A. Fujisawa

Viscous damping and effective helical ripple have been quantitatively estimated for a proposed quasi-axisymmetric stellarator CHS-qa configuration. It is predicted that viscous damping against plasma flow in CHS-qa is low enough to sustain large E × B shear as observed in the H-modes and internal transport barriers in many tokamaks. Effective helical ripple in CHS-qa is adjusted to be on the order of 10^-2 or lower, which is suitable to trigger transport barriers due to non-ambipolar neoclassical fluxes peculiar to conventional helical devices. For experimental flexibility, viscous damping and effective helical ripple in CHS-qa can be extensively controlled over a range of about one order by modifying modular coil currents.

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) (n_e(0)>0.5×10¹⁹ m^-3, T_e(0)n_e19 m^-3) with ECRH and co-NBI. The NC ITB mode is characterized by steeply peaked electron temperature profile (T_e(0)>2.0 keV) in the core domain and large positive radial electric field (E_r~ + 10 kV m^-1). The dependence of toroidal net current on magnetic axis position R_ax was investigated for ECRH discharges without ITB. There is good agreement in the tendency of R_ax 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.

The bifurcated nature of the potential profile of a toroidal helical plasma is investigated in CHS using a heavy ion beam probe. The measurements reveal that three main branches of potential profile exist in ECR heated plasmas with a low density of _e ≈ 0.5 × 10¹⁹m^-3. The branches with higher central potential exhibit a rather strong radial electric field shear that should result in fluctuation reduction and formation of a transport barrier. An analysis of the pulsating behaviour in the potential suggests the existence of a fine structure in the potential profiles between the main branches. Lissajous and histogram expressions are useful in extracting the bifurcation characteristics of the potential structure.