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Title: Integration of uncooled scraper elements and its diagnostics into Wendelstein 7-X

Abstract

The modular stellarator Wendelstein 7-X in Greifswald (Germany) successfully started operation in 2015 with short pulse limiter plasmas. In 2017, the next operation phase (OP) OP1.2 will start once 10 uncooled test divertor units (TDU) with graphite armor will be installed. The TDUs allow for plasma pulses of 10 s with 8 MW heating. OP2, allowing for steady state operation, is planned for 2020 after the TDUs will be replaced by 10 water cooled CFC armored divertors. Due to the development of plasma currents like bootstrap currents in long pulse plasmas in OP2, the plasma could hit the edge of the divertor targets which has a reduced cooling capacity compared to the central part of the target tiles. To prevent overloading of these edges, a so-called scraper element can be positioned in front of the divertor, intersecting those strike lines that would otherwise hit the divertor edges. As a result, these edges are protected but as a drawback the pumping efficiency of neutrals is also reduced. As a test an uncooled scraper element with graphite tiles will be placed in two out of ten half modules in OP1.2. A decision to install ten water cooled scraper elements for OP2 ismore » pending on the results of this test in OP1.2. To monitor the impact of the scraper element on the plasma, Langmuir probes are integrated in the plasma facing surface, and a neutral gas manometer measures the neutral density directly behind the plasma facing surface. Moreover, IR and VIS cameras observe the plasma facing surface and thermocouples monitor the temperatures of the graphite tiles and underlying support structure. This paper describes the integration of the scraper element and its diagnostics in Wendelstein 7-X.« less

Authors:
 [1];  [2];  [2];  [3];  [3];  [3];  [4];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [2];  [2];  [2];  [2]
  1. Max Planck Inst. for Plasmaphysik, Greifswald (Germany)
  2. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  3. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  4. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1414695
Grant/Contract Number:
AC05-00OR22725; 633053
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Fusion Engineering and Design
Additional Journal Information:
Journal Volume: 124; Journal Issue: C; Journal ID: ISSN 0920-3796
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Wendelstein 7-X; strike lines; heat loads; scraper element; Langmuir probes; neutral gas manometer

Citation Formats

Fellinger, Joris, Loesser, Doug, Neilson, Hutch, Lumsdaine, Arnie, McGinnis, Dean, Lore, Jeremy, Wurden, Glen, Wendorf, Jörg, Klose, Sören, Wenzel, Uwe, Grosser, Klaus, Rummel, Kerstin, Hölbe, Hauke, Pedersen, Thomas Sunn, Mitchell, John, Sibilia, Marc, Zhang, Han, and Titus, Peter. Integration of uncooled scraper elements and its diagnostics into Wendelstein 7-X. United States: N. p., 2017. Web. doi:10.1016/j.fusengdes.2017.05.115.
Fellinger, Joris, Loesser, Doug, Neilson, Hutch, Lumsdaine, Arnie, McGinnis, Dean, Lore, Jeremy, Wurden, Glen, Wendorf, Jörg, Klose, Sören, Wenzel, Uwe, Grosser, Klaus, Rummel, Kerstin, Hölbe, Hauke, Pedersen, Thomas Sunn, Mitchell, John, Sibilia, Marc, Zhang, Han, & Titus, Peter. Integration of uncooled scraper elements and its diagnostics into Wendelstein 7-X. United States. doi:10.1016/j.fusengdes.2017.05.115.
Fellinger, Joris, Loesser, Doug, Neilson, Hutch, Lumsdaine, Arnie, McGinnis, Dean, Lore, Jeremy, Wurden, Glen, Wendorf, Jörg, Klose, Sören, Wenzel, Uwe, Grosser, Klaus, Rummel, Kerstin, Hölbe, Hauke, Pedersen, Thomas Sunn, Mitchell, John, Sibilia, Marc, Zhang, Han, and Titus, Peter. 2017. "Integration of uncooled scraper elements and its diagnostics into Wendelstein 7-X". United States. doi:10.1016/j.fusengdes.2017.05.115.
@article{osti_1414695,
title = {Integration of uncooled scraper elements and its diagnostics into Wendelstein 7-X},
author = {Fellinger, Joris and Loesser, Doug and Neilson, Hutch and Lumsdaine, Arnie and McGinnis, Dean and Lore, Jeremy and Wurden, Glen and Wendorf, Jörg and Klose, Sören and Wenzel, Uwe and Grosser, Klaus and Rummel, Kerstin and Hölbe, Hauke and Pedersen, Thomas Sunn and Mitchell, John and Sibilia, Marc and Zhang, Han and Titus, Peter},
abstractNote = {The modular stellarator Wendelstein 7-X in Greifswald (Germany) successfully started operation in 2015 with short pulse limiter plasmas. In 2017, the next operation phase (OP) OP1.2 will start once 10 uncooled test divertor units (TDU) with graphite armor will be installed. The TDUs allow for plasma pulses of 10 s with 8 MW heating. OP2, allowing for steady state operation, is planned for 2020 after the TDUs will be replaced by 10 water cooled CFC armored divertors. Due to the development of plasma currents like bootstrap currents in long pulse plasmas in OP2, the plasma could hit the edge of the divertor targets which has a reduced cooling capacity compared to the central part of the target tiles. To prevent overloading of these edges, a so-called scraper element can be positioned in front of the divertor, intersecting those strike lines that would otherwise hit the divertor edges. As a result, these edges are protected but as a drawback the pumping efficiency of neutrals is also reduced. As a test an uncooled scraper element with graphite tiles will be placed in two out of ten half modules in OP1.2. A decision to install ten water cooled scraper elements for OP2 is pending on the results of this test in OP1.2. To monitor the impact of the scraper element on the plasma, Langmuir probes are integrated in the plasma facing surface, and a neutral gas manometer measures the neutral density directly behind the plasma facing surface. Moreover, IR and VIS cameras observe the plasma facing surface and thermocouples monitor the temperatures of the graphite tiles and underlying support structure. This paper describes the integration of the scraper element and its diagnostics in Wendelstein 7-X.},
doi = {10.1016/j.fusengdes.2017.05.115},
journal = {Fusion Engineering and Design},
number = C,
volume = 124,
place = {United States},
year = 2017,
month = 8
}

