Search

Radial electric field (Er) properties in LHD have been investigated based on the neoclassical transport theory and have also been applied to LHD experimental results. The effects of the helicity of the magnetic configuration on the condition required to realize the electron root are examined. The larger helicity makes the threshold temperature lower for the same electron density. A higher threshold temperature is anticipated to be required in the plasma core region based on this fact and also due to the larger density there. This high electron temperature (Te) has been successfully obtained with a centerfocused ECH. There is a threshold for the ECH power to achieve a steep gradient of Te, and it seems to be qualitatively consistent with the transition of Er, at least in the sense that the abrupt increase of Te occurs after entering the anticipated electron root regime. These experimental results, consistent with those of analysis of the neoclassical ambipolar Er, indicate that the transition phenomena of Er in LHD are predominantly governed by neoclassical features.

Electron cyclotron emission (ECE) diagnostics have been developed in the Large Helical Device (LHD). The ECE is transmitted from the antenna to the spectrometer for 80 m by means of a corrugated waveguide system, which has a small transmission loss (˜30%) in LHD. Although the field angle changes from −40 to +30 degrees on the sight-line of the ECE antenna, it has been observed that the polarization of ECE, which is determined at the plasma edge in LHD, is identical for all frequencies. Thus, the electron temperature profile can be measured using the second harmonic X-mode of ECE. The ECE measured by the Michelson is calibrated with a hot source in order to reduce noise. The ECE temperature measurement agrees well with the Thomson scattering measurement over a wide range of electron densities. The radiometer is cross-calibrated to the Michelson, and is employed for the study of fast phenomena such as internal transport barrier (ITB) formation. Tangentially emitted ECE, in addition to the normal ECE, is investigated in LHD in order to explore the potential of ECE diagnostics.

A highly space-resolved Thomson scattering system installed on the large helical device revealed that the electron temperature profile of the plasma occasionally has bulged regions at the location where the iota is close to 1.

Electron heat transport in low-collisionality LHD plasma is investigated in order to study the neoclassical transport optimization effect on thermal plasma transport with an optimization level typical of so-called "advanced stellarators". In the central region, a higher electron temperature is obtained in the optimized configuration, and transport analysis suggests the considerable effect of neoclassical transport on the electron heat transport assuming the ion-root level of radial electric field. The obtained experimental results support a future reactor design in which the neoclassical and/or anomalous transports are reduced by magnetic field optimization in a non-axisymmetric configuration.