Two-Dimensional Measurement of Electron Density Profile in the Edge Region of the Large Helical Device Plasma by a Sheet-Shaped Thermal Lithium Beam

Abstract

In the Large Helical Device (LHD), of which the magnetic surfaces are surrounded by an ergodic magnetic field layer, the two-dimensional profile of electron density was for the first time measured by a newly developed sheet-shaped thermal lithium beam probe. The sheet beam was formed by guiding lithium vapor through three orifices placed in front of an aperture of an oven and injecting the vapor nearly vertically into the LHD plasma. The two-dimensional (2D) image of the LiI line was measured by using a CCD camera with an interference filter for LiI (670.8 nm). From the 2D image of LiI, we successfully obtained a two-dimensional profile of electron density in the plasma edge region. Detailed measurement and control of the edge plasma region are very important for tokamaks and helical devices to achieve a high confinement mode such as H-mode where an edge transport barrier (ETB) is formed and is partially destroyed by so-called edge-localized modes (ELMs). Recently, ETB formation by LH transition was observed in LHD [1,2]. The magnetic configuration of LHD is threedimensional and has a helical divertor, where the nested magnetic surfaces are surrounded by a complicated ergodic magnetic field layer near the plasma edge [3]. The complicated edge structure is measured by using an accelerated lithium beam, and is partly clarified [4]. Twodimensional measurement of the edge plasma structure is necessary and effective for understanding edge plasma behavior in this magnetic configuration. For the purpose of two-dimensional measurement, we have developed a new type of lithium beam probing (LiBP) system using a sheet-shaped thermal beam and applied the system to lowdensity plasmas in LHD.In this newly developed lithium beam probe, pure lithium ingot in an oven is heated to about 800 K, and generated lithium vapor is conducted to an LHD plasma through three horizontally-elongated orifices. A sheetshaped thermal beam was successfully produced by this simple technique. The beam expansions in the direction of the major radius and toroidal direction are respectively about 680 mm and 50 mm on the mid plane of LHD, as shown in Fig. 1 (a)

Properties of Intermittent Bursts in Edge Plasma of LHD

“…To observe such phenomena effectively, we have developed a two-dimensional(2D) thermal lithium beam probe (2D-LiBP) [11][12][13][14] to two-dimensionally measure the edge density and its fluctuations. In this paper, blob characteristics observed in the Large Helical Device (LHD) [15] with the 2D-LiBP are presented.…”