M. Endler

Max Planck Institute for Plasma Physics, Arching, Bavaria, Germany

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Publications (98)148.24 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Wendelstein 7-X, currently under commissioning at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimized properties of the plasma. Most of the envisaged magnetic configurations of the machine are rather sensitive to symmetry breaking perturbations which are the consequence of unavoidable manufacturing and assembly tolerances. This overview describes the successive tracking of the Wendelstein 7-X magnet system geometry starting from the manufacturing of the winding packs up to the modelling of the influence of operation loads. The deviations found were used to calculate the resulting error fields and to compare them with the compensation capacity of the trim coils.
    Nuclear Fusion 06/2015; 55(6):063025. DOI:10.1088/0029-5515/55/6/063025 · 3.24 Impact Factor
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    ABSTRACT: An overview of the diagnostics which are essential for the first operational phase of Wendelstein 7-X and the set of diagnostics expected to be ready for operation at this time are presented. The ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating (ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described
    Review of Scientific Instruments 11/2014; 85(11):11D818. DOI:10.1063/1.4889905 · 1.58 Impact Factor
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    ABSTRACT: Microscopic turbulence properties in the edge of toroidally confined fusion plasmas are studied by comparative analysis of experimental data from seven devices, collected in an international edge turbulence database. The database contains Langmuir probe measurements of fluctuations in the floating potential and ion saturation current across the last closed flux surface. They are used to address statistical properties and particle transport. Universal features of plasma edge turbulence such as an increase in skewness across the scrape-off layer (SOL) as footprints of density blobs are recovered in all devices. Analysis of the correlation lengths and times reveals power law scaling relations with macroscopic drift-wave parameters, albeit weaker than would be expected for drift-wave turbulence. As a result, the turbulent diffusivity scales with the inverse of the magnetic field strength, which is closer to Bohm-like scaling than to gyro-Bohm scaling. Nearly identical scaling relations are determined in the confined plasma edge and the SOL, pointing to a strong connection between drift-wave turbulence in the edge and blobs in the SOL. The contributions of blobs and holes (negative density spikes) to the radial particle transport are analyzed qualitatively with a conditional averaging approach. Blobs are connected to outward transport in the SOL of all devices whereas holes exhibit no uniform propagation pattern.
    Plasma Physics and Controlled Fusion 07/2014; 56(9):095015. DOI:10.1088/0741-3335/56/9/095015 · 2.39 Impact Factor
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    ABSTRACT: The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.
    Nuclear Fusion 12/2013; 53(12):126001. DOI:10.1088/0029-5515/53/12/126001 · 3.24 Impact Factor
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    ABSTRACT: Mirnov coils are used to measure fluctuations of the magnetic field which are in particular generated by magnetohydrodynamic (MHD) modes. The underlying plasma currents have a multipolar structure in a poloidal cross-section. Therefore the amplitude of the magnetic fluctuations decays quickly with increasing distance from the plasma edge. It is hence important to place the Mirnov coils as close to the plasma edge as possible where they are exposed to high thermal loads. Two types of Mirnov coils are proposed to be used in Wendelstein 7-X (W7-X). Type 1 (44 Mirnov coils) should be mounted on the plasma side of wall protection panels with a graphite cap to shield them from direct plasma exposure. Type 2 (137 Mirnov coils) will be located behind the tiles of the heat shields. An important issue concerning the design of these Mirnov coils is to verify their suitability for steady state operation from the thermal point of view. Both steady state and transient finite element thermal analyses were performed for the Mirnov coils under different conditions and with different designs. The paper presents detailed thermal analyses of the Mirnov coils.
