Publications (17)29.02 Total impact
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Article: Study of density fluctuations during MHD activity, soft landing discharges and major disruptions in TEXTOR using CO2 laser collective scattering
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ABSTRACT: A modulation of microturbulence is observed in TEXTOR during low mode number MHD activity using CO2 laser collective scattering. This is accompanied by a strong enhancement of density fluctuations near kas ≈ 3 at the end of soft landing discharges and a displacement of the frequency spectrum towards lower frequencies. The increase is most significant for rapid rampdown of the plasma current accompanied by strong MHD activity but also occurs when the latter is not detected. The evolution of microturbulence is also studied during major plasma disruptions. It was found that disruptions without MHD precursor oscillations are characterized by a rapid increase in the density fluctuations starting ~100 ms before plasma disruption.Nuclear Fusion 01/2011; 27(1):109. · 4.09 Impact Factor -
Article: Enhanced small-scale turbulence oscillations correlated with sawtooth relaxations in the TEXTOR tokamak
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ABSTRACT: A periodic enhancement of the microturbulence level by sawtooth relaxations has been detected by CO2 laser forward scattering in the TEXTOR tokamak. This feature is reproduced quantitatively by a heat transport code in which the anomalous electron transport coefficient is calculated self-consistently, following a theoretical model of the saturation of the dissipative trapped electron instability. The code also predicts a strong modulation of the heat flux throughout the plasma and a strong 'profile consistency' as demonstrated elsewhere by continuous temperature measurements. A simple interpretation of these results is given. The calculated global plasma parameters, such as the energy confinement time and the loop voltage, are in good agreement with the measured values.Nuclear Fusion 01/2011; 26(6):797. · 4.09 Impact Factor -
Article: Transport through dissipative trapped electron mode and toroidal ion temperature gradient mode in TEXTOR
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ABSTRACT: A self-consistent transport code is used to evaluate how plasma confinement in tokamaks is influenced by the microturbulent fields excited by the dissipative trapped electron (DTE) instability. As shown previously, the saturation theory on which the code is based has been developed from first principles. The numerical results reproduce well the Neo-Alcator scaling law observed experimentally (for example in TEXTOR) in non-detached Ohmic discharges, the confinement degradation resulting when auxiliary heating is applied and a large number of other experimental observations. The potential impact of the toroidal ion temperature gradient (ηi) mode on energy confinement is assessed by estimating the ion thermal flux with the help of the mixing length approximation. The temperature and density profiles measured in TEXTOR (qa 2.45) are compared at either variable mean density or variable additional power, and their stability against DTE and ηi modes is checked; in the latter case, a new criterion is used, which is valid for arbitrary curvature. All profiles examined are marginally unstable for the two modes, essentially between the q = 1 and q = 2 magnetic surfaces. The code results and the stability analysis lead to the following conclusions and suggestions: (1) The DTE instability is sufficient to explain the anomalous heat transport in low density discharges (attached plasmas) with or without additional heating; the marginal instability for the DTE mode thus follows from heat flux constraints. (2) The observed marginal instability against the ηi mode must then follow from particle flux constraints. (3) The condition that both γηi and γDTE (γ = growth rate) must be small is the restriction which determines the profiles that correspond to the experimental conditions and which determines, to a large extent, profile consistency. (4) Finally, it is suggested that the deviation from Neo-Alcator scaling, the density limit and the phenomenon of plasma detachment are interrelated effects which arise at high densities, when the constraint on the electron heat flux becomes harder to satisfy.Nuclear Fusion 01/2011; 28(6):1053. · 4.09 Impact Factor -
Article: Overview of JET results
