Adamek, J.

Artola, F.

Barton, P.

Bousek, M.

Dejarnac, R.

Havlicek, J.

Havranek, A.

Jerab, M.

Lehnen, M.

Matveeva, E.

Pitts, R.

Roccella, R.

Sestak, D.

Weinzettl, V.

English

MPG.PuRe

Current flows towards the divertor during VDEs at COMPASSE. Matveeva1,2, F.J. Artola3,4, J. Havlicek1, M. Lehnen3, R. Pitts3, R. Roccella3, R. Dejarnac1,M. Jerab1, D. Sestak1, P. Barton1, J. Adamek1, M. Bousek1, A. Havranek1, V. Weinzettl11 Institute of Plasma Physics of the CAS, Prague, Czech Republic2 Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic3 ITER Organization, Cedex, France4 Max-Planck Institute for Plasma Physics, Garching b. M., GermanyIntroduction. Consistent theoretical understanding of current flows to the vacuum vessel(VV) and their paths in the wall during disruptions is essential for electromagnetic loads es-timation when designing future machines such as ITER [1]. A number of models including"source and sink" [2], Hiro currents [3] and Asymmetric Toroidal Eddy currents (ATEC) [4] fo-cus on plasma-vessel electromagnetic interaction. The aim of this work is to test ATEC modelthat predicts that toroidal currents between plasma facing components (short-circuited throughthe plasma) can significantly modify the current flow path between the plasma and the vacuumvessel and thus the resulting forces by VDEs.Special divertor tiles. Two special divertor tiles (Fig. 1) were installed at the COMPASStokamak [5] in order to investigate, whether part of the toroidal addy current can flow to thedivertor tiles and through the gaps between them forming a parallel current circuit. The tilesare toroidally separated by 135◦. The tiles have identical design consisting of 8 segments (theirlabels are shown in Fig. 1 (a, b)). The segments are insulated from the VV by Boron-Nitridebackplate. Three pairs of segments have 2.5 mm gap between them. These toroidally split seg-ments measure current flows from the plasma to the VV by Rogowski coils located outside ofthe VV. The signal difference between left and right segments of each pair allows interpretationof halo and eddy currents paths.Current flows towards divertor. Current flows were measured during a dedicated campaignhaving 37 downward disruptions (in the direction of the divertor). The discharge parameterswere kept unchanged (Ip = 300kA) with the exception of Ip direction which was reversed forsome discharges. In addition to this, the tiles are left in the VV vessel until the end of COM-PASS operation and regularly perform measurement during unintentional disruptions. The tiles’segments have 4 possible connection configurations (Fig. 2): grounded (cable leading from thesegment is grounded to the VV on the feedthrough outside of the VV), floating (left and rightsegments are connected to each other with no contact to the VV), disconnected (the segment hasno connection to the VV nor to the neighboring segments) and cross-floating (LFS left segment47th EPS Conference on Plasma Physics P4.1034Figure 1: (a) Tile #1. Measuring pairs of segments are labeled as High Field Side (HFS), Middle andLow Field Side (LFS) according to their position in the VV. "Left" and "Right" notation is used to distin-guish the segments in each pair; (b) Tile #2; (c) Tiles’ toroidal position and accompanying diagnostics:Ip measurement by full Rogowski coil, 3 rings of Mirnov coils (MC_A, MC_B and MC_C) and InternalPartial Rogowski coils (IPR); Plasma-wall contact point position by divertor Langmuir probes (LP andOld LP).is connected to the Middle right segment and LFS right segment is connected to Middle leftone).Typical signal in grounded configuration is shown in Fig. 3. HFS segments have positivesignals, while Middle and LFS are negative due to the halo current direction. Halo current flowsalong open magnetic field lines and then closes its path through the VV. For standard toroidalfield (Bt) direction it flows inside the VV on the HFS and flows out of the VV on the LFS.Significant asymmetry is observed between left and right neighboring segments. It is suspectedthat this might be due to eddy currents partially flowing to the divertor tiles and short-circuitingthe gaps between them as schematically shown in Fig. 2(a). In this case left segment wouldmeasure IL = Ihalo+ Ieddy while the right segment IR = Ihalo− Ieddy. Therefore, the asymmetry isexpected to be dependent on eddy current direction which can be changed by Ip reversal. It wasobserved that IR becomes larger than IL at reversed Ip, which is consistent with ATEC modelpredictions of gaps short-circuiting.Current flows in floating configuration are shown in Fig. 4. According to ATEC model thecurrent should flow in co-Ip direction when the segments are connected to each other. LFSand HFS segments signals confirm this hypothesis, also changing the