    Fusion Engineering and Design 10/2013; 88(9-10):1844-1847. DOI:10.1016/j.fusengdes.2013.05.087 · 1.15 Impact Factor
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    ABSTRACT: The main heating system of W7-X will be a 10 MW, 140 GHz ECRH. It is expected that the non-absorbed ECRH power will be up to 1 MW. Multiple reflections of the ECRH beam result in a nearly isotropic radiation background. The non-absorbed ECRH power has to be considered as an additional thermal load of microwave stray radiation for all W7-X in-vessel components.Several in-vessel diagnostics, especially magnetic diagnostics, partly consist of microwave absorbing materials. The microwave stray radiation could damage these components or, by evaporation due to overheating, perturb the operation of W7-X. Usually these diagnostics are protected against direct plasma radiation and are thermally connected to actively cooled structures. Nevertheless, ECRH stray radiation can cause a critical temperature increase.We present examples of ECRH stray radiation protection by metallic housing. The use of such housing may be in conflict with vacuum compatibility and in particular for magnetic diagnostics, with the requirement to avoid eddy currents limiting the time resolution of magnetic measurements.Details are given on the protection against ECRH stray radiation as well as on thermal connection concepts. Acceptance tests are presented on full-scale mockups.
    Fusion Engineering and Design 10/2013; 88(6-8):1232-1235. DOI:10.1016/j.fusengdes.2013.01.003 · 1.15 Impact Factor
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    ABSTRACT: The critical issues in the development of diagnostics, which need to work robust and reliable under quasi-steady state conditions for the discharge durations of 30 min and which cannot be maintained throughout the one week duration of each operation phase of the Wendelstein 7-X stellarator, are being discussed.
    The Review of scientific instruments 10/2012; 83(10):10D730. DOI:10.1063/1.4733531 · 1.58 Impact Factor
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    ABSTRACT: Long pulse operation considerably increases the thermal load on in-vessel components. Diagnostic front-ends formerly employed at short pulse machines therefore have to be considerably re-designed for installation in the stellarator W7-X that is currently being built at Greifswald, Germany. The strategy applied to cope with the thermal load is threefold: to reduce the influx of heat on the component, to conduct the heat inside the component to suitable heat sinks and to choose suitable materials for sensitive components. The first is achieved by the shielding against microwave stray radiation, plasma radiation, thermal radiation and particle fluxes and by absorbing residual microwave stray radiation in the immediate vicinity of sensitive components. The second task, suitable heat conduction, enforces severe restrictions on the use of any thin parts like foils or meshes. Thirdly, in order for a component to survive the residual loads, materials must be chosen that absorb only a small fraction of the microwave stray radiation flux, conduct heat well enough, and survive high temperatures and large temperature gradients. Examples are provided from bolometry, magnetic diagnostics, soft X-ray diagnostics and Thomson scattering. Measurements of microwave stray radiation effects are presented, in particular the effectiveness of several shielding concepts.
    Fusion Engineering and Design 08/2012; 87(s 5–6):392–394. DOI:10.1016/j.fusengdes.2011.11.004 · 1.15 Impact Factor
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    ABSTRACT: The gain in experience with long-pulse operation under fusion relevant plasma conditions is an important step towards successful sustainment of future steady state reactor plasmas. The stellarator Wendelstein 7-X (W7-X) [1], presently being under construction, is already equipped with a superconducting coil system and principally capable of quasi-continuous operation. Like other long pulse devices, W7-X is faced with new enhanced technical requirements which have to be met by plasma facing components as well as the diagnostic systems in general. Water-cooled windows were qualified up to 60 kW/m2 thermal load, and shutters against evaporation of sensitive elements outside the attended time of the diagnostic as well as for baseline control had been developed. Shielding of in-vessel components against damage by absorption of microwave stray radiation originating from the heating systems appears to be one of the most challenging tasks. Experiments using a microwave test chamber identify critical materials and approved the necessity for careful shielding of both, sensitive diagnostics and cables. Spectroscopic systems for monitoring the impurity content and divertor load as well as for robust density measurement are presented (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Beiträge aus der Plasmaphysik 03/2011; 51(2‐3). DOI:10.1002/ctpp.201000053
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    ABSTRACT: The status of the diagnostic developments for the quasistationary operable stellarator Wendelstein 7-X (maximum pulse length of 30 min at 10 MW ECRH heating at 140 GHz) will be reported on. Significant emphasis is being given to the issue of ECRH stray radiation shielding of in-vessel diagnostic components, which will be critical at high density operation requiring O2 and OXB heating.
