Impurity emission characteristics of long pulse discharges in Large Helical Device

Line spectra from intrinsic impurity ions have been monitored during the three kinds of long-pulse discharges (ICH, ECH, NBI). Constant emission from the iron impurity shows no preferential accumulation of iron ion during the long-pulse operations. Stable Doppler ion temperature has been also measured from Fe XX, C V and C III spectra

Recent Results from LHD Experiment with Emphasis on Relation to Theory from Experimentalist’s View

he Large Helical Device (LHD) has been extending an operational regime of net-current free plasmas towardsthe fusion relevant condition with taking advantage of a net current-free heliotron concept and employing a superconducting coil system. Heating capability has exceeded 10 MW and the central ion and electron temperatureshave reached 7 and 10 keV, respectively. The maximum value of β and pulse length have been extended to 3.2% and 150 s, respectively. Many encouraging physical findings have been obtained. Topics from recent experiments, which should be emphasized from the aspect of theoretical approaches, are reviewed. Those are (1) Prominent features in the inward shifted configuration, i.e., mitigation of an ideal interchange mode in the configuration with magnetic hill, and confinement improvement due to suppression of both anomalous and neoclassical transport, (2) Demonstration ofbifurcation of radial electric field and associated formation of an internal transport barrier, and (3) Dynamics of magnetic islands and clarification of the role of separatrix

Design for X-Ray Imaging Spectroscopy in Large Helical Device

An assembly of pin-hole camera has been designed for x-ray imaging spectroscopy in Large Helical Device (LHD). In the design the camera basically consists of a 2-D-pixel-array detector, a metallic filter, and a pin hole. The detector measures gray scale images through the filter, while the thickness of the filter changes shot by shot. The gray scale images are mathematically transformed to a color spectral image by means of Mellin transform. The available spectral range is depending on the material of the filter. The spectral energy resolution is also depending on the material of the filter and the intensity of incident x-ray. In the case of the filter made of aluminum, the spectral range is theoretically predicted to be from 3.0 keV to 30 keV. It is also theoretically expected in the case of typical LHD plasmas that 50 flames of color spectral 32 × 32-pixel-image is obtained per second with a spectral resolution of E/ΔE = 4.3 at a photon energy of E = 5.0 keV

Impurity Transport Study with TESPEL Injection and Simulation

Impurity behaviors in LHD are studied by a Tracer-encapsulated Solid Pellet (TESPEL) injection. By containing multiple tracers in a TESPEL, the different tracer species have been compared simultaneously under the same plasma condition. The density disturbance on the bulk plasma by TESPEL is typically less than 10 %. The amount of the tracer particles deposited locally inside a plasma is about a few 1017 particles which is smaller than that of the bulk plasma by a factor of three orders of magnitude. Triple tracers, V, Mn and Co are used, because the charges of nuclei of intrinsic impurities, Cr and Fe are in between those of the tracers. The impurity confinement behavior depends substantially on the electron density. In case of the density higher than ne = 5 × 1019 m?3, the tracer impurity in the plasma core was kept for a long time, while it decays in order of 500 ms in the medium density case. Such temporal behavior is compared with a STRAHL simulation code assuming diffusion coefficient and convection. The general behavior fits well with the emissivity value integrated along the sight line