Publications (61)111.48 Total impact
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Article: Temporally resolved plasma composition measurements by collective Thomson scattering in TEXTOR (invited).
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ABSTRACT: Fusion plasma composition measurements by collective Thomson scattering (CTS) were demonstrated in recent proof-of-principle measurements in TEXTOR [S. B. Korsholm et al., Phys. Rev. Lett. 106, 165004 (2011)]. Such measurements rely on the ability to resolve and interpret ion cyclotron structure in CTS spectra. Here, we extend these techniques to enable temporally resolved plasma composition measurements by CTS in TEXTOR, and we discuss the prospect for such measurements with newly installed hardware upgrades for the CTS system on ASDEX Upgrade.The Review of scientific instruments 10/2012; 83(10):10E307. · 1.52 Impact Factor -
Article: Measurements of plasma composition in the TEXTOR tokamak by collective Thomson scattering
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ABSTRACT: We demonstrate the use of collective Thomson scattering (CTS) for spatially localized measurements of the isotopic composition of magnetically confined fusion plasmas. The experiments were conducted in the TEXTOR tokamak by scattering millimeter-wave probe radiation off plasma fluctuations with wave vector components nearly perpendicular to the magnetic field. Under such conditions the sensitivity of the CTS spectrum to plasma composition is enhanced by the spectral signatures of the ion cyclotron motion and of weakly damped ion Bernstein waves. Recent experiments on TEXTOR demonstrated the ability to resolve these signatures in the CTS spectrum as well as their sensitivity to the ion species mix in the plasma. This paper shows that the plasma composition can be inferred from the measurements through forward modeling of the CTS spectrum. We demonstrate that spectra measured in plasmas consisting of hydrogen, deuterium and 3He can be accurately reproduced by theory and yield inferred plasma compositions consistent with expectations. The potential to use CTS for measurements of plasma composition is of significant interest since CTS is well suited for reactor environments and since there is at present no established method to measure the fuel ion density ratio in the core of a burning fusion plasma.Plasma Physics and Controlled Fusion 12/2011; 54(1):015008. · 2.42 Impact Factor -
Article: Feedback control of the sawtooth period through real time control of the ion cyclotron resonance frequency
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ABSTRACT: Modification of the sawtooth period through ion cyclotron resonance frequency (ICRF) heating and current drive has been demonstrated in a number of experiments. The effect has been seen to depend critically on the location of the ICRF absorption region with respect to the q = 1 flux surface. Consequently, for ICRF to be a viable tool for sawtooth control, one must be able to control the ICRF absorption location in real time so as to follow variations in the location of the q = 1 surface. To achieve this, the JET ICRF system has been modified to allow the JET real time central controller to control the frequency of the ICRF generators. An algorithm for real time determination of the sawtooth period has been developed and a closed loop controller, which modifies the frequency of the ICRF generators to bring the measured sawtooth period to the desired reference value, has been implemented. This paper shows the first experimental demonstration of closed loop sawtooth period control by real time variation of the ICRF wave frequency.Nuclear Fusion 06/2011; 51(7):073032. · 4.09 Impact Factor -
Article: Measurements of intrinsic ion Bernstein waves in a tokamak by collective Thomson scattering.
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ABSTRACT: In this Letter we report measurements of collective Thomson scattering (CTS) spectra with clear signatures of ion Bernstein waves and ion cyclotron motion in tokamak plasmas. The measured spectra are in accordance with theoretical predictions and show clear sensitivity to variation in the density ratio of the main ion species in the plasma. Measurements with this novel diagnostic demonstrate that CTS can be used as a fuel ion ratio diagnostic in burning fusion plasma devices.Physical Review Letters 04/2011; 106(16):165004. · 7.37 Impact Factor -
Article: Collective Thomson scattering measurements with high frequency resolution at TEXTOR.
