A. "Sanpei

A. Higashi

D. Fukahori

H. Himura

H. Motoi

K. Oki

K.Y. Watanabe

M. Nakamura

S. Masamune

S." Ohdachi

T.F. Ming

Y. Suzuki

National Institute for Fusion Science (NIFS-Repository)

The measurement of bremsstrahlung soft X-ray(SXR) radiation is one of the useful passive methods fordiagnosing high-temperature plasmas, because contoursof the SXR emissivity correspond to magnetic surfacesof the plasmas. Tangential SXR imaging has been ap-plied to high-temperature toroidal plasma experimentsfor the study of pressure ﬂuctuations either in the coreor at the edge1). The reversed ﬁeld pinch (RFP) is ahigh-temperature and high-beta toroidal plasma. In theRFP, studies on the behavior of magnetic islands due tothe tearing modes are quite important, because the RFPconﬁguration is self-organized and sustained by nonlinearinteraction of the tearing modes, which lead to magneticchaos. One of the important issues of this study is thedevelopment of three dimensional (3-D) SXR measure-ment system, which will be applied for physics study on3-D helical equilibrium on LHD.Consecutive imaging measurement has been a usefultool for understanding the plasma dynamics and insta-bilities. We are therefore developing an SXR imagingsystem consisting of a SXR pin-hole camera and a high-speed CMOS camera (Photoron FASTCAM SA-4 500K-M1AS) for the study of time evolution of 3-D structure.A schematic drawing of the developing imaging systemis illustrated in Fig.1. Plural unit is located tangentialview of the plasma to have a view of nearly the entireplasma diameter and most of the vertical extent of theplasma. This SXR imaging system utilizes a microchan-nel plate (MCP) to record a higher-resolution distribu-tion of two-dimensional (2-D) luminosity on a phosphorplate. The projected images strongly depend on the equi-librium magnetic ﬁeld. As the A of RFP is lowered, thesafety factor q on axis increases and n of dominant inter-nally resonant mode is to be lower. In this sense, the coreof a low-A RFP is favorable for this kind of tangentialmeasurement.In a low-A RFP machine RELAX2), (R = 0.51 m/a= 0.25 m (A = 2)), a quasi-periodic transition to quasi-single helicity (QSH) state has been observed3). In thisexperiment, we have taken a series of tangential imagesof the plasma with 10 ? sec time intervals, to iden-tify time evolution of SXR emissivity structure in theQSH state in RELAX. 2-D luminosity distributions cor-responding to integrated SXR emissivity are measuredwith the high-speed camera with 320 × 192 pixels ar-ray and 12-bit of dynamic range. Raw images of SXRemission contain both the equilibrium and ﬂuctuatingcomponent. To clearly illustrate the ﬂuctuating com-Fig. 1: Arrangement of the SXR pin-hole camera andthe high-speed camera. Solid lines and a dashed