Studies of HRS H-mode plasma in the JFT-2M tokamak

Full text: An attractive operational regime without large ELMs, namely 'High Recycling Steady' (HRS) H-mode, was found on JFT-2M during Y2000 campaign after boronization. We have newly extended the HRS operational regime toward higher energy confinement utilizing the capability of both high triangularity (δ > 0.7) and elongation (κ > 1.6) operation under the double-null (DN) configuration. The energy confinement time enhancement factor (H89P) in the HRS plasma under the DN configuration increased up to ∼2 at around ne/nGW∼4, which was close to that of the standard ELMy H-mode regime having large ELMs under the single-null (SN) configuration (δ ∼ 0.4/κ ∼ 1.4). A series of parameter scan was also performed in the q95-DELTA space under the limiter (LIM) configuration. The HRS H-mode was easily reproduced in the D-shaped plasmas, while the large ELMs appeared in the circular cross-sectional shape, indicating an important role of plasma shaping to access the HRS regime. A key feature of the HRS H-mode is a reduction of the transient heat load to the divertor target due to large ELMs. It was estimated to be about 0.3MW/m2 in the typical HRS H-mode plasmas, which was significantly small in comparison with the transient heat load of ELMy H-mode discharge. The edge MHD activities in the frequency range of the order of 10-100 kHz were recognized to be important for an enhanced particle transport and steady H-mode edge condition in the absence of large ELMs. (author)