Multi-Machine Comparisons of Divertor Heat Flux Mitigation by Radiative Cooling with Nitrogen

The exhaust characteristics and performance of nitrogen seeded H-mode discharges in Alcator C-Mod, ASDEX Upgrade and JET-C (carbon plasma facing components) have been analysed by regression analysis of plasma parameters relevant for power exhaust and simple analytic models. The highest divertor radiation levels observed lie around or below the maximum level expected from simple analytic considerations, which suggest a P/R metric or size scaling. Regression of the divertor radiation versus Zeff, the divertor neutral pressure, the expected power width and the major radius suggest exponents R1.5 for the full dataset including unseeded, low intrinsic divertor radiation levels and R1.1 if only nitrogen seeded discharges are taken into account. Extrapolated ITER divertor radiation levels for dZeff = 1 by nitrogen are 75 and 25MW if a small power width of 1.6mm mapped to the outer midplane is assumed at the entrance of the radiating zone. The ts suggest that a higher neutral pressure in the JET divertor would be benecial for a higher radiation level. Normalised connement improves with bN, suggesting improvements for higher heating powers for C-Mod and JET.
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