Power deposition on misaligned edges in COMPASS

•Power deposition on small leading edges is well described by the optical approximation.•Mini-SOL in front of the leading edge does not affect the deposited power.•The ion Larmor smoothing effect under the ambipolar assumption at the leading edge is weak.•The deposited power seems to be electron-dom...

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Bibliographic Details
Published in:Nuclear materials and energy 2017-08, Vol.12, p.1374-1378
Main Authors: Dejarnac, R., Corre, Y., Vondracek, P., Gardarein, J-L., Gaspar, J., Gauthier, E., Gunn, J.P., Horacek, J., Hron, M., Komm, M., Panek, R., Pitts, R.A., the COMPASS team
Format: Article
Language:English
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Summary:•Power deposition on small leading edges is well described by the optical approximation.•Mini-SOL in front of the leading edge does not affect the deposited power.•The ion Larmor smoothing effect under the ambipolar assumption at the leading edge is weak.•The deposited power seems to be electron-dominated. If the decision is made not to apply a toroidal chamfer to tungsten monoblocks at ITER divertor vertical targets, exposed leading edges will arise as a result of assembly tolerances between adjacent plasma-facing components. Then, the advantage of glancing magnetic field angles for spreading plasma heat flux on top surfaces is lost at the misaligned edges with an interaction occurring at near normal incidence, which can drive melting for the expected inter-ELM heat fluxes. A dedicated experiment has been performed on the COMPASS tokamak to thoroughly study power deposition on misaligned edges using inner-wall limited discharges on a special graphite tile presenting gaps and leading edges directly viewed by a high resolution infra-red camera. The parallel power flux deducted from the unperturbed measurement far from the gap is fully consistent with the observed temperature increase at the leading edge, respecting the power balance. All the power flowing into the gap is deposited at the leading edge and no mitigation factor is required to explain the thermal response. Particle-in-cell simulations show that the ion Larmor smoothing effect is weak and that the power deposition on misaligned edges is well described by the optical approximation because of an electron dominated regime associated with non-ambipolar parallel current flow.
ISSN:2352-1791
2352-1791
DOI:10.1016/j.nme.2016.09.009