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ABSTRACT: We discuss the development and first results of a receiver system for the collective Thomson scattering (CTS) diagnostic at TEXTOR with frequency resolution in the megahertz range or better. The improved frequency resolution expands the diagnostic range and utility of CTS measurements in general and is a prerequisite for measurements of ion Bernstein wave signatures in CTS spectra. The first results from the new acquisition system are shown to be consistent with theory and with simultaneous measurements by the standard receiver system.The Review of scientific instruments 10/2010; 81(10):10D515. · 1.52 Impact Factor -
Conference Proceeding: An overview of the ITER electron cyclotron H&CD system
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ABSTRACT: This paper reviews the design and functionality of the 24 MW 170 GHz electron cyclotron heating and current drive system being planned for the ITER Tokamak. The sub-systems (power supplies, gyrotrons, transmission lines and launcher antennas) are described based on present day technologies, while on-going R&D provides component and sub-system testing with the possibility of increasing the reliability of the overall EC system. Modifications to the steering ranges of the launching antennas are under investigation that can improve the functional capabilities of the EC system without increasing cost and relaxing the engineering constraints.Infrared, Millimeter, and Terahertz Waves, 2009. IRMMW-THz 2009. 34th International Conference on; 10/2009 -
Article: Strong scattering of high power millimeter waves in tokamak plasmas with tearing modes.
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ABSTRACT: In tokamak plasmas with a tearing mode, strong scattering of high power millimeter waves, as used for heating and noninductive current drive, is shown to occur. This new wave scattering phenomenon is shown to be related to the passage of the O point of a magnetic island through the high power heating beam. The density determines the detailed phasing of the scattered radiation relative to the O-point passage. The scattering power depends strongly nonlinearly on the heating beam power.Physical Review Letters 09/2009; 103(12):125001. · 7.37 Impact Factor -
Conference Proceeding: Design and testing of the ECH upper port plug for ITER
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ABSTRACT: Four ECH Upper Port Plugs are foreseen at ITER for counteracting plasma instabilities based on the injection of up to 20 MW mm-wave power at 170 GHz into the plasma. The required targeting of flux surfaces will be achieved by angular steering in the poloidal direction. The paper describes the main components of the mm-wave and structural system for the current reference design of the extended physics launcher (EPL). The mm-wave system is formed by waveguide and quasi-optical sections with a front steering system driven by a friction-less and backlash-free pneumatic system. The first tritium barrier is formed by a CVD diamond window with an indirect cooling concept that avoids direct water contact to the diamond disk and brazing material. The structure consists of the blanket shield module with the plasma facing first wall panel, the port plug frame, and the internal shield that provides adequate neutron shielding towards the launcher back-end. The key design requirements for the main structure are discussed with respect to efficient baking, to rigidity towards launcher deflection and to extraction of thermal loads. The current status of fabrication studies is presented demonstrating the feasibility of manufacturing routes for complex double wall structures. Testing of major port plug components is described in the context of dedicated test facilities and maintenance requirements.Plasma Science, 2008. ICOPS 2008. IEEE 35th International Conference on; 07/2008 -
Article: Pellet injection and high density ITB formation in JET advanced tokamak plasmas