current direction after Ipreversal. Middle segments are not discussed here, as they are located in the vicinity of plasma-wall contact point where magnetic field lines have very shallow incident angle. Therefore, smallmisalignment or shadowing of the segments will result in significantly asymmetrical wetted area47th EPS Conference on Plasma Physics P4.1034and halo currents. HFS and LFS segments are not prone to this effect.Cross-floating configuration aims to distinguish contributions from halo and eddy currents.As can be seen from Fig. 2(b), the current should flow from the Middle to the LFS segmentsif halo contribution is dominant. In case of dominant eddy currents, current flow from right toleft segment is expected at standard Ip. Experimental results shown in Fig. 5 indicate that thesegments measure primarily halo currents. However, the current magnitude from the Middle leftto the LFS right segment is smaller than the one from the Middle right to the LFS left segment.This might be due to eddy current contribution as expected from ATEC model. The asymmetryreverses with Ip reversal for Tile #1, but the dependence is not clear for Tile #2. It should betaken into account that the Middle segments might have non-uniform halo current and affect theasymmetry.Additional measurement were performed during dedicated VDE experiments. Divertor Lang-muir probes [6] allow estimation of plasma-wall contact point position. The tile were observedby two fast visible cameras. Both diagnostics show significant 3D asymmetry of the plasma thatmight affect the ration of the asymmetry between left and right segments.Figure 2: (a) Tiles segments’ connection config-urations: grounded and floating (green and bluedashed lines). Eddy current flow according toATEC and halo current (black and red arrows)are shown at LFS segments for standard Ip and Btdirection; (b) cross-floating segments connectionand related halo and eddy currents flows (red andblack arrows) shown for standard and reversed Ipdirection.Figure 3: Current flows to the divertor tiles ingrounded configuration during disruption. Fromtop to bottom: HFS, Middle and LFS segments oftile #1 (left column) and tile #1 (right column).Left and right segments are indicated by blue andred curves.Conclusions. Current flows towards the special divertor tiles were measured during VDEs.The current flow reaches 2 kA in discharges with Ip = 300kA , but may vary from discharge47th EPS Conference on Plasma Physics P4.1034Figure 4: Current flows to the left segment infloating configuration for tile #1 (left column) andtile #2 (right column). Signals at standard and re-versed direction of Ip are represented by magentaand black curves. Negative signal correspond tothe current flowing towards the segment.Figure 5: Current flows to the left segment incross-floating configuration for tile #1 (left col-umn) and tile #2 (right column). Signals at stan-dard and reversed direction of Ip are representedby magenta and black curves. Negative signalcorrespond to the current flowing towards thesegment.to discharge with similar plasma parameters. Both tiles exhibit a certain pattern of asymmetrybetween neighboring left and right segments. The asymmetry dependence on Ip direction isconsistent with ATEC model prediction in most cases in grounded, floating and cross-floatingconfigurations. However, it is not clear yet why the two identical tiles consistently show anopposite behavior in terms of left and right segments ratio. In addition to this, some outliersnon-consistent with ATEC model are present. A possible explanation could be a strong 3Dasymmetry of the plasma during disruptions, that is detected by additional measurements in-cluding contact point position and fast camera videos.Acknowledgment. This work has been done under service contracts IO/20/CT/4300002104(13.05.2020 – 13.05.2021). This work has been carried out within the framework of the EU-ROfusion Consortium and has received funding from the Euratom research and training pro-gramme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinionsexpressed herein do not necessarily reflect those of the European Commission. Co-funded byMEYS project number 8D15001. Co-funded by MEYS project LM2018117.References[1] C. Bachmann et al., Fusion Engineering and Design, 86 (2011)[2] V. Riccardo et al., Nucl. Fusion, 40, (2000)[3] L.E. Zakharov et al., Physics of Plasmas, 19, (2012)[4] R. Roccella et al., Nucl. Fusion, 56, (2016)[5] M. Hron et al., IAEA FEC, (2020)[6] J. Adamek et al., Nucl. Fusion, 57, (2017) 11601747th EPS Conference on Plasma Physics P4.1034

Current flows towards the divertor during VDEs at COMPASS

http://hdl.handle.net/21.11116/0000-0009-AA94-0

Current flows towards the divertor during VDEs at COMPASS

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