Nuclear Materials and Energy (Dec 2023)

Performance of tungsten plasma facing components in the stellarator experiment W7-X: Recent results from the first OP2 campaign

  • Dirk Naujoks,
  • Chandra-Prakash Dhard,
  • Yuhe Feng,
  • Yu Gao,
  • Torsten Stange,
  • Birger Buttenschön,
  • Sergey A. Bozhenkov,
  • Sebastijan Brezinsek,
  • Kai Jakob Brunner,
  • Gábor Cseh,
  • Andreas Dinklage,
  • David Ennis,
  • Joris Fellinger,
  • Eric Flom,
  • Dorothea Gradic,
  • Eduard Grigore,
  • Dirk Hartmann,
  • Frederik Henke,
  • Marcin Jakubowski,
  • Amit Kharwandikar,
  • Mikhail Khokhlov,
  • Jens Knauer,
  • Gábor Kocsis,
  • Petra Kornejew,
  • Maciej Krychowiak,
  • Matej Mayer,
  • Paul McNeely,
  • Daniel Medina,
  • Rudolf Neu,
  • Kian Rahbarnia,
  • Cristian Ruset,
  • Norbert Rust,
  • Peter Scholz,
  • Thomas Sieber,
  • Ivan Stepanov,
  • Naoki Tamura,
  • Erhui Wang,
  • Thomas Wegner,
  • Daihong Zhang

Journal volume & issue
Vol. 37
p. 101514

Abstract

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The transition to reactor-relevant materials for the plasma facing components (PFCs) is an important and necessary step to provide a proof of principle that the stellarator concept can meet the requirements of a future fusion reactor by demonstrating high performance steady-state operation. As a first step to gain experience with tungsten as plasma-facing material in the Wendelstein 7-X (W7-X) stellarator, graphite tiles coated with an approximately 10 µm MedC tungsten layer (NILPRP Bucharest) were installed to complete the ECRH beam dump area in two of the five plasma vessel modules over an area of approximately one square meter each. In addition, tungsten baffle tiles are installed (40 tiles in total) with either bulk tungsten as part of NBI shine-through target or with a tungsten heavy alloy (W95-Ni3.5-Cu1.5) to replace the graphite tiles that were previously thermally overloaded. Based on an advanced diffusive field line tracing method and EMC3-Eirene simulations, the overloaded baffle tiles were redesigned by making the tiles thinner (i.e. moving the plasma-facing surface (PFS) away from the hot plasma region) and by reducing the local angle of incidence through toroidal displacement of the watershed. Significant erosion of the tungsten tiles can only be expected if sputtering by impurity ions such as carbon or oxygen ions contributes. However, the resulting central concentration of tungsten and the corresponding radiation losses are expected to be marginal. The expected deposition of carbon on the tungsten surfaces in the baffle regions mitigates further the possible tungsten enrichment in the core plasma. In OP2.1, no adverse effects of the installed tungsten PFCs on the plasma performance were observed during normal plasma operation. With the design changes made in the baffle area, the predicted heat load reductions could be experimentally confirmed.

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