TEXTOR Team

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (119)176.65 Total impact

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    ABSTRACT: A new physical mechanism of the formation of runaway electron (RE) beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between $q=1$ and the closest low--order rational magnetic surfaces [$q=3/2$, $q=4/3$, \dots]. It results in that runaway electron beam current has a helical nature with a predominant $m/n=1/1$ component. The thermal quench and current decay times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms of the decay of runaway electron current due to an outward drift electron orbits and resonance interaction of high--energy electrons with the $m/n=1/1$ MHD mode are discussed.
    Physics of Plasmas 01/2015; 22(4). DOI:10.1063/1.4919253 · 2.25 Impact Factor
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    ABSTRACT: Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak mechanisms of the formation of runaway electron beams and their losses are proposed. The plasma disruption is caused by strong stochastic magnetic field formed due to nonlinearly excited low-mode number MHD modes. It is hypothesized that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between $q=1$ and the closest low--order rational [$q=4/3$ or $q=3/2$] magnetic surfaces. The thermal quench time caused by the fast electron transport in a stochastic magnetic field is calculated using the collisional transport model. The current decay stage is due to the ambipolar particle transport in a stochastic magnetic field. The runaway electron beam in the confined plasma region is formed due to their acceleration the inductive toroidal electric field. The runaway electron beam current is modeled as a sum of toroidally symmetric part and a small amplitude helical current with a predominant $m/n=1/1$ component. The runaway electrons are lost due to two effects: ($i$) by outward drift of electrons in a toroidal electric field until they touch wall and ($ii$)by the formation of stochastic layer of runaway electrons at the beam edge. Such a stochastic layer for high--energy runaway electrons is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to wall. The effect of external resonant magnetic perturbations on runaway electron loss is discussed. A possible cause of the sudden MHD signals accompanied by runaway electron bursts is explained by the redistribution of runaway current during the resonant interaction of high--energetic electron orbits with the $m/n=1/1$ MHD mode.
    Journal of Plasma Physics 01/2015; 81(05). DOI:10.1017/S0022377815000501 · 0.74 Impact Factor
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    A Krämer-Flecken · H Arnichand · S Hacquin · R Sabot · Y Xu · Textor Team
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    ABSTRACT: Today's simulation of turbulence and its propagation in plasmas shows a complex 3-dimensional picture [1]. However, experimental evidence for turbulence is often observed by 1-dimensional diagnostics [2], only. Integration along lines of sight complicate the analysis further. In addition turbulence diagnostics have a limited sensitivity with respect to the wave number spectrum which allows to diagnose only a small window of the variety of micro structures in plasmas. Therefore a comparison of simulations and experiments is difficult. However, it is indispensable for simulations to have 3-dimensional information on turbulence and structures to compare with. The paper discusses the 3-dim analysis of quasi coherent modes visible on a major part of the plasma radius and especially on the q = 1 surface. Figure 1: Coherence for LRC during flat top conditions showing the enhanced coherence in the range 80 kHz to 120 kHz At TEXTOR a multi location reflectometry sys-tem [3] with good radial resolution was used to study short and long range correlations of den-sity fluctuations in the range k ⊥ ≤ 3 cm −1 . The different locations of the antenna arrays yield in-formation on the toroidal extent of structures in the plasma. Therefore the plasma conditions are chosen in such a manner that a field line con-nects both antennae arrays. In the case under in-vestigation I p is in counter clock wise direction and magnetic field in clock wise direction con-necting the low field side (LFS) array with the top array. The toroidal separation is ∆φ = 247.5 • and the poloidal separation ∆θ is in the same range matching the q = 1 condition. To reach the q = 1 surface a low density high current ohmic plasma was used to extent the operation range of the reflectometer (26 GHz to 40 GHz) towards the plasma center. In all cases reported here, the reflectometer was operated in O-mode polar-ization to make sure that the reflection comes from the same iso density surface. With a second reflectometer the radial separation between top and LFS array as well as within the top array
    41st EPS Conference on Plasma Physics, Berlin, Germany; 06/2014
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    ABSTRACT: Neutral gas breakdown by the standard ICRF antenna operated in the Ion Cyclotron Wall Conditioning (ICWC) mode is a major issue for the antenna safety and the RF discharge optimization. Consistent modelling with a 1-D full wave RF code and a 0-D transport code was undertaken to simulate the gas breakdown threshold for two AUG ICWC discharges in hydrogen with different antenna phasing in the light of slow (SW) and fast (FW) waves excitation. The present study clearly indicates that SW excitation in vicinity of the low hybrid resonance (LHR) at the antenna side, independently on antenna phasing, may be considered as the trigger for gas breakdown (ne-LHR/ne-bd model≈0.8−0.85). Monopole phasing suggests an additional benefit: low toroidal modes (ntor=2) of the FW, excited at the gas breakdown moment (ne>10^16 m^-3), dramatically improve the antenna coupling, reduce the antenna RF voltage and, finally, promote fast, robust and safe breakdown compared to dipole phasing. The possible contribution to the gas ionization of the high energy resonant protons usually generated at ICR is also investigated.