    The Review of scientific instruments 10/2010; 81(10):10E133. DOI:10.1063/1.3483210 · 1.58 Impact Factor
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    J. Geiger, M. Endler, A. Werner
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    ABSTRACT: For W7-X a set of magnetic diagnostic coils is foreseen to assess plasma properties such as toroidal net current and energy content for real-time monitoring and also for equilibrium reconstruction. In addition to the set of full Rogowski-Coils, diamagnetic coils and saddle loops, a set of segmented Rogowski-Coils is envisaged to access profile information. The planned scheme of equilibrium reconstruction is described and the status of the analysis of the magnetic diagnostics is presented and its prospects are discussed. It is found that the plasma energy may be retrieved with good accuracy. However, the recovery of profile information shows limited accuracy based on the statistical analyses and modeling done so far (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Beiträge aus der Plasmaphysik 08/2010; 50(8):736 - 740. DOI:10.1002/ctpp.200900025
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    McKenna, C.: 37th European Physical Society Conference on Plasma Physics. Contributed Papers, European Physical Society (2010), Dublin, Ireland; 06/2010
  • S. Marsen, M. Endler, M. Otte, F Wagner
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    ABSTRACT: The three-dimensional structure of turbulence in the edge (inside the last closed flux surface) of the WEGA stellarator is studied focusing on the parallel dynamics. WEGA as a small stellarator with moderate plasma parameters offers the opportunity to study turbulence with Langmuir probes providing high spatial and temporal resolution. Multiple probes with radial, poloidal and toroidal resolution are used to measure density fluctuations. Correlation analysis is used to reconstruct a 3D picture of turbulent structures. We find that these structures originate predominantly on the low field side and have a three-dimensional character with a finite averaged parallel wavenumber. The ratio between the parallel and perpendicular wavenumber component is in the order of 10-2. The parallel dynamics are compared at magnetic inductions of 57 and 500 mT. At 500 mT, the parallel wavelength is in the order of the field line connection length 2\pi R/\iotabar . A frequency resolved measure of kpar/ktheta shows a constant ratio in this case. At 57 mT the observed kpar is much smaller than at 500 mT. However, the observed small average value is due to an averaging over positive and negative components pointing parallel and antiparallel to the magnetic field vector.
    Plasma Physics and Controlled Fusion 08/2009; DOI:10.1088/0741-3335/51/8/085005 · 2.39 Impact Factor
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    ABSTRACT: One manufacturing step of the Wendelstein 7-X non-planar coils is intended to generate a mechanical prestress between winding pack and casing. To document this “embedding” procedure, we performed several temperature and electric resistivity measurements and geometric surveys before, during and after the embedding. We also investigated the behaviour of quartz sand, used to fill the gap between winding pack and casing, under pressure. The results of the whole body of investigations are consistent with the assumption that the achieved prestress is significantly lower than initially intended.
    Fusion Engineering and Design 06/2009; 84(2-6):743-746. DOI:10.1016/j.fusengdes.2009.02.024 · 1.15 Impact Factor
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    ABSTRACT: Wendelstein 7-X, currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimised properties of the plasma. The magnet system of the machine consists of 50 non-planar and 20 planar superconducting coils which are arranged in five identical modules, forming a toroidal five-fold symmetric system. The majority of operational magnetic configurations will have rotational transform ι/2π = 1 at the boundary. Such configurations are very sensitive to symmetry breaking perturbations, which are the consequence of imprecisely manufactured coils or assembly errors. To date, all 70 coils have been fabricated, and the first two half-modules of the machine have been assembled. The comparative analysis of manufactured winding packs and estimates of the corresponding level of magnetic field perturbation are presented. The dependency of the error fields on the coil assembly sequence is considered, as well as the impact of the first assembly errors. The influence of different construction uncertainties is discussed, and measures to minimise the magnetic field perturbation are suggested.
