Okamura Shoichi

Improvement in Plasma Heating at Higher-Density Region Using 106.4 GHz Electron Cyclotron Heating System for Compact Helical System

A 106.4 GHz electron cyclotron heating (ECH) system for compact helical system (CHS) has been newly constructed. Initial experiments to confirm the systems performance showed good controllability of beam direction and polarization for optimized heating. In the beam direction and polarization scan experiments, on-axis beam injection with second-harmonic X-mode polarization resulted in the highest stored energy and central electron temperature. Additional heating becomes possible for the plasmas sustained by neutral beam injection with a density higher than the plasma cutoff density of 3.5×10 19 m -3 for the 53.2 GHz waves of the existing ECH system. Applying the 106.4 GHz heating waves extends the CHS plasma parameter to a high electron temperature of the keV order at a high density up to the second-harmonic X-mode cutoff density of 7×10 19 m -3 .

The drift reversal capability in quasi-axisymmetric stellarators

The drift reversal capability of trapped particles has been investigated in quasi-axisymmetric (QA) configurations for the confinement improvement as has been experimentally demonstrated in other axisymmetric configurations. The properties of J (second adiabatic invariance), its profile and maximum-J capability are examined for this purpose. Due to the non-axisymmetric nature of the magnetic field strength, a region of local maximum of J is created to cause the drift reversal. This region can be controlled radially and also toroidally with the introduction of symmetric breaking components of B, externally. The increase of magnetic shear in finite beta equilibria also has a favourable effect on the drift reversal. Thus, maximum-J capability and its external controllability are demonstrated for QA stellarators, by which the impact of magnetic configuration on turbulent transport can be studied.