Access to edge scenarios for testing a scraper element in early operation phases of Wendelstein 7-X

Abstract

For long-pulse operation of Wendelstein 7-X (W-7X) installation of an additional divertor component is being considered to protect the edges of the water-cooled divertor target plates. In this paper, the impact on plasma performance of such a protection element, the scraper element (SE), is analyzed using the state-of-the-art edge transport code EMC3-Eirene, combined with magnetohydrodynamic equilibrium magnetic fields calculated with the code package VMEC-Extender. From this numerical analysis, which is significantly more sophisticated than previous analyses of the SE performance [1] , the following conclusions can be drawn: First, the expected protection capability of the SE is confirmed. Second, the calculated heat load patterns are such that overload of other parts of the plasma-facing components may result from the planned scenario. Third, the SE will cause a significant reduction in the particle exhaust capability of the divertor. These results show that a decision to add SE is not straightforward. It is prudent to test its pros and cons by installation of a SE prototype as early as possible. That possibility is described in a recent paper [2].

Infrared imaging systems for wall protection in the W7-X stellarator (invited)

“…Another object for infrared monitoring has been added with the installation of a divertor extension in two modules, so called scraper elements (SE) 20 . In OP1.2b, a separate imaging system 21 , will be used with a view towards the upper divertor and its scraper element in The control software is a distributed system of 10 acquisition computers and one central server computer in the control room.…”

Measurement and modeling of magnetic configurations to mimic overload scenarios in the W7-X stellarator

“…The SE-RS is not directly accessible in the early operational phases of W7-X (OP1.2) due to limits on the injected power and pulse length. To overcome this, a set of new magnetic configurations were developed to approximately reproduce the important features in the deposited flux to the target edges and the scraper element [10]. These 'mimic configurations' use the planar coil set to control the edge transform and the control coils to adjust the edge island size, to mimic the effect of a net toroidal current and plasma beta, respectively.…”

Preparation of erosion and deposition investigations on plasma facing components in Wendelstein 7-X

“…In the upcoming Operation Phase (OP1.2), starting Sept. 2017, the divertor consists of 10 inertially cooled, fine grain graphite Test Divertor Units (TDU), which are identical in shape to the actively water cooled CFC high heat flux divertor, being used in the next operation phase OP2. Between OP1.2a and OP1.2b, in addition two so called scraper elements (essentially target modules) will be installed just in front of one upper and one lower TDU divertor module, to investigate their ability to mitigate possible thermal overloads of target tiles at the pumping gap edge in certain magnetic configurations when the bootstrap current evolution may drive strike lines from the horizontal onto the vertical target area [5]. To reach sufficient pumping capability for quasi-continuous plasma operation in OP2, ten cryo-vacuum pumps will be installed behind the divertors.…”