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@PHDTHESIS{Ritz:14086,
      author       = {Ritz, Guillaume Henri},
      title        = {{P}erformance of {T}ungsten-{B}ased {M}aterials and
                      {C}omponents {U}nder {ITER} and {DEMO} {R}elevant
                      {S}teady-{S}tate {T}hermal {L}oads},
      volume       = {128},
      issn         = {1866-1793},
      school       = {RWTH Aachen},
      type         = {Dr. (Univ.)},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {PreJuSER-14086},
      isbn         = {978-3-89336-755-9},
      series       = {Schriften des Forschungszentrums Jülich : Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {X, 128 S.},
      year         = {2010},
      note         = {Record converted from JUWEL: 18.07.2013; RWTH Aachen,
                      Diss., 2010},
      abstract     = {In nuclear fusion devices the surfaces directly facing the
                      plasma are irradiated with high energy fluxes. The most
                      intense loads are deposited on the divertor located at the
                      bottom of the plasma chamber, which has to withstand
                      continuous heat loads with a power density of several
                      MW$\cdot$m$^{−2}$ as well as transient events. These are
                      much shorter (in the millisecond and sub-millisecond regime)
                      but deposit a higher power densities of a few
                      GW$\cdot$m$^{−2}$. The search for materials that can
                      survive to those severe loading conditions led to the choice
                      of tungsten which possesses advantageous attributes such as
                      a high melting point, high thermal conductivity, low thermal
                      expansion and an acceptable activation rate. These
                      properties made it an attractive and promising candidate as
                      armor material for divertors of future fusion devices such
                      as ITER and DEMO. For the DEMO divertor, conceptual studies
                      on helium-cooled tungsten plasma-facing components were
                      performed. The concept was realized and tested under DEMO
                      specific cyclic thermal loads. The examination of the
                      plasma-facing components by microstructural analyses before
                      and after thermal loading enabled to determine the
                      mechanisms for components failure. Among others, it clearly
                      showed the impact of the tungsten grade and the thermal
                      stress induced crack formation on the performance of the
                      armor material and in general of the plasma-facing component
                      under high heat loads. A tungsten qualification program was
                      launched to study the behaviour of various tungsten grades,
                      in particular the crack formation, under fusion relevant
                      steady-state thermal loads. In total, seven commercially
                      available materials from two industrial suppliers were
                      investigated. As the material’s thermal response is
                      strongly related to its microstructure, this program
                      comprised different material geometries and manufacturing
                      technologies. It also included the utilization of an
                      actively cooled specimen holder which has been designed to
                      perform sophisticated material tests at different surface
                      temperatures. The steady-state thermal loading with
                      superimposed transient thermal loading was induced by high
                      frequency scanning of the electron beam. The steady-state
                      thermal loading was performed with different power
                      densities, surface temperatures and cycle numbers. The
                      cracking threshold was investigated in a temperature range
                      of 1000 to 1900$^{\circ}$C. Once cracks occurred, the
                      surface temperature had no impact on the crack network of
                      the loaded surface. The cracks grew in depth with increasing
                      the cycle number. However, under all loading conditions,
                      crack depths were still limited in a shallow region, namely
                      below 100 $\mu$m. One disadvantage of tungsten is its high
                      brittleness at room temperature which makes the
                      manufacturing of tungsten parts challenging as it requires
                      suitable machining techniques. The examination of the
                      helium-cooled tungsten plasma-facing components revealed
                      cracks in as-machined surfaces. For a better understanding
                      of the [...]},
      cin          = {IEK-2},
      ddc          = {500},
      cid          = {I:(DE-Juel1)IEK-2-20101013},
      pnm          = {Fusion},
      pid          = {G:(DE-Juel1)FUEK403},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/14086},
}