEXTOR-94 by more than a factor of 2. High densities have been obtained by means of normal gas feed. Maximum central densities of n(e)(0) = 1.3 x 10(20) m(-3) have been obtained. The maximum achievable density scales with the input power and plasma current. Non-disruptive discharges, with a stationary (t > 25 tau(E)) density a factor of 1.93 above the Greenwald limit have been produced in L mode. The radiative losses and impurity concentration have been maintained at a relatively low level during the entire high density phase.

An overview is presented of recent experiments with radiating mantles on limiter and divertor machines, realizing simultaneously high confinement and high density at high-radiation levels. A variety of operational regimes has been observed and the characteristics of each are documented. High-performance plasmas (i.e. edge localized mode (ELM)-free H-mode confinement quality f(H93) greater than or equal to 1 and normalized beta values beta(n) greater than or equal to 2 simultaneously) with radiating mantles have been demonstrated under quasistationary conditions during the maximum flattop time of the machine (equal to tens of confinement times) on DIII-D and TEXTOR-94. Maximum values for beta(n) up to 4 and for the advanced tokamak confinement-stability product beta(n) x f(L89) up to 13, have been obtained in very high confinement mode (VH-mode) like discharges with radiating mantles in DIII-D. There is a striking similarity between improved ohmic confinement discharges (with or without Ne seeding) and radiating mantle discharges, indicating a possible common origin for the confinement improvement observed. Possible scenarios for the application of radiating mantles on larger machines such as JET and JT-60U are indicated.

An experimental investigation into electron temperature profile perturbations due to MHD tearing modes is performed on the TEXTOR tokamak. By means of an ECE diagnostic with a good temporal and radial resolution, m = 1 and m = 2 precursors to sawtooth crashes and major disruptions, respectively, can be studied. The specific temperature perturbations due to a rotating m = 2 magnetic island prior to a disruption have been studied. The phase of the rn = 2 MHD oscillation reverses at the rational q surface. The amplitude distribution of the perturbation has been determined. A frequency analysis of the MHD oscillations on different radial positions close to the rational surface shows an increase of higher harmonics in the frequency spectrum due to the difference in magnetic field topology. Temperature profiles over magnetic islands could be measured. A secondary temperature maximum in the centre of the m = 2 magnetic island has been found. Because of the good radial resolution of the ECE diagnostic an estimate of the island width and the position of the q = 2 surface could be obtained. In one case a movement of the q = 2 surface, due to peaking of the current profile, of more than 7 cm, i.e. 15% of the minor radius, was found. Island widths of w/a = 0.07-0.2 have been observed.

Direct measurements of residual stress (force) have been executed at the edge of the TEXTOR tokamak using multitip Langmuir and Mach probes, together with counter-current NBI torque to balance the existing toroidal rotation. Substantial residual stress and force have been observed at the plasma boundary, confirming the existence of a finite residual stress as possible mechanisms to drive the intrinsic toroidal rotation. In low-density discharges, the residual stress displays a quasi-linear dependence on the local pressure gradient, consistent with theoretical predictions. At high-density shots the residual stress and torque are strongly suppressed. The results show close correlation between the residual stress and the Er × B flow shear rate, suggesting a minimum threshold of the E × B flow shear required for the k∥ symmetry breaking. These findings provide the first experimental evidence of the role of Er × B sheared flows in the development of residual stresses and intrinsic rotation.

The experiments on plasma heating and confinement in a medium size tokamak are reported. A high neutron production is obtained due to reaching the I‐mode of the confinement operation better energy storage and the efficient use of RF reactor systems. (AIP)