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Runaway electrons can cause severe damage to plasma facing components of large tokamaks. The designs proposed for the first wall and divertor of the next large fusion experiment, ITER (International Thermonuclear Experimental Reactor), are investigated. Energies of up to 300 MeV per electron and surface energy depositions of 30 MJ/m2 are assumed. The GEANT code originating from high energy physics is used to model the energy deposition quantitatively as a function of the penetration depth and material. The magnetic field causes bending back of reflected charged particles and reduced penetration depth of the electrons, due to the gyration of the electrons around the magnetic field lines. The energy deposition in the bulk material for a given surface energy load is roughly independent of the incident angle and energy (above 100 MeV) since the main physical process of the energy loss is the formation of an electromagnetic shower. Typical divertor designs protect the cooling tubes with a 1 cm thick graphite layer. Melting of such molybdenum (copper) cooling tubes occurs at a heat load of 50 (25) MJ/m2. Since it is proposed to operate ITER with low cooling water temperatures (below 150 C), water pressurisation due to runaway electron impact is not a serious problem if the cooling pipes do not melt. If the first material facing the plasma is metallic, melting must be expected for heat loads of around 20 MJ/m2.

Additional information

Authors: BARTELS H-W, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Report: IPP 1/269 EN (1992) 27 pp.
Availability: Available from Max-Planck-Institut für Plasmaphysik, 8046 Garching bei München (DE)
Record Number: 199211255 / Last updated on: 1994-11-29
Original language: en
Available languages: en
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