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ABSTRACT: High density internal transport barriers with peaked density profiles have been produced at JET by optimizing the pellet fuelling, heating power, current drive and plasma start-up. The scenario was developed with moderate momentum input and comparable ion and electron temperatures. Both features aim at matching the conditions envisaged for a burning plasma regime. Current and density profiles, which both appear to influence transport barrier formation, were independently controlled. The optimization recipe included early lower hybrid current drive followed by an Ohmic or neutral beam heated pellet fuelling gap. The internal transport barrier was then formed at the start of a high power additionally heated phase. Typical plasmas were at a toroidal field of 3 T and plasma current of 2 MA, corresponding to a peripheral safety factor q95 ≈ 5. Particle and energy transport were analysed by a 1.5 dimensional code including a pellet ablation module and a criterion for the barrier formation. Results of turbulence simulations with global electrostatic and electromagnetic fluid codes and a description of the observed magnetohydrodynamic instabilities are also reported.Nuclear Fusion 01/2007; 47(2):74. · 4.09 Impact Factor -
Article: Study of slow n = 1, m = 1 reconnection in JET discharges with low central magnetic shear
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ABSTRACT: Experiments designed for the attainment of the hybrid advanced mode of operation in JET revealed the existence of a new form of reconnecting modes with toroidal number n = 1 and poloidal number m = 1, which have the same radial structure as sawtooth precursors but are characterized by small growth rate and low saturation amplitude. These modes develop in repetitive cycles with typical growth times of 100 ms. Temperature perturbations are not in the form of sawteeth; in fact, there is slow and mild erosion of the central temperature without any crash. Comparison with theory of m = 1 modes indicates that the slow growth observed in the hybrid regime is caused by low magnetic shear (a measure of the radial variation of field line inclination) and possibly by diamagnetic effects in the plasma core.Plasma Physics and Controlled Fusion 06/2006; 48(7):1005. · 2.42 Impact Factor -
Article: Cross-machine comparison of resonant field amplification and resistive wall mode stabilization by plasma rotation
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ABSTRACT: Dedicated experiments in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion, 42, 614 (2002)], the Joint European Torus (JET) [P. H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)], and the National Spherical Torus Experiment (NSTX) [M. Ono, S. M. Kaye, Y.-K. M. Peng et al., Nucl. Fusion 40, 557 (2000)] reveal the commonalities of resistive wall mode (RWM) stabilization by sufficiently fast toroidal plasma rotation in devices of different size and aspect ratio. In each device the weakly damped n = 1 RWM manifests itself by resonant field amplification (RFA) of externally applied n = 1 magnetic fields, which increases with the plasma pressure. Probing DIII-D and JET plasmas with similar ideal magnetohydrodynamic (MHD) stability properties with externally applied magnetic n = 1 fields, shows that the resulting RFA is independent of the machine size. In each device the drag resulting from RFA slows the toroidal plasma rotation and can lead to the onset of an unstable RWM. The critical plasma rotation required for stable operation in the plasma center decreases with increasing q95, which is explained by the inward shift of q surfaces where the critical rotation remains constant. The quantitative agreement of the critical rotation normalized to the inverse Alfvén time at the q = 2 surface in similar DIII-D and JET plasmas supports the independence of the RWM stabilization mechanism of machine size and indicates the importance of the q = 2 surface. At low aspect ratio the required fraction of the Alfvén velocity increases significantly. The ratio of the critical rotation in similar NSTX and DIII-D plasmas can be explained by trapped particles not contributing to the RWM stabilization, which is consistent with stabilization mechanisms that are based on ion Landau damping. Alternatively, the ratio of the required rotation to the sound wave velocity remains independent of aspect ratio.Physics of Plasmas 05/2006; 13(5):056107-056107-11. · 2.15 Impact Factor -
Article: Expanding the operating space of ICRF on JET with a view to ITER
