Fusion Science and Technology / Volume 46 / Number 2 / September 2004 / Pages 288-298
Technical Papers / Stellarators
The H-mode transition properties of 70-GHz, 0.4-MW electron cyclotron heating (ECH) plasmas in Heliotron J have been studied with special reference to their magnetic configuration dependences, such as the edge iota dependences. Two edge iota windows for the H-mode transition were observed to be (a) 0.54 < (a)/2 < 0.56 in separatrix discharge plasmas and (b) 0.62 < (a)/2 < 0.63 in partial wall-limiter discharge plasmas if a certain threshold line-averaged electron density ([overbar]ne = 1.2-1.6 × 1019 m-3) is achieved, where (a) is the vacuum edge iota value and a is the plasma minor radius, respectively. A strong dependence of the quality of the H-mode on the edge topology conditions was revealed. The energy confinement time for the separatrix discharge plasmas was found to be enhanced beyond the normal ISS95 scaling in the transient H-mode phase, being 50% longer than that in the "before transition" phase. The window characteristics are discussed on the basis of the calculated geometrical poloidal viscous damping rate coefficient in a collisional plasma, indicating that the behavior of the viscous damping rate coefficient alone could not explain the observed characteristics. The bootstrap current properties of ECH plasmas and the relevant electron cyclotron current drive experimental results are also discussed.