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

An attractive operational regime without any large ELMs, 'High Recycling Steady' (HRS) H-mode, has been found on JFT-2M during Y2000 campaign after a boronization. Recent experiments have been concentrated on the studies of the access conditions for the HRS regime, in terms of the pedestal parameters. The HRS regime was more likely at higher edge density and lower edge temperature, which corresponds to the normalized electron collisionality of ve* ≥ 1 at the edge pedestal. The effects of the plasma shape were also investigated. Then, 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 ∼ 0.4, which was close to that of the standard ELMy H-mode regime having large ELMs under the single-null (SN) configuration (δ/κ ∼ 0.4/1.4). A series of parameter scan was also performed in the q95-δ 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. These observations indicate an important role of plasma shaping to access the HRS regime as well as edge cpllisionality. 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 ∼0.3 MW/m2 in the typical HRS 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)