Journal Article:
Free Publicly Available Full Text
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  • The edge topology of magnetic fusion devices is decisive for the control of the plasma exhaust. In Wendelstein 7-X, the island divertor concept will be used, for which the edge topology can change significantly as the internal currents in a plasma discharge evolve towards steady-state. Consequently, the device has been optimized to minimize such internal currents, in particular the bootstrap current [1]. Nonetheless, there are predicted pulse scenarios where effects of the remaining internal currents could potentially lead to overload of plasma-facing components. These internal currents are predicted to evolve on long time scales (tens of seconds) so their effectsmore » on the edge topology and the divertor heat loads may not be experimentally accessible in the first years of W7-X operation, where only relatively short pulses are possible. However, we show here that for at least one important long-pulse divertor operation issue, relevant physics experiments can be performed already in short-pulse operation, through judicious adjustment of the edge topology by the use of the existing coil sets. The specific issue studied here is a potential overload of the divertor element edges. This overload might be mitigated by the installation of an extra set of plasma-facing components, so-called scraper elements, as suggested in earlier publications. It is shown here that by a targeted control of edge topology, the effectiveness of such scraper elements can be tested already with uncooled test-scraper elements in short-pulse operation. Furthermore, this will allow an early and well-informed decision on whether long-pulse-capable (actively cooled) scraper elements should be built and installed.« less
  • The water-cooled high heat flux scraper element aims to reduce excessive heat loads on the target element ends of the actively cooled divertor of Wendelstein 7-X. Its purpose is to intercept some of the plasma fluxes both upstream and downstream before they reach the divertor surface. The scraper element has 24 identical plasma facing components (PFCs) divided into 6 modules. One module has 4 PFCs hydraulically connected in series by 2 water boxes. A PFC, 247 mm long and 28 mm wide, has 13 monoblocks made of CFC NB31 bonded by hot isostatic pressing onto a CuCrZr cooling tube equippedmore » with a copper twisted tape. 4 full-scale prototypes of PFCs have been successfully tested in the GLADIS facility up to 20 MW/m 2. The difference observed between measured and calculated surface temperatures is probably due to the inhomogeneity of CFC properties. The design of the water box prototypes has been detailed to allow the junction between the cooling pipe of the PFCs and the water boxes by internal orbital welding. In conclusion, the prototypes are presently under fabrication.« less
  • The use of superconducting coils in W7-X will allow to run very long pulses (up to 30 min). Therefore enhanced demands have to be met for plasma diagnostics with respect to hardware components exposed to steady state heat fluxes, but also to the data acquisition and analysis. A multicamera x-ray tomography system inside the vacuum vessel--because of lack of suitable ports--with about 400 viewing chords distributed along the poloidal direction is proposed for magneto hydrodynamics investigations. The system will consist of about 20 compact cameras containing linear photodiode arrays, beryllium-transmission foils, and preamplifiers. A particular technical issue is the controlmore » of detector/amplifier offset and gains in steady state plasmas. Another multipurpose x-ray camera system with interchangeable filters will be installed using a particular port combination. In addition x-ray pulse height analysis will be used to deduce spectroscopic information and to provide the central electron temperature.« less
  • The steady state operation of Wendelstein 7-X stellarator presently under construction in Greifswald poses special challenges to the diagnostics development [1, 2]. A critical issue is the heat load on plasma facing components ({approx}500 kW/m{sup 2}) over a long discharge time (up to 30 min), which leads to the necessity of active cooling. As result, the design of the 400 channel soft X-Ray Multi Camera Tomography System (XMCTS) [2, 3] has to cope with dark currents and amplifier drifts due to the heating of active components like photo diodes and in-vessel preamplifiers. In order to allow for a quantitative measurementmore » of dynamic drifts and offsets, a shutter system and blind diodes are considered to compensate these effects. Another important issue is the large amount of data gathered by the XMCT system during long pulse discharges. A fast but less precise online reconstruction is planned, which will give information on the plasma shape and position on a human time scale. The two options under investigation are a Cormack-Inversion method and an approach based on neural networks [4]. Dependent on the available hardware, as much information as possible should be stored for more accurate offline-analysis. An intelligent way of marking interesting data is required. In case that the steady-state storage of all measured data is not feasible, at least this marked data will be stored in high time resolution.« less
  • 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.