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

• Published in: Fusion engineering and design Vol. 124; no. C; pp. 226 - 230
• Main Authors: 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
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-11-2017; Elsevier Science Ltd; Elsevier
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Cables; Fusion; Heat; Heat loads; Iridium; Langmuir probes; Magnetic Fusion Energy; Manometers; Neutral gas manometer; Neutral gases; Overloading; Plasma; Plasmas; Scraper element; Strike lines; Thermocouples; Wendelstein 7-X; Strike lines; Heat loads; Scraper element; Langmuir probes; Neutral gas manometer; Wendelstein 7-X
• ISSN: 0920-3796; 1873-7196
• DOI: 10.1016/j.fusengdes.2017.05.115

•Uncooled scraper element in front of island divertor.•Shielding of divertor edges versus loss of neutrals pumping.•Fast installation procedure to minimise oprataional break.•Simple in-vessel plug design to connect diagnostics.
The modular stellarator Wendelstein 7-X in Greifswald (Germany) was successfully taken into operation in 2015. In the next experimental campaign in 2017, divertor plasmas are planned up to 80MJ input power limited by the largely uncooled first wall. In 2020, long pulse plasmas are foreseen once the water cooled first wall has been completed.
In long pulses, bootstrap currents gradually develop in the plasma, which causes a shift of the strike lines onto the relatively poorly cooled edge of the divertor. 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.
To assess the impact, two uncooled scraper elements will be installed in the mid-break of the next campaign, one at the top of module 5 and one at the bottom of module 3. The divertor and scraper elements are monitored with Langmuir probes, neutral gas manometers, thermocouples and IR and VIS cameras viewing the plasma facing surface.
This paper describes the integration of the uncooled scraper elements and their diagnostics in Wendelstein 7-X. A dedicated low power plasma scenario was developed to evaluate the uncooled scraper element, allowing for reduced heat loads and design simplifications. It is demonstrated that the diagnostics are still capable of measuring the impact of the scraper element. An easily accessible socket to plug in the cables of the diagnostics integrated in the scraper was designed and installed up front to allow for a fast and simple installation of the scraper element in the short mid-break.