    Fusion Engineering and Design 06/2009; 84:408-412. DOI:10.1016/j.fusengdes.2008.12.073 · 1.15 Impact Factor
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    ABSTRACT: Stellerators can be operated without the need of significant plasma current for generation of poloidal magnetic field components. Wendelstein 7-X (W7-X) is optimized for stiff magnetic configurations by minimizing pressure driven currents, such as bootstrap and Pfirsch–Schlüter currents. In Addition to the largely reduced plasma currents, the HELIAS-type device is a low magnetic shear configuration required for magnetic boundary islands. However, the diverting magnetic field structure is already influenced by small net plasma currents. In particular, the W7-AS divertor results revealed a significant sensitivity of the X -point location with respect to favorable edge plasma states. Thus, a precise X -point control for W7-X is envisaged and, although expected net plasma currents range in the order of 10–100 kA only, stabilization by adaptive controllers is foreseen to an accuracy of about 1 kA . This stabilization should be reliable even after 2000 s of plasma operation. A precondition for achieving this is a reliable current detection. For this purpose, W7-X will be equipped with Rogowski of which the first has now been assembled onto one of the plasma vessel sectors. The integral performance and sensitivity inhomogeneities have been tested by employing the digital integrator of W7-X [A. Werner, Rev. Sci. Instrum. 77, 10E307 (2006)] and a cable energized with 200 A , representing the plasma current.
    Review of Scientific Instruments 11/2008; DOI:10.1063/1.2957933 · 1.58 Impact Factor
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    ABSTRACT: Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9 T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared successfully with their theoretical predictions. Besides, new heating schemes of overdense plasmas were developed such as RF mode conversion heating—Ordinary mode, Extraordinary mode, Bernstein-wave (OXB) heating—or 2nd harmonic O-mode (O2) heating. The energy confinement was about a factor of 2 above ISS95 without degradation near operational boundaries. A number of improved confinement regimes such as core electron-root confinement with central Te ≤ 7 keV and regimes with strongly sheared radial electric field at the plasma edge resulting in Ti ≤ 1.7 keV were obtained. As the first non-tokamak device, W7-AS achieved the H-mode and moreover developed a high density H-mode regime (HDH) with strongly reduced impurity confinement that allowed quasi-steady-state operation (τ ≈ 65 τE) at densities (at 2.5 T). The first island divertor was tested successfully and operated with stable partial detachment in agreement with numerical simulations. With these results W7-AS laid the physics background for operation of an optimized low-shear steady-state stellarator.
    Plasma Physics and Controlled Fusion 03/2008; 50(5):053001. DOI:10.1088/0741-3335/50/5/053001 · 2.39 Impact Factor
  • M Schubert, M Endler, H Thomsen
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    ABSTRACT: Stationary Langmuir probe measurements of ion saturation current and floating potential in a plasma cannot give direct information on density and plasma potential fluctuations in the presence of temperature fluctuations. This problem can be avoided if the probe bias voltage is continuously swept faster than the fluctuation time scale, recording the current-voltage characteristic. This article reports the development of a spatiotemporal highly resolving Langmuir probe array with 15 fast swept tips, operating in the strongly magnetized, collisionless edge plasma of the Wendelstein 7-AS stellarator [Plasma Phys. Controlled Fusion 31, 1579 (1989)]. The probe tips are aligned in the poloidal direction, the tip spacing is 2 mm, and the sweeping frequency is 1.4 MHz. Current and voltage data are sampled with 50 MHz. The high bandwidth of the measurement is achieved by placing miniaturized differential amplifiers close to the probe tips in order to do an impedance transform. The surface-mounting technology and an additional inverse feedback module are utilized, allowing for an input voltage range of +/-100 V, and a common mode rejection rate of 55 dB at 4 MHz, which is sufficient to resolve the nonlinear probe characteristic. For the evaluation of the data, a fit model for stationary probes is employed and found adequate. Changes of the plasma parameters during one voltage sweep are taken into account by a linear interpolation of the fit parameters. Spatio-temporal fluctuation data gained by a fast swept Langmuir probe array, which can be relevant for the turbulent radial transport of particles and energy, are presented.