    41st EPS Conference on Plasma Physics, Berlin 2014; 01/2014
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    41st EPS Conference on Plasma Physics, Berlin 2014; 01/2014
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    ABSTRACT: The radio-frequency (RF) plasma production technique in the ion cyclotron range of frequency (ICRF) attracts growing attention among fusion experts because of its high potential for solving several basic problems of reactor-oriented superconducting fusion machines, such as ICRF wall conditioning in tokamaks and stellarators (Te =3-5eV, ne <1012 cm􀀀3), ICRF-assisted tokamak start-up and target plasma production (ne = 1013 cm􀀀3) in stellarators. Plasma initiation by ICRF has been studied intensively using single particle descriptions and basic analytic models. To further improve the present understanding on plasma production employing the vacuum RF field of ICRF antennas in toroidal devices in presence of the toroidal magnetic field, and its parametric dependencies a Monte Carlo code has been developed. The 1D code RFdinity1D describes the motion of electrons, accelerated by the RF field in front of the ICRF antenna, along one toroidal magnetic field line. Dependent on their individual energies and the related electron collision cross sections (ionisation, excitation and dissociation) weighted by a Monte Carlo procedure, an electron avalanche may occur. Breakdown conditions are discussed as function of RF discharge parameters (i) RF vacuum electric field strength, (ii) RF frequency and (iii) neutral pressure (H2). The slope of the exponential density increase, taken as measure for the breakdown speed, shows qualitative agreement to experimental breakdown times as found in literature and experimental data of the ASDEX upgrade and TEXTOR tokamak, and is interpreted by studying the characteristic electron velocity distribution functions.
    20th Topical Conference on Radio Frequency Power in Plasmas, Sorrento 2013; 01/2014
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    20th Topical Conference on Radio Frequency Power in Plasmas, Sorrento 2013; 01/2014
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    41st EPS Conference on Plasma Physics, Berlin 2014; 01/2014
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    ABSTRACT: Recent experiments on Ion Cyclotron Wall Conditioning (ICWC) performed in tokamaks TEXTOR and ASDEX Upgrade with standard ICRF antennas operated at fixed frequencies but variable toroidal magnetic field demonstrated rather contrasting parameters of ICWC discharge in scenarios with on-axis fundamental ion cyclotron resonance (ICR) for protons, 􀈦=􀈦cH+, and with its high cyclotron harmonics (HCH), 􀈦=10􀈦cH+. HCH scenario: very high antenna coupling to low density RF plasmas (Ppl􀁼0.9PRF-G) and low energy Maxwellian distribution of CX hydrogen atoms with temperature TH􀁼350 eV. Fundamental ICR: lower antenna-plasma coupling efficiency (by factor of about 1.5 times) and generation of high energy non-Maxwellian CX hydrogen atoms (with local energy E􀁁H 􀂕􀀔􀀑􀀓 keV). In the present paper, we analyze the obtained experimental results numerically using (i) newly developed 0-D transport code describing the process of plasma production with electron and ion collisional ionization in helium-hydrogen gas mixture and (ii) earlier developed 1-D Dispersion Relation Solver accounting for finite temperature effects and collision absorption mechanisms for all plasma species in addition to conventionally examined Landau/TTPM damping for electrons and cyclotron absorption for ions. The numerical study of plasma production in helium with minor hydrogen content in low and high toroidal magnetic fields is presented. The investigation of the excitation, conversion and absorption of plasma waves as function of BT-field suggests that only fast waves (FW) may give a crucial impact on antenna coupling and characteristics of the ICWC discharge using standard poloidally polarized ICRF antennas designed to couple RF power mainly to FW. The collisional (non-resonant) absorption by electrons and ions and IC absorption by resonant ions of minor concentration in low Te plasmas is studied at fundamental ICR and its high harmonics.
