ELM mitigation by nitrogen seeding in the JET gas box divertor

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.