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@ARTICLE{Sereda:891391,
      author       = {Sereda, S. and Brezinsek, S. and Wang, E. and Barbui, T.
                      and Brakel, R. and Buttenschön, B. and Goriaev, A. and
                      Hergenhahn, U. and Höfel, U. and Jakubowski, M. and Knieps,
                      A. and König, R. and Krychowiak, M. and Kwak, S. and Liang,
                      Yunfeng and Naujoks, D. and Pavone, A. and Rasinski, M. and
                      Rudischhauser, L. and Ślęczka, M. and Svensson, J. and
                      Viebke, H. and Wauters, T. and Wei, Y. and Winters, V. and
                      Zhang, D.},
      title        = {{I}mpact of boronizations on impurity sources and
                      performance in {W}endelstein 7-{X}},
      journal      = {Nuclear fusion},
      volume       = {60},
      number       = {8},
      issn         = {1741-4326},
      address      = {Vienna},
      publisher    = {IAEA},
      reportid     = {FZJ-2021-01480},
      pages        = {086007 -},
      year         = {2020},
      abstract     = {The low-Z oxygen and carbon were the main plasma impurities
                      in the Wendelstein 7-X (W7-X) stellarator in the last
                      experimental campaign with the passively cooled graphite
                      divertor. To tackle this issue boronization (Winter et al
                      1989 J. Nucl. Mater. 162–4 712–23) was applied, which
                      has led to one of the main achievements of the campaign:
                      plasma operation at high core densities of more than 1020
                      m−3 in hydrogen fueled plasmas due to the reduced
                      radiation-induced density limit.In total three boronizations
                      were applied. After the first boronization the oxygen to
                      hydrogen flux ratio (normalized influx of oxygen) at the
                      divertor substantially decreased by a factor of 10 and the
                      carbon to hydrogen flux ratio (normalized influx of carbon)
                      decreased by a factor of 4 as obtained from spectroscopy. In
                      the same time, boron emission appeared in the spectra.
                      Between the boronizations oxygen and carbon normalized
                      influxes increased but never reached the pre-boronization
                      values. With each subsequent boronization O level decreased
                      even more, reaching the lowest values after the third
                      boronization which were more than a factor of 100 lower than
                      before the first boronization.Such a decrease in low-Z
                      impurity concentration significantly extended the operation
                      window of W7-X in terms of line-integrated electron density
                      (from 4ċ1019 m−2 to more than 1ċ1020 m−2) and
                      diamagnetic energy (from 330 kJ up to 510 kJ). Zeff
                      decreased from 4.5 down to values close to 1.2 as obtained
                      from bremsstrahlung measurements. The above mentioned values
                      are given for the two reference discharges before and after
                      boronization.},
      cin          = {IEK-4},
      ddc          = {620},
      cid          = {I:(DE-Juel1)IEK-4-20101013},
      pnm          = {174 - Plasma-Wall-Interaction (POF3-174)},
      pid          = {G:(DE-HGF)POF3-174},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000552681900001},
      doi          = {10.1088/1741-4326/ab937b},
      url          = {https://juser.fz-juelich.de/record/891391},
}