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ABSTRACT: This paper reports on ITER-relevant ion cyclotron resonance frequency (ICRF) physics investigated on JET in 2003 and early 2004. Minority heating of helium three in hydrogen plasmas—(3He)H—was systematically explored by varying the 3He concentration and the toroidal phasing of the antenna arrays. The best heating performance (a maximum electron temperature of 6.2 keV with 5 MW of ICRF power) was obtained with a preferential wave launch in the direction of the plasma current. A clear experimental demonstration was made of the sharp and reproducible transition to the mode conversion heating regime when the 3He concentration increased above ~2%. In the latter regime the best heating performance (a maximum electron temperature of 8 keV with 5 MW of ICRF power) was achieved with dipole array phasing, i.e. a symmetric antenna power spectrum. Minority heating of deuterium in hydrogen plasmas—(D)H—was also investigated but was found inaccessible because this scenario is too sensitive to impurity ions with Z/A = 1/2 such as C6+, small amounts of which directly lead into the mode conversion regime. Minority heating of up to 3% of tritium in deuterium plasmas was systematically investigated during the JET trace tritium experimental campaign (TTE). This required operating JET at its highest possible magnetic field (3.9 to 4 T) and the ICRF system at its lowest frequency (23 MHz). The interest of this scenario for ICRF heating at these low concentrations and its efficiency at boosting the suprathermal neutron yield were confirmed, and the measured neutron and gammay ray spectra permit interesting comparisons with advanced ICRF code simulations. Investigations of finite Larmor radius effects on the RF-induced high-energy tails during second harmonic (ω = 2ωc) heating of a hydrogen minority in D plasmas clearly demonstrated a strong decrease in the RF diffusion coefficient at proton energies ~ 1 MeV, in agreement with theoretical expectations. Fast wave heating and current drive experiments in deuterium plasmas showed effective direct electron heating with dipole phasing of the antennas, but only small changes of the central plasma current density were observed with the directive phasings, in particular at low single pass damping. New investigations of the heating efficiency of ICRF antennas confirmed its strong dependence on the parallel wavenumber spectrum. Advances in topics of a more technological nature are also summarized: ELM studies using fast RF measurements, the successful experimental demonstration of a new ELM-tolerant antenna matching scheme and technical enhancements planned on the JET ICRF system for 2006, they being equally strongly driven by the preparation for ITER.Nuclear Fusion 01/2006; 46(2):391. · 4.09 Impact Factor -
Article: Alfvén cascades in JET discharges with NBI-heating
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ABSTRACT: Alfvén cascade (AC) eigenmodes excited by energetic ions accelerated with ion-cyclotron resonance heating in JET reversed-shear discharges are studied experimentally in high-density plasmas fuelled by neutral beam injection (NBI) and by deuterium pellets. The recently developed O-mode interferometry technique and Mirnov coils are employed for detecting ACs. The spontaneous improvements in plasma confinement (internal transport barrier (ITB) triggering events) and grand ACs are found to correlate within 0.2 s in JET plasmas with densities up to ∼5 × 10 19 m −3 . Measurements with high time resolution show that ITB triggering events happen before 'grand' ACs in the majority of JET discharges, indicating that this improvement in confinement is likely to be associated with the decrease in the density of rational magnetic surfaces just before q min (t) passes an integer value. Experimentally observed ACs excited by sub-Alfvénic NBI-produced ions with parallel velocities as low as V NBI ≈ 0.2 · V A are found to be most likely associated with the geodesic acoustic effect that significantly modifies the shear-Alfvén dispersion relation at low frequency. Experiments were performed with a tritium NBI-blip (short time pulse) into JET plasmas with NBI-driven ACs. Although considerable NBI-driven AC activity was present, good agreement was found both in the radial profile and in the time evolution of DT neutrons between the neutron measurements and the TRANSP code modelling based on the Coulomb collision model, indicating the ACs have at most a small effect on fast particle confinement in this case.Nucl. Fusion. 01/2006; 468735(52). -
Article: Development of internal transport barrier scenarios at ITER-relevant high triangularity in JET
