Observation of High Recycling Steady H-mode edge and compatibility with improved core confinement mode on JFT-2M

A new operational regime has been discovered on JFT-2M under the boronized first wall condition to produce the High Recycling Steady (HRS) H-mode, which is characterized by good energy confinement (H89P ∼ 1.6) at a high density of around 70% of the Greenwald density (ne/nGW), low radiated power fraction, and complete disappearance of large ELMs. Accompanying the HRS H-mode transition, coherent magnetic fluctuation is seen on magnetic probes at the vessel wall. These coherent fluctuations have a frequency of the order of 10–100 kHz with significant variation. They are recognized as important in enhancing particle transport, whose characteristics are similar to the Enhanced Dα (EDA) H-mode regime reported from Alcator C-Mod. The H'-mode, previously observed on JFT-2M, had features in common with the EDA mode in terms of the coherent density fluctuation, but it appeared transiently. The HRS operating regime is also similar to the EDA mode, except that HRS is seen even at low q95 (2 < q95 ⩽ 3). The most important feature of the HRS H-mode edge condition is compatibility with an improved core confinement mode at a high density without large ELMs. We have demonstrated that an internal transport barrier can be produced under a HRS H-mode edge condition, achieving βNH89P ∼ 6.2 at ne/nGW ∼ 70%, transiently.