    20th Topical Conference on Radio Frequency Power in Plasmas, Sorrento 2013; 01/2014
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    ABSTRACT: This paper focuses on a study of the principal operation aspects of standard ICRF heating antennas in the ion cyclotron wall conditioning (ICWC) mode: (i) ability of the antenna to ignite the cleaning discharge safely and reliably in different gases including those most likely to be used in ITER – He, H2, D2 and their mixtures, (ii) the antenna capacity to couple a large fraction of the RF generator power (>50%) to low density (�1016–1018 m-�3) plasmas and (iii) the RF power absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. The ICWC discharge optimization in terms of RF plasma wave excitation/absorption resulted in successful simulation of the conditioning scenarios for ITER operation at full field (JET) and half-field (TEXTOR, TORE SUPRA, ASDEX Upgrade).
    Journal of Nuclear Materials 08/2013; 419:S1029–S1032. · 2.02 Impact Factor
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    ABSTRACT: This paper reports on the recent assessment of the Ion Cyclotron Wall Conditioning (ICWC) technique for isotopic ratio control, fuel removal and recovery after disruptions, which has been performed on TORE SUPRA, TEXTOR, ASDEX Upgrade and JET. ICWC discharges were produced using the standard ICRF heating antennas of each device, at different frequencies and toroidal fields, either in continuous or pulsed mode. Intrinsic ICWC discharge inhomogeneities could be partly compensated by applying a small vertical magnetic field, resulting in the vertical extension of the discharge in JET and TEXTOR. The conditioning efficiency was assessed from the flux of desorbed and retained species, measured by means of mass spectrometry. In Helium ICWC discharges, fuel removal rates between 1016D.m-2.s-1 to 3.1017D.m-2.s-1 were measured, with a linear dependence on the coupled RF power and on the He + density. ICWC scenarios have been developed in D or H plasmas for isotopic exchange. The H (or D) outgassing was found to increase with the D (resp. H) partial pressure. In continuous mode, wall retention is on the average two to ten times higher than desorption , due to the high reionization probability of desorbed species in ICWC discharges, where the electron density is about 1018m-3. Retention can be minimized in pulsed ICWC discharges without severely reducing outpumping. Pulsed He-ICWC discharges have been successfully used on TORE SUPRA to recover normal operation after disruptions, when subsequent plasma initiation would not have been possible without conditioning.
    Journal of Nuclear Materials 08/2013; 415:S1021–S1028. · 2.02 Impact Factor
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    ABSTRACT: The 1.1-1.5 mm wide gaps between tiles of the main toroidal belt limiter in TEXTOR were utilized to study the long-term impurity deposition and fuel retention in gaps. The tiles were exposed during a full tokamak campaign of 9365 s of plasma to various discharge conditions and wall conditioning, accumulating of up to 30 μm thick layers at the gap entrance. It was found that (i) gaps trap impurities twice as efficient as the top surface, (ii) the deposition in the toroidal gaps is twice as high as in the poloidal, (iii) carbon deposition decays with a fall-off length of about 0.7 mm towards the gap bottom, (iv) deposition on the bottom is significantly higher than on the adjacent side walls of gaps, and (v) the amount of deuterium scales with the amount of carbon with D/C varying from 3% to 30% depending on the surface temperature.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.159 · 2.02 Impact Factor
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    ABSTRACT: Resonant Magnetic Perturbations (RMPs) are applied with the Dynamic Ergodic Divertor (DED) at TEXTOR to control the plasma edge transport and the plasma surface interaction. This leads to the formation of a three-dimensional (3D) topology of the scrape-off layer (SOL). To quantify the erosion/deposition balance and the material migration in this 3D boundary, spherical test limiters were exposed to plasmas with and without RMP fields applied. Methane doped with 13C as tracer element was injected through a gas inlet in the test limiter. The local gas source was monitored by spatially resolving spectroscopy and the resulting deposition patterns on the limiters were analysed with colourimetry and nuclear reaction analysis. These measurements were compared to simulations of the magnetic field topology simulations. The data provide evidence of a particle migration dominated by an ExB drift within stochastic zones of the 3D plasma boundary.
