Publications (33)26.06 Total impact
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Article: Fuelling efficiency of massive gas injection in TEXTOR: mass scaling and importance of gas flow dynamics
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ABSTRACT: Fuelling efficiency is an important parameter in designing a massive gas injection system for suppression of runaway electrons in ITER. In this work Z-dependence of fuelling efficiency is measured for TEXTOR. The dependence covers the following gases: He, Ne, Ar, Kr, Xe and a 10% Ar–D2 mixture. It is shown that the fuelling efficiency significantly decreases with the gas mass, from above 0.5 for He to below 0.03 for Xe.To explain the variation of the efficiency with the gas mass and pressure a simple model of gas flow from the valve to the plasma edge is developed. The flow model is validated using available laboratory flow measurements of a TEXTOR-like injection system. An unsteady gas flow and a premature plasma disruption are shown to explain the mass dependence of the efficiency.Nuclear Fusion 07/2011; 51(8):083033. · 4.09 Impact Factor -
Article: Active control over carbon deposition on diagnostic components and in remote areas of ITER
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ABSTRACT: a b s t r a c t In future fusion devices like ITER deposition of impurities will likely occur in areas, remote from plasma and on the sensitive components of optical diagnostics, like mirrors and windows. Deposition in remote areas may lead to the tritium retention and therefore represent a safety issue. Deposition on optical com-ponents will severely deteriorate their optical properties potentially leading to the shutdown of the respective diagnostic systems. An active control over deposition is therefore highly desirable. The paper contains an overview of experiments on active control over carbon deposition undertaken in TEXTOR tokamak. A prototype of diagnostic duct was exposed in the scrape-off layer plasmas of TEXTOR where several techniques were applied to mitigate carbon deposition on diagnostic mirrors located inside this prototype. The complete suppression of carbon deposition on the surface of diagnostic mirror was achieved by feeding the deuterium gas inside the diagnostic duct.Journal of Nuclear Materials 01/2011; · 2.05 Impact Factor -
Article: Toroidal pump limiter experiments in the TEXTOR tokamak
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ABSTRACT: The Advanced Limiter Test-II (ALT-II) is a large area toroidal pump limiter in the TEXTOR tokamak. Discrete target plates located in channels at eight toroidal positions behind the main limiter surface neutralize a portion of the plasma efflux from the core. The resulting gas is exhausted by eight external pumps. The primary experimental goals of ALT-II are aimed at power loading studies and plasma density control during long pulse (4 s), high power (6 MW) tokamak discharges. It is found that both the power and the plasma flow to the limiter are asymmetric and depend on line density. Peak neutral pressures of 0.8 m torr and removal rates of up to 0.15 torrL/s per pump station are achieved in the Ohmic phase. The projected exhaust efficiency of ALT-II with full pumping is 5-10%. During ion cyclotron resonance frequency heating, the particle removal rate exceeds 0.4 torrL/s per blade, and the exhaust efficiency is 4-5% for power levels up to 2.6 MW.Nuclear Fusion 01/2011; 31(6):1067. · 4.09 Impact Factor -
Article: Thomson scattering system on the TEXTOR tokamak using a multi-pass laser beam configuration
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ABSTRACT: The main challenge for the Thomson scattering (TS) diagnostic on the TEXTOR tokamak is the detailed study of fast plasma events at a high spatial resolution and a high repetition rate of the measurements. The diagnostic uses intra-cavity probing of the plasma with a repetitively pulsed ruby laser and a fast CMOS camera as detectors. Since 2004, the TS system on TEXTOR has been gradually and systematically enhanced for the measurements of fast plasma events. For that it has recently been upgraded to obtain a multi-pass configuration. Two spherical mirrors have been installed that force the laser beam to probe the plasma a specified number of times before it is directed back into the laser medium. The diagnostics with the upgraded probing system have achieved the measurement accuracy of 3% for the electron temperature and 1.5% for the electron density at <1 cm spatial resolution and 3 × 1019 m−3 plasma density and can measure at 5 kHz during an interval up to 8 ms. This makes it possible to detect, amongst others, fine structures of magnetic islands and variations of the edge pedestal in the ELMy limiter H-mode.Plasma Physics and Controlled Fusion 02/2009; 51(5):055002. · 2.42 Impact Factor -
Article: Poloidal asymmetry in perpendicular plasma rotation and radial electric field measured with correlation reflectometry at TEXTOR
