Highlights 2011

Research news from the division Plasma Edge and Wall

New scaling for divertor peak power loads confirmed at JET and ASDEX Upgrade.

One of the major challenges on the way to a fusion reactor is the power exhaust in the divertor where the plasma interacts with the so called plasma facing components. These components are highly optimized to withstand  peak power loads in the range of 10 - 20 MW/m2. Therefore a reliable extrapolation of the power loads in present experiments to that in future devices is  crucial. A first-order approximation which uses the ratio of the power produced by the fusion reaction and the size of the power exhaust area was not sufficiently understood to be extrapolated to a large future device. In particular understanding was missing about the transport processes determining the radial width of this area.

Joint experiments by the scientific teams at the fusion devices ASDEX Upgrade in Garching and JET in Oxford have now improved the database which can be used to scale to larger devices. These experiments involved dedicated power exhaust studies and made use of state-of-the-art thermographic infra-red systems. The most important property of the resulting scaling is that the power exhaust area increases only linearly with the dimension of the device. Further dependencies were also found on the magnetic field itself and the plasma current, both important parameters for efficient magnetic confinement. These experimental results were compared in close collaboration with the Princeton Plasma Physics Laboratory to a transport model, providing a theoretical basis for the experimental result.

A fusion reactor will need to have a major radius of 6 meters or more to sustain the fusion process and will therefore be about 2 - 4 times larger than the two largest experiments in Europe, ASDEX Upgrade and JET. As a result, the peak power fluxes would exceed a value of 10 - 20 MW/m2 in large reactor-like devices. This finding motivates intensive research of techniques to reduce the peak power flux by additional means like radiative power exhaust or the application of resonant-magnetic perturbations to increase the plasma boundary region and so the power exhaust area. Both topics are priority areas of experimental research in ASDEX Upgrade.

The results have been published in T. Eich et al., Physical Review Letters 107 (2011), 215001.

High resolution divertor thermography measurements of the powerflux in the divertor together with their peak and integrated values.
The agreement of predicted to experimental power decay lengths is excellent.     


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