Requirements in the edge for H-mode transitions

In JFT-2M, conditions for H-mode transitions in the periphery of the plasma were studied. It was found that edge electron temperature does not always determines the H-mode transition. In usual sawtooth triggered H-mode cases, the minimum edge electron temperature needed for H-mode transition decreased with the increase of the electron density. And this edge electron temperature and the density seems to control the transition phenomena in this case. However, there are cases when these parameters are not determining the transition. It was found that the threshold beam heating power for H-mode was found to decrease by lowering the beam acceleration voltage. In this case, the transition took place in lower edge temperature and lower electron density compared with higher beam voltage case. Power deposition calculation shows that the lower energy beam did not preferentially heat edge electrons, instead, it delivered more power to ions in the periphery. (author)