Correlation of the tokamak H-mode density limit with ballooning stability at the separatrix

摘要

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_(sep) increases about linearly with the separatrix density normalized to Greenwald density, n_(e,sep)_(GW) for a wide range of discharge parameters in both devices. The observed operational space is found to reach at maximum n_(e,sep)_(GW) ≈ 0.4-0.5 at values for α_(sep) ≈ 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.