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A new operational regime has been observed in neutral-beam-heated JFT-2M limiter discharges. This regime is characterized by confinement times close to those of Ohmic and divertor //-mode (good confinement mode) discharges. The properties of these discharges are similar to the divertor H mode. The incremental energy-confinement time due to the heating is found to be independent of the plasma density up to the maximum density.

Significant improvement of energy confinement has been observed on JFT-2M during ICRF heating. This improvement is preceded by a sudden drop in the H a /D a emission and a successive increase in stored plasma energy, electron density and radiation loss. This is believed to be the same phenomenon as the H-mode transition observed in ASDEX, and in PDX divertor experiments with neutral beam injection. However, in JFT-2M, this transition is observed both in limiter discharges and in open divertor configurations.