    Review of Scientific Instruments 06/2007; 78(5):053505. DOI:10.1063/1.2740785 · 1.58 Impact Factor
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    ABSTRACT: Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX and W7-AS by Langmuir probes and by observation of Hα light and light emitted from a fast Li atom beam with high spatial and temporal resolution. It was demonstrated that these fluctuations contribute a significant, if not dominant, fraction of the anomalous radial particle transport. The basic properties of the fluctuations are the same in both experiments. A model for an instability mechanism specific to the SOL is presented including density, temperature and electric potential fluctuations. From this model mixing length estimates for the radial transport and resulting density and pressure gradients in the SOL are derived and compared to measurements in the mid-plane and in the divertor of ASDEX. In spite of several simplifications in the model a quantitative agreement up to factors of 1-3 and a qualitative agreement for variations of discharge parameters is achieved between the model predictions and the measurements. Data from poloidal multi-pin probe arrays are decomposed into a sum of spatial-temporal "events" by means of a fitting procedure. Centres of selected events serve as reference points for the analysis of the dynamics in their surroundings. Averaging shows that positive and negative potential events appear mostly in pairs with the E x B drift in between directed radially outward.
    Physica Scripta 07/2006; 51(5):610. DOI:10.1088/0031-8949/51/5/011 · 1.30 Impact Factor
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    ABSTRACT: This paper is a summary of some of the major results from the Wendelstein 7-AS stellarator W7-AS. W7-AS G. Grieger et al., Phys. Fluids B 4, 2081 1992 has demonstrated the feasibility of modular coils and has pioneered the island divertor and the modeling of its three-dimensional characteristics with the EMC3/EIRENE code Y. Feng, F. Sardei et al., Plasma Phys. Controlled Fusion 44, 611 2002. It has extended the operational range to high density 4 10 20 m −3 at 2.5 T and high 3.4% at 0.9 T; it has demonstrated successfully the application of electron cyclotron resonance heating ECRH beyond cutoff via electron Bernstein wave heating, and it has utilized the toroidal variation of the magnetic field strength for ion cyclotron resonance frequency beach-wave heating. In preparation of W7-X J. Nührenberg et al., Trans. Fusion Technol. 27, 71 1995, aspects of the optimization concept of the magnetic design have been successfully tested. W7-AS has accessed the H-mode, the first time in a "non-tokamak" and has extended H-mode operation toward high density by the discovery of the high-density H-mode HDH, characterized by H-mode energy and L-mode-level impurity confinement. In the HDH-mode quasisteady state operation is possible close to operational limits without noticeable degradation in the plasma properties. High-phases up to t pulse / E = 65 have been achieved, which can already be taken as an indication of the intrinsic stellarator capability of steady-state operation. Confinement issues will be discussed with emphasis on the similarities to tokamak confinement general transport properties, H-mode transition physics but also with respect to distinct differences no confinement degradation toward operational boundaries, positive density scaling, lack of profile resilience, no distinct isotope effect, H-mode operational window. W7-AS turned out to be an important step in the development of the Wendelstein stellarator line towards an independent fusion power plant concept. © 2005 American Institute of Physics.
    Physics of Plasmas 06/2005; 12(7). DOI:10.1063/1.1927100 · 2.25 Impact Factor

Publication Stats

1k Citations
148.24 Total Impact Points

Institutions

  • 1995–2014
    • Max Planck Institute for Plasma Physics
      • Max Planck Institute for Plasma Physics, Greifswald
      Arching, Bavaria, Germany
  • 2006
    • National Center for Scientific Research Demokritos
      Athínai, Attica, Greece
  • 2002
    • University of Wisconsin–Madison
      • Department of Physics
      Madison, Wisconsin, United States
  • 1999
    • Oak Ridge National Laboratory
      • Fusion Energy Division
      Oak Ridge, Florida, United States
  • 1998
    • Universität Stuttgart
      Stuttgart, Baden-Württemberg, Germany