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ABSTRACT: Measurements of plasma rotation and electric field are crucial for the study of plasma confinement and transport. The present paper is devoted to experimental observations of poloidal asymmetry in perpendicular plasma rotation with correlation reflectometry on TEXTOR.Plasma Physics Reports 08/2008; 34(9):736-740. · 0.65 Impact Factor -
Article: Experimental observation of m/n = 1/1 mode behaviour during sawtooth activity and its manifestations in tokamak plasmas
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ABSTRACT: 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.Plasma Physics and Controlled Fusion 07/2005; 47(8):1111. · 2.42 Impact Factor -
Article: Plasma exhaust and density control in tokamak fusion experiments with neutral beam or ICRF auxiliary heating
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ABSTRACT: Particle exhaust studies have been carried out with the pump limiter ALT-II in the TEXTOR tokamak, under ohmic conditions as well as with NBI and with ICRF auxiliary heating, and the pumping effectiveness is shown to meet the requirements for a fusion reactor. Quantitative measurements of Dα emission, made with a CCD camera, have been used to determine the particle efflux from the plasma. Roughly one third of the Dα emission occurs in a diffuse `halo' that surrounds the limiter belt. The particle confinement time is less than the energy confinement time by a factor of typically 4. Modelling in 2-D of plasma and neutral flows in the TEXTOR boundary has been performed. The source of D+ ions can be related to the Dα emission by a factor that is found to depend on the location of the emission and on the discharge density. The predicted total Dα emission agrees with the measurements within a factor of about 2. Pumping of ALT-II allows for density control; with NBI, the density can be increased well beyond the ohmic limit without the discharge ending in disruption. The plasma particle efflux and the pumped flux both increase with density as well as with heating power. The exhaust efficiency is typically ~2%, with the highest values observed in high density NBI discharges. Higher exhaust rates are observed with NBI than with ICRF. Plasma and neutral flows in the ALT-II scoops have been simulated, making use of a simple plasma model. The scoop may be viewed as a non-linear amplifier of the plasma particle flux; the amplification is found to range from about 2 to 3 for most cases. Flow reversal in the scoop is found in some of the NBI cases and particularly in the highest density case.Nuclear Fusion 05/2002; 38(11):1585. · 4.09 Impact Factor -
Article: Turbulent transport and turbulence in radiative I mode plasmas in TEXTOR-94
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ABSTRACT: First measurements of turbulence levels and turbulence induced transport in the outer edge of the plasma of TEXTOR-94 during radiative improved mode discharges show a reduction by a factor of 4-7 of the radial particle turbulent transport. The quenching is most evident on the normalized potential fluctuations and is strongest above 100 kHz. Non-linear gyrokinetic particle-in-cell simulations of these discharges show impurity induced suppression of the electrostatic fluctuations associated with the ion temperature gradient driven mode over most of the cross-section, including the edge. Such a mechanism is proposed as the explanation for the improved confinement and turbulence reduction. The reduction in the edge turbulent transport levels is consistent with increased particle confinement time and the reduction of the SOL thickness. Particle and energy fluxes to the limiter are reduced by an order of magnitude. A concomitant increase of the measured energy and particle confinement times τE and τp versus radiated fraction suggests a common underlying suppression mechanism.Nuclear Fusion 05/2002; 40(2):209. · 4.09 Impact Factor -
Article: Study of the power exhaust and the role of impurities in the Torus Experiment for Technological Oriented Research (TEXTOR)
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ABSTRACT: Controlled application of radiating impurities in the boundary layer can help to solve the problem of power exhaust in a fusion reactor. Experiments in the Torus Experiment for Technological Oriented Research (TEXTOR) [J. Nucl. Mater. 145-147,3 (1987)] are presented, which show that impurities with sufficiently high atomic number (~1O) are well suited for this purpose. Injection of neon, a gas recycled at the wall, enabled the establishment of a quasistationary radiating boundary layer, from which more than 90% of the input power was emitted. The required neon density was established by means of a feedback control for the neon influx, which was made possible by the toroidal pump limiter Advanced Limiter Technology (ALT-II) [J. Nucl. Mater. 162-164, 115 (1989)]. Alternatively, or in addition silicon was introduced as a condensing element-either by surface reactions from siliconized walls or by silane [SiH(D)4] injection-which revealed self-controlling mechanisms effective with changing plasma parameters. In neither case was a significant increase in central impurity concentration observed and good energy confinement time was maintained up to the highest plasma densities. Based on the information from various refined edge diagnostics, the underlying mechanisms for the buildup of a radiating plasma mantle and the interdependences of neon and silicon on other impurities are discussed. © 1995 American Institute of Physics.Physics of Plasmas 01/1995; 2(6):2272. · 2.15 Impact Factor -