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ABSTRACT: Scientific and technical activities on JET focus on the issues likely to affect the ITER design and operation. Our understanding of the ITER reference mode of operation, the ELMy H-mode, has progressed significantly. The extrapolation of ELM size to ITER has been re-evaluated. Neoclassical tearing modes have been shown to be meta-stable in JET, and their beta limits can be raised by destabilization (modification) of sawteeth by ion cyclotron radio frequency heating (ICRH). Alpha simulation experiments with ICRH accelerated injected 4 (He) beam ions provide a new tool for fast particle and magnetohydrodynamic studies, with up to 80-90% of plasma heating by fast 4 He ions. With or without impurity seeding, a quasi-steady-state high confinement (H-98 = 1), high density(n(e)/n(GW) = 0.9-1) and high beta (betaN = 2) ELMy H-mode has been achieved by operating near the ITER triangularity ( similar to 0.40-0.5) and safety factor (q(95) similar to 3), at Z(eff) similar to 1.5-2. In advanced tokamak (AT) scenarios, internal transport barriers (ITBs) are now characterized in real time with a new criterion, rhoT(*). Tailoring of the current profile with T lower hybrid current drive provides reliable access to a variety of q profiles, lowering access power for barrier formation. Rational q surfaces appear to be associated with ITB formation. Alfven cascades were observed in reversed shear plasmas, providing identification of q profile evolution. Plasmas with 'current holes' were observed and modelled. Transient high confinement AT regimes with H-89 = 3.3, beta(N) = 2.4 and ITER-relevant q < 5 were achieved with reversed magnetic shear. Quasi-stationary ITBs are developed with full non-inductive current drive, including similar to 50% bootstrap current. A record duration of ITBs was achieved, up to 11 s, approaching the resistive time. For the first time, pressure and current profiles of AT regimes are controlled by a real-time feedback system, in separate experiments. Erosion and co-deposition studies with a quartz micro-balance show reduced co-deposition. Measured divertor thermal loads during disruptions in JET could modify ITER assumptions.Nuclear Fusion 01/2003; · 4.09 Impact Factor -
Article: Plasma rotation induced by the Dynamic Ergodic Divertor Plasma rotation induced by the Dynamic Ergodic Divertor
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ABSTRACT: After introduction of the experimental options available with the Dynamic Ergodic Diver-tor (DED) and a discussion of the static aspects of the ergodic and laminar zones, the dynamic aspects of the rotating DED field are emphasized. The rotating perturbation field induces a shielding current which is modelled under different assumptions. Interaction of the shielding current with that of the DED coils results in a torque exerted by the coils on the plasma. The location of the maximum of this torque with respect to the frequency depends critically on the width of the shielding current, and for the TEXTOR-DED conditions it is in the frequency band of 1–30 kHz. The DED will have the option of operation with full power in this band so that the basic investigations on the field line penetration can be attempted. The force transferred to the plasma has two components, a weaker toroidal one and a dominant poloidal one. The toroidal force component has about the same value as the one from NBI; from the experience with NBI induced plasma rotation, a substantial plasma acceleration in the toroidal direction is expected. For neoclassical reasons it is not yet clear whether the dominant poloidal force component will result in a poloidal plasma rotation or a radial force. If the poloidal rotation is inhibited, a static radial electric field is estimated on the basis of a revisited neoclassical theory to be of the order of several kilovolts per metre.Nuclear Fusion 01/2001; 41:503-511. · 4.09 Impact Factor -
Article: Properties, control and ICR-heating of the plasma in TEXTOR
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ABSTRACT: The TEXTOR research programme focuses on the systematic analysis of plasma wall interaction, the development of a suitable wall system and the production of quasi-stationary long-pulse high-temperature plasmas with tolerable impurity concentrations, with well-defined boundary layer and with relevant particle and power fluxes through the boundary. The authors describe the method of wall carbonization developed in Julich and some characteristic features of the TEXTOR plasma obtained with this method for both metal limiters and graphite limiters. Moreover, the effect of ICRH heating (2 MW, 1 sec) on plasma parameters and on the boundary layer is discussed, together with the application of the single head pump limiter ALT-I and of the localized set of magnetic perturbation coils for boundary 'ergodization'.Plasma Physics and Controlled Fusion 12/2000; 28(9A):1413. · 2.42 Impact Factor -
Conference Proceeding: High-Frequency Operation of the Dynamic Ergodic Divertor (DED)
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ABSTRACT: After the introduction of the experimental options with the DED, the dynamic features of the rotating DED field are emphasised. The rotating perturbation field induces a shielding current which is modelled under different assumptions. A result of the interaction of the shielding current with that of the DED coils is a torque exerted by the coils on the plasma. The location of the maximum of this torque with respect to the frequency depends critically on the width of the shielding current. The toroidal velocity of the plasma is first computed from the torque, then the radial electric field is estimated on the basis of a revisited neoclassical theory.18-th IAEA Fusion Energy Conference, Sorrento, Italy October 4-10, 2000; 10/2000 -
Article: A scheme for a dynamic ergodic divertor in TEXTOR
