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Abstract

One of the most severe problems for fusion reactors is the power load on the divertor target plates. Technically only power loads of less than 10 MW m(-2) are acceptable. However, strong edge localized mode (ELM) activity can lead to power loads in excess of 800 MW m-2. In order to reduce the steady- state heat flux and the transient heat flux due to ELMS, radiation cooling experiments were performed at JET Nitrogen was puffed into the divertor up to a radiative power fraction of 90%. This was achieved at a density of 0.85 times the Greenwald density (Greenwald M 1988 Nucl. Fusion 28 2199), while maintaining an H-factor of f(H98) = 0.82. The Z(eff) in all those discharges stayed around 2.0. At approximately 55% radiative power fraction, the ELM characteristic changes from type I to type III, resulting in a loss of confinement of about 25% due to a degradation of the edge pedestal and hence a reduction of the ELM power load to the divertor tiles. By increasing the radiative power fraction to values of about 90%, the heat flux is reduced to 2 MW m(-2).