Article: 10 kHz repetitive high-resolution TV Thomson scattering on TEXTOR: Design and performance (invited)
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ABSTRACT: In late 2003 a 10 kHz multiposition Thomson scattering diagnostic with high spatial resolution became operational on the TEXTOR tokamak. In the initial phase of operation, one burst of 18 pulses of 12 J each with a repetition rate of 5 kHz could be extracted from the laser system. The installation of a low-dope ruby rod spring 2005 resulted in a system, which can deliver higher pulse energy and moreover a divergence of better than 0.7 mrad, leading to a big improvement in the detection of Thomson scattering photons. Furthermore, the number of laser pulses in one burst could be extended to even more than 30. The achieved laser energy of more than 15 J / pulse makes it possible to measure electron temperature and density profiles with an observational error of 8% on the electron temperature T e and 4% on the electron density n e at n e = 2.5 10 19 m −3 , per spatial element of 7.5 mm. The viewing optics enables sampling of either the full plasma diameter of 900 mm with 120 spatial channels of 7.5 mm each or a 160 mm long edge chord with 98 spatial channels of 1.7 mm each. The system, which has recently become available for physics exploration, has already been used to study the structure of m = 2 magnetic islands and the response of the plasma to off-axis electron cyclotron resonance heating. © 2006 American Institute of Physics. -
Article: Protection of limiter surfaces by films locally deposited during TEXTOR discharges
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ABSTRACT: The erosion of first wall material is a serious problem in future fusion devices. A concept is suggested to compensate erosion by the local injection of reactive gases through divertor plates or limiters forming a suitable deposit on their surfaces. The technique can be implemented during plasma operation with feed back control. Initial experiments were performed in TEXTOR in which silane gas (SiD4) was blown through a hole in a graphite test limiter during ohmic discharges. The gas was ionized and partly deposited in the vicinity of the interaction hole, at areas subject to the highest heat load. Without gas puffing these are erosion dominated zones. The deposition of a silicon layer over an area of about 700 mm2, peaked at the injection hole with a maximum thickness of about 900 nm and with a poloidal extend of about 20 mm was observed. It contained a fraction of about 4–5% of the total number of injected Si atoms. In addition, about the same quantity of carbon was co-deposited, significantly exceeding the C fluence arriving from the plasma during normal discharges. An additional local carbon source during silane injection must be invoked, caused e.g. by carbon erosion due to Frank-Condon or charge exchange atoms impinging on the limiter itself on nearby wall areas.Journal of Nuclear Materials. -
Article: Influence of the Dynamic Ergodic Divertor on the heat deposition pattern in TEXTOR at different collisionalities
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ABSTRACT: The definitive version is available at http://www.elsevier.com/locate/jnucmat . -
Article: Quasistationary high confinement discharges with Trans-Greenwald density in TEXTOR-94
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ABSTRACT: This version is available at the following Publisher URL: http://prl.aps.org -
Article: Suppression of temperature fluctuations and energy barrier generation by velocity shear
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ABSTRACT: This version is available at the following Publisher URL: http://prl.aps.org -
Article: Enhanced particle confinement and turbulence reduction due to Exb shear in the TEXTOR-tokamak
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ABSTRACT: This version is available at the following URL: http://dx.doi.org/10.1088/0029-5515/40/7/309 -
Article: Turbulent transport and turbulence in radiative I-mode plasmas in TEXTOR-94
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ABSTRACT: This version is available at the following URL: http://dx.doi.org/10.1088/0029-5515/40/2/306 -
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. -
Article: Transport of argon and iron during a resonant magnetic perturbation at TEXTOR-DED
Plasma Physics and Controlled Fusion, v.51 (2009). -
Article: Comparison of 13C2H4 and 13CH4 injection through C and W limiters in TEXTOR
Physica Scripta, v.T138 (2009). -
Article: Reduction of the turbulent blob transport in the scrape-off layer by a resonant magnetic perturbation in TEXTOR
Nuclear Fusion, v.49 (2009).
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2002–2011
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Forschungszentrum Jülich
Düren, North Rhine-Westphalia, Germany
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