Bootstrap current analysis for neoclassical internal transport barrier discharge of CHS

Abstract

The bootstrap current analyses have been carried out for two different kinds of discharges of compact helical system (CHS) heliotron/torsatron. One is the electron cyclotron resonance heating (ECRH) discharge with moderate density and without internal transport barrier (ITB) (ne(0)>0.5×1019 m-3, Te(0)<1.2 keV). Another is the neoclassical ITB (NC ITB) discharge, which has been achieved in the rather low electron density condition (ne<0.4×1019 m-3) with ECRH and co-NBI. The NC ITB mode is characterized by steeply peaked electron temperature profile (Te(0)>2.0 keV) in the core domain and large positive radial electric field (Er~ + 10 kV m-1). The dependence of toroidal net current on magnetic axis position Rax was investigated for ECRH discharges without ITB. There is good agreement in the tendency of Rax dependence of the net toroidal current between experiment and neoclassical calculation. In the NC ITB discharges, the experimental net current including both bootstrap and beam-driven current is largely reduced compared with that in the L-mode and it drops to almost zero. It seems as if the reversal of bootstrap current takes place in the NC ITB phase but such a view is inconsistent with neoclassical prediction. The observed reduction of the plasma current in the ITB phase is not explained by the neoclassical theory.