Correlation of the tokamak H-mode density limit with ballooning stability at the separatrix. (15th January 2018)

• Record Type: Journal Article
• Authors: Eich, T.; Goldston, R.J.; Kallenbach, A.; Sieglin, B.; Sun, H.J.
• Other Names: collab.; collab.
• Is Part Of: Nuclear fusion. Volume 58:Number 3(2018:Mar.)
• Publication Date: 2018-01-15
• Subjects: H-mode density limit -- power width scaling -- ballooning instability -- SOL decay lengths; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/aaa340
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: We show for JET and ASDEX Upgrade, based on Thomson-scattering measurements, a clear correlation of the density limit of the tokamak H-mode high-confinement regime with the approach to the ideal ballooning instability threshold at the periphery of the plasma. It is shown that the MHD ballooning parameter at the separatrix position α s e p increases about linearly with the separatrix density normalized to Greenwald density, n e, s e p / n G W for a wide range of discharge parameters in both devices. The observed operational space is found to reach at maximum n e, s e p / n G W ≈ 0.4 –0.5 at values for α s e p ≈ 2 –2.5, in the range of theoretical predictions for ballooning instability. This work supports the hypothesis that the H-mode density limit may be set by ballooning stability at the separatrix.