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ABSTRACT: The development of scenarios characterized by H-mode confinement and internal transport barriers (ITBs) in high triangularity, δ ~ 0.4–0.5, discharges is of particular interest for ITER advanced tokamak operation. Previous JET experiments have shown that high triangularity favours H-modes which are ELM-free or develop type I edge localized mode (ELM) activity, which inhibits long lasting ITBs. The recent experiments reported here concentrate on integrated optimization of edge and core conditions. The stability of the edge pedestal was controlled using gas injection, deuterium or light impurities, and plasma current ramps. Both methods yield more ITB-friendly edge pedestal conditions, varying from small type I to type III ELMs and, in extreme cases, resulting in L-mode. In parallel, the conditions for triggering and sustaining ITBs encompassing a large proportion of the plasma volume (outer ITBs) were optimized, as opposed to less performing ITBs located closer to the plasma centre (inner ITB). These plasmas have deeply reversed target current profiles with qmin ~ 3 and a narrow inner ITB, located typically at a small normalized radius ρ < 0.5 and close to the reversed shear region, is routinely observed. Large radius outer ITBs are only triggered at an input power in excess of 20 MW, but they do not usually survive the transition into H-mode. The best results, in terms of sustained high confinement, have been obtained with neon injection; an outer ITB is triggered during the phase with L-mode edge and survives into H-mode for about 2 s, corresponding to ~10 times the global energy confinement time τE, at q95 = 7.5, H89βN ~ 3.5–4 and ~60% of the Greenwald density limit. In summary, a high triangularity scenario has been developed, which combines the desirable characteristics of controlled edge, long lasting wide ITBs and high performance at density higher than the low triangularity JET scenarios.Nuclear Fusion 11/2005; 45(12):1481. · 4.09 Impact Factor -
Article: Overview of transport, fast particle and heating and current drive physics using tritium in JET plasmas
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ABSTRACT: Results are presented from the JET Trace Tritium Experimental (TTE) campaign using minority tritium (T) plasmas (nT/nD < 3%). Thermal tritium particle transport coefficients (DT, vT) are found to exceed neo-classical values in all regimes, except in ELMy H-modes at high densities and in the region of internal transport barriers (ITBs) in reversed shear plasmas. In ELMy H-mode dimensionless parameter scans, at q95 ~ 2.8 and triangularity δ = 0.2, the T particle transport scales in a gyro-Bohm manner in the inner plasma (r/a < 0.4), whilst the outer plasma particle transport scaling is more Bohm-like. Dimensionless parameter scans show contrasting behaviour for the trace particle confinement (increases with collisionality, ν* and β) and bulk energy confinement (decreases with ν* and is independent of β). In an extended ELMy H-mode data set, with ρ*, ν*, β and q varied but with neo-classical tearing modes (NTMs) either absent or limited to weak, benign core modes (4/3 or above), the multiparameter fit to the normalized diffusion coefficient in the outer plasma (0.65 < r/a < 0.8) gives DT/B ~ ρ*2.46ν*−0.23β−1.01q2.03. In hybrid scenarios (qmin ~ 1, low positive shear, no sawteeth), the T particle confinement is found to scale with increasing triangularity and plasma current. Comparing regimes (ELMy H-mode, ITB plasma and hybrid scenarios) in the outer plasma region, a correlation of high values of DT with high values of vT is seen. The normalized diffusion coefficients for the hybrid and ITB scenarios do not fit the scaling derived for ELMy H-modes. The normalized tritium diffusion scales with normalized poloidal Larmor radius in a manner close to gyro-Bohm , with an added inverse β dependence. The effects of ELMs, sawteeth and NTMs on the T particle transport are described. Fast-ion confinement in current-hole (CH) plasmas was tested in TTE by tritium neutral beam injection into JET CH plasmas. γ-rays from the reactions of fusion alpha and beryllium impurities (9Be(α, nγ)12C) characterized the fast fusion-alpha population evolution. The γ-decay times are consistent with classical alpha plus parent fast triton slowing down times (τTs + ταs) for high plasma currents (Ip > 2 MA) and monotonic q-profiles. In CH discharges the γ-ray emission decay times are much lower than classical (τTs+ταs), indicating alpha confinement degradation, due to the orbit losses and particle orbit drift predicted by a 3-D Fokker–Planck numerical code and modelled using TRANSP.Nuclear Fusion 10/2005; 45(10):S181. · 4.09 Impact Factor -
Conference Proceeding: Availability of Present Fusion Devices
