[en] An in situ density calibration for a Thomson scattering measurement by a microwave reflectometry based on Bayesian estimation is proposed. A simultaneous calibration with a plasma experiment is feasible, because it is not necessary for a vacuum vessel to be filled with N₂, Ar or H₂ gases. The Bayesian estimation provides an accurate density calibration factor from a few reflectometer measurements, which is confirmed by a Monte-Carlo simulation. (author)

[en] An edge transport barrier (ETB) formation and an improved L-mode (IL mode) have been simultaneously realized in high density region (n-bar_e - 1.2x10²⁰m^-3) on Compact Helical System (CHS). When the ETB is formed during the IL mode, the density reduction in the edge region is suppressed by the barrier formation. As a result of the continuous increasing of the temperature by the IL mode, the stored energy during the combined mode increased up to the maximum stored energy (W_p - 9.4 kJ) recorded in CHS experiments. The plasma pressure in the peripheral region increases up to three times larger than that of the L-mode, and the large edge plasma pressure gradient is formed accompanying the pedestal structure. That is caused by the anomalous transport reduction that is confirmed from the sharp drop of the density fluctuation in the edge region. The neutral particle reduction in the peripheral region and the metallic impurity accumulation in the core plasma are simultaneously observed during the high density ETB formation. (author)

No abstract available

[en] An improved confinement mode with reheat and edge transport barrier is observed in the Compact Helical System (CHS). This mode provides enhanced confinement in high density region (n-bar_e - 1.2 x 10²⁰ m^-3) due to the increase in temperature while high density is maintained in the peripheral region. (author)

[en] The high ion temperature mode for neutral beam heated plasma is observed in CHS heliotron/torsatron device. Transport analysis for this mode shows that increase in ion temperature is due to improvement of ion heat transport. On the other hand, electron diffusivity in the high ion temperature mode is larger than that in the L-mode. There is a correlation between thermal diffusivity and density gradient. (author)

[en] A clear transition of edge particle transport was observed for the neutral beam heated plasmas in CHS. The heating power threshold for the transition is about 1 MW for a plasma with 2x10¹⁹ m^-3 average density. The H_α emission drops within 1 msec and the increase of the local edge density at the transition was confirmed by means of YAG Thomson scattering and beam emission spectroscopy. When the heating power is well above the threshold, the transport barrier is maintained for the full duration of NBI heating (100 msec). A clear back transition appears when the heating power is decreased during the discharge. (author)

[en] High ion temperature mode (high T_i mode) is observed for neutral beam heated plasmas in Compact Helical System (CHS) Heliotron/torsatron plasmas. The high T_i mode plasma is characterized by a peaked ion temperature profile and is associated with a peaked electron density profile produced by neutral beam fueling. The reduction of ion thermal diffusivity is observed at the plasma core (ρ=0.1) in the high T_i mode by a factor of two. The high T_i mode discharge is observed only in the low density and the upper limit of n_e (critical n_e) for the high T_i mode depends on the density peaking factor. (author)

[en] Bifurcation properties in radial electric field have been studied using a heavy ion beam probe in a toroidal helical plasma of Compact Helical System. This paper presents an overview of the bifurcation phenomena observed in potential structure and its dynamics, such as potential patterns with transport barrier formation, and self-organized oscillation termed electric pulsation. Detailed analysis of this oscillatory state provides an interesting result of spatio-temporal evolutions of potential and density profile during a pulse-like event. (author)

[en] An improvement of plasma confinement by the addition of electron cyclotron heating (P_inj - 130 kW) to neutral beam injection sustained plasmas has been observed on the CHS heliotron/torsatron device. In the core region inside ρ=0.5, the temperature is increased for both electrons (2300 eV from 300 eV) and ion (400 eV from 200 eV). The change of the electric field structure by transition from ion to electron root is correlated to the improvement of the plasma performance. The condition for this phenomenon occurring depends sensitively on the plasma density. (author)

[en] In the local Electron Cyclotron Heating (ECH) experiments, high electron temperature plasmas have been obtained in Compact Helical System (CHS), and recently in the Large Helical Device (LHD). The internal transport barrier (ITB) with strong positive radial electric field has been experimentally observed in CHS, which reduces neo-classical ripple transport and anomalous transport losses. The same physics pictures are expected in LHD high temperature plasmas. Several ion temperature profiles are assumed for analyzing LHD experimental data, and it is found that the experimentally obtained electron thermal transport coefficients seem to roughly agree with neoclassical ripple transport outside the ITB region. However, around ITB region, about ten times higher than the neoclassical coefficients with strong ambipolar electric field prediction are obtained. The anomalous transport losses might be dominant and be reduced by this strong electric field shear around the ITB region. (author)