    Journal of Nuclear Materials 07/2013; 438:602-. DOI:10.1016/j.jnucmat.2013.01.126 · 2.02 Impact Factor
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    ABSTRACT: Laser-based methods are investigated for the development of an in situ diagnostic for spatially and temporally resolved characterization of the first wall in fusion devices. Here we report on the first systematic laser-induced ablation spectroscopy (LIAS) measurements carried out on various surface layers in the TEXTOR tokamak. These materials include a-C:D, mixed W/C/Al/D2, Oerlikon Balzers ‘Balinit’ diamond-like carbon layers and EK98 fine-grain graphite. In LIAS, the bulk or deposited material is evaporated during the plasma discharge by intense laser radiation. The light emitted by particles entering the edge of the ionizing tokamak plasma is then observed by optical spectroscopy. In the measurements taken, it was found that the studied layers can be identified by their characteristic line emission. A good correlation between the observed line intensity and layer thickness is found. The observed plumes show target material dependence. To analyze layers formed during tokamak operation, further investigation of the ablation process and reference materials for cross calibration is required.
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    ABSTRACT: Experiments have been carried out in the TEXTOR, ASDEX Upgrade (AUG) and Alcator C-Mod (C-Mod) tokamaks to study melt-layer motion, macroscopic W-erosion from the melt as well as the changes of material properties such as grain-size and voids. In addition the effect of multiple exposures is studied to judge the potential amelioration of inflicted melt damage. The parallel heat flux at the radial position of the PFCs in the plasma ranges from around q∥ ∼ 45 MW/m2 at TEXTOR up to q∥ ∼ 500 MW/m2 at C-Mod which covers scenarios close to ITER parameters, allowing samples to be exposed and molten even at shallow divertor angles. Melt-layer motion perpendicular to the magnetic field is observed consistent with a Lorentz-force originating from thermoelectric emission of the hot sample. While melting in the limiter geometry at TEXTOR is rather quiescent causing no severe impact on plasma operation, exposure in the divertors of AUG and C-Mod shows significant impact on operation, leading to subsequent disruptions. The power-handling capabilities are severely degraded by forming exposed hill structures and changing the material structure by re-solidifying and re-crystallizing the original material. Melting of W seems highly unfavorable and needs to be avoided especially in light of uncontrolled transients and misaligned PFCs.
    Journal of Nuclear Materials 01/2013; DOI:10.1016/j.jnucmat.2013.01.005 · 2.02 Impact Factor
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    ABSTRACT: Ion cyclotron wall conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the limiter tokamak TEXTOR to explore safe operational regimes for the experimental parameters for possible ICWC-discharge cleaning in International Thermonuclear Experimental Reactor (ITER) at half field. Antenna coupling properties obtained during the ion cyclotron range of frequencies (ICRF) wall conditioning experiments performed in helium–hydrogen mixture in TEXTOR were analysed in relation to the obtained ICWC-plasma characterization results. Satisfactory antenna coupling in the mode conversion scenario along with reproducible generation of ICRF plasmas for wall conditioning, were achieved by coupling radio frequency (RF) power from one or two ICRF antennas. The plasma breakdown results obtained in the TEXTOR tokamak have been compared with the predictions of a zero-dimensional RF plasma production model. The present study of ICWC emphasizes the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (B V ≪ B T) or oscillating poloidal magnetic field (B P ≪ B T) on antenna coupling and relevant plasma parameters.
    Pramana 01/2013; 80(1):121. DOI:10.1007/s12043-012-0459-2 · 0.72 Impact Factor

Publication Stats

652 Citations
176.65 Total Impact Points

Institutions

  • 1995–2011
    • Forschungszentrum Jülich
      • Zentralabteilung für Chemische Analysen (ZCH)
      Jülich, North Rhine-Westphalia, Germany
  • 2010
    • Technische Universiteit Eindhoven
      • Control Systems Technology
      Eindhoven, North Brabant, Netherlands
  • 2009
    • KTH Royal Institute of Technology
      • Department of Physics
      Tukholma, Stockholm, Sweden
  • 2006–2009
    • Ghent University
      • Department of Applied Physics
      Gent, VLG, Belgium
    • Nagoya University
      • Department of Energy Engineering and Science
      Nagoya, Aichi, Japan
  • 2003–2008
    • Kurchatov Institute
      • Institute of Tokamak Physics
      Moskva, Moscow, Russia
  • 2005
    • National Institute for Fusion Science
      Tokitsu-chō, Gifu, Japan