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ABSTRACT: Operation of tokamaks has now reached few hundred device-years. Availability is a top level parameter vital for the efficient management of complex plants, like fusion devices, for decision making and to judge the quality of design, manufacturing and operation. There is no standard way to analyze the operating experience: different techniques are adopted, various levels of detail are reached and also the kind of analysis and corrective actions are different. Nevertheless most of the results for the various tokamaks are expressed in similar terms like number of plasma pulses, operating time and delays, experimental sessions lost. An overall analysis of JET, JT-60, DIII-D, ASDEX-U, Tore Supra, FTU and RFX operating experiences has been performed to find out main results, lessons learned and possible suggestions to improve availability of the devices themselves and of those under design or construction, like ITER. The main results are here presented and discussedFusion Engineering 2005, Twenty-First IEEE/NPS Symposium on; 10/2005 -
Article: The 'hybrid' scenario in JET: towards its validation for ITER
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ABSTRACT: In 2003, the performance of the 'hybrid' regime was successfully validated in JET experiments up to βN = 2.8 at low toroidal field (1.7 T), with plasma triangularity and normalized Larmor radius (ρ*) corresponding to identical ASDEX Upgrade discharges. Stationary conditions have been achieved with the fusion figure of merit ( ) reaching 0.42 at q95 = 3.9. The JET discharges show similar MHD, edge and current profile behaviour, when compared with the ASDEX Upgrade. In addition, the JET experiments have extended the hybrid scenario operation at higher toroidal field of 2.4 T and lower ρ* towards the projected ITER values. Using this database, transport and confinement properties are characterized with respect to the standard H-mode regime. Moreover, trace tritium has been injected to assess the diffusion and convective coefficients of the fusion fuel. The maximization of confinement and stability properties provides, to this scenario, a good probability of achieving a high fusion gain at reduced plasma current for durations of up to 2000 s in ITER.Nuclear Fusion 06/2005; 45(7):626. · 4.09 Impact Factor -
Article: Effect of hysteresis in JET ITB plasma with LHCD
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ABSTRACT: A strong ITB was sustained in a reversed shear discharge in JET during a long time interval after a significant reduction in plasma heating power. The observed ITB evolution is reminiscent of the effect of hysteresis, associated in theory with turbulence suppression due to shear of the plasma rotation. The mechanism of ITB sustainment was analysed. Modelling of the plasma heating and current profile was done using the TRANSP and JETTO transport codes. The resulting q-profile evolution was verified by comparison with the pitch angle δ = Bp/Bt deduced from motional Stark effect measurements (where Bp and Bt are the poloidal and toroidal magnetic field, respectively). Turbulence stability was analysed using the gyro-kinetic code Kinezero. It was shown that the strong negative magnetic shear produced by LHCD and sustained mainly by the bootstrap current in the plasma core is responsible for the turbulence stabilization. Stabilization due to shear of the plasma rotation and finite β -stabilization play a complimentary role. The effect of bootstrap and LH driven currents on magnetic shear in JET discharges was analysed.Plasma Physics and Controlled Fusion 05/2005; 47(7):975. · 2.42 Impact Factor -
Article: Toroidal plasma rotation induced by the dynamic ergodic divertor in the TEXTOR tokamak.
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ABSTRACT: 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.Physical Review Letters 02/2005; 94(1):015003. · 7.37 Impact Factor -
Article: Toroidal Plasma Rotation Induced by the Dynamic Ergodic Divertor in the TEXTOR Tokamak
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ABSTRACT: 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.Physical Review Letters 01/2005; 94:015003. · 7.37 Impact Factor
Top co-authors
Top Journals
Institutions
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2010–2012
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Technical University of Denmark
- • Department of Physics
- • National Laboratory for Sustainable Energy
Copenhagen, Capital Region, Denmark
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2006–2007
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Culham Centre for Fusion Energy
Abingdon, ENG, United Kingdom
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