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967 J. Plasma Fusion Res. Vol.81, No.12 (2005) 967 968 In the Large Helical Device (LHD), increase in the plasma beta (i.e., the ratio of volume-averaged plasma pressure to the toroidal magnetic pressure) causes appreciable excitement in MHD modes of m/n = 1/1, 3/4, 2/3 and 1/2 (m and n are poloidal and toroidal mode numbers, respectively) near the plasma edge region of the magnetic hill [1]. These edge MHD modes sometimes interrupt the increase in the stored energy in high beta regime. In particular, the edge MHD modes are suddenly destabilized immediately after the L-H transition which leads to the formation of edge transport barrier having a steep pressure gradient [1,2]. To clarify the characteris tics of edge MHD modes and minimize the effects on plasma confi nement, measurements of the internal structure, growth rate and so on are crucially required. In the LHD, we have employed seven sets of a 20channel soft-X-ray (SX) detector array in order to measure the internal structure of the edge MHD modes. SX emission measurement is applicable for the study of MHD instabilities in high beta plasmas without any limitations in magnetic fi eld and density, although the path integral effect should be taken into account. An example of these SX detector array systems is shown in Fig. 1. This system is installed inside the vacuum vessel in the vertically elongated section of the LHD. The detector array is a silicon PIN photodiode array which consists of 20 ch active areas arranged in one dimension and was developed by Hamamatsu Photonics. A beryllium foil of 8 μm or 15 μm thickness is attached to the front of the system to shut out visible and vacuum ultra violet emissions. The viewing sight of the detector system is adjusted through a collimator slit, as shown in Fig. 1. A typical time evolution of high beta hydrogen plasma heated by NBI heating is shown in Fig. 2. The condition of discharge is as follows, where the magnetic axis position of the vacuum fi eld Rax = 3.6 m, the toroidal magnetic fi eld strength Bt = 0.5 T and an index of the aspect ratio of the last closed magnetic surface γ = 1.2. In this discharge, the vol umeaveraged beta value obtained from diamagnetic measurement reaches up to 〈βdia〉 ~ 2.5% by the ramping of line averaged electron density. In the latter half of the discharge where 〈βdia〉 exceeds about 2%, the Hα signal exhibits fl uctuations and Fig. 1 SX detector array system in the LHD. 40 lines of sight by two SX-ray detector arrays are drawn. Observation of Internal Structure of Edge MHD Modes in High Beta Plasmas on the Large Helical Device