Confinement characteristics of ECH plasmas in Heliotron J

摘要

Studies of global energy confinement and toroidal plasma current behaviour for the second harmonic 70 GHz ECH at B = 1-1.5 T are described with emphasis on the magnetic configuration effects in the helical-axis heliotron 'Heliotron J'. At low densitites of (n-bar)_e < 0.4 * 10~(19) m~(-3), the electron temperature reached T_e ≈ 1 keV in the core region, indicating the production of collision-less plasmas of electron collisionality v~* 0.1, where v~* = v/(ν_e/π R_(0q)). For medium densities of 0.5 * 10~(19) m~(-3) < (n-bar)_e < 2 * 10~(19) m~(-3), the preferable energy confinement time, 1.5-2 times larger than that of the ISS95 scaling, was obtained under the condition of localizaed central heating at B ≈ 1.25 T for the standard configuration of Heliotron J. The measurements of the toroidal current under perpendicular microwave injection revealed the change of the current flow direction as a function of the poloidal magnetic field. The measured current behaviour was found to be qualitatively consistent with that of the bootstrap current predicted from neoclassical theory. The observed flow reversal showed that a proper selection of the field configuration could control the bootstrap current in the helical-axis heliotron. In addition, the current control through the electron cyclotron current drieve scenario with oblique injection of microwaves was experimentally examined.