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ABSTRACT: A helical multipolar coil system for dynamic edge ergodization of the TEXTOR plasma is proposed being energized by a three phase ac-current. This would permit a sweeping of the perturbation pattern with the aim (I) to establish a uniform distribution of the heat load at the wall components, (II) to increase the thickness of the scrape-off layer, (III) to improve the helium exhaust and to enhance impurity shielding for controlled edge radiation cooling, and (IV) to possibly influence plasma rotation. Resonant interaction with internal modes may be foreseen in a later phase.Journal of Nuclear Materials 04/1995; 220-222:448-451. · 2.05 Impact Factor -
Article: ASYMMETRY OF RADIAL HEAT PULSE PROPAGATION AT TEXTOR
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ABSTRACT: Electron heat pulse propagation following sawtooth collapse has been simultaneously observed along two opposite radial directions. For a given flux surface, the arrival of the heat pulse at the high field side (HFS) is delayed by up to 1 ms as compared with the arrival at the low field side (LFS). The observed delay agrees with an estimate of the time required for parallel electron heat transport along the magnetic field lines connecting the corresponding LFS and HFS isobars. This suggests that the primary radial heat transport occurs on the LFS, whereas the HFS is fed subsequently from the LFS by temperature equilibration along the connecting magnetic field lines.Nuclear Fusion 06/1993; 33(6):921. · 4.09 Impact Factor -
Article: High-Frequency Operation of the Dynamic Ergodic Divertor (DED)
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ABSTRACT: After the introduction of the experimental options with the DED, the dynamic features of the rotating DED field are emphasised. The rotating perturbation field induces a shielding current which is modelled under different assumptions. A result of the interaction of the shielding current with that of the DED coils is a torque exerted by the coils on the plasma. The location of the maximum of this torque with respect to the frequency depends critically on the width of the shielding current. The toroidal velocity of the plasma is first computed from the torque, then the radial electric field is estimated on the basis of a revisited neoclassical theory. -
Article: Asymmetry of Radial Heat Pulse Propagation at TEXTOR
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ABSTRACT: This version is available at http://dx.doi.org/10.1088/0029-5515/33/6/I09 Copyright © IOP Publishing Ltd. -
Article: Plasma rotation induced by the dynamic ergodic divertor
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ABSTRACT: This version is available at the following URL: http://dx.doi.org/10.1088/0029-5515/41/5/303 -
Article: Properties, Control and ICR-Heating of the Plasma in TEXTOR
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ABSTRACT: This version is available at http://dx.doi.org/10.1088/0741-3335/28/9A/018 Copyright © IOP Publishing Ltd. -
Article: Nonlinear structures in interchange mode turbulence
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ABSTRACT: This version is available at the following Publisher URL: http://pre.aps.org -
Article: Washboard modes as ELM-related events in JET
Plasma Physics and Controlled Fusion, v.46, 61-87 (2004). -
Article: A scheme for a dynamic ergodic divertor in TEXTOR
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ABSTRACT: A helical multipolar coil system for dynamic edge ergodization of the TEXTOR plasma is proposed being energized by a three phase ac-current. This would permit a sweeping of the perturbation pattern with the aim (I) to establish a uniform distribution of the heat load at the wall components, (II) to increase the thickness of the scrape-off layer, (III) to improve the helium exhaust and to enhance impurity shielding for controlled edge radiation cooling, and (IV) to possibly influence plasma rotation. Resonant interaction with internal modes may be foreseen in a later phase.Journal of Nuclear Materials. -
Article: Plasma-wall interaction and plasma performance in textor — A review
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ABSTRACT: TEXTOR, a tokamak primarily devoted to plasma-wall interaction studies, has been operated during the last 2–3 years under a wide variety of discharge conditions. Discharges with Ohmic heating only and with additional ICR heating in the MW-range were produced over an extended range of plasma parameters. Different limiter materials were used — Inconel, stainless steel, graphite, TiC coated graphite. The surface of the first wall (liner) consisted either of Inconel or of a carbon layer deposited upon the Inconel. Characteristic features of the discharge have been modified by making use of the modular pump limiter ALT-I and by applying a localized perturbation coil for “ergodization” of the magnetic field structure in the boundary.The characteristics of plasma-wall interaction under these conditions and their influence on the global performance of the TEXTOR plasma are described and discussed. Use has been made of novel diagnostics specifically developed for edge plasma investigation.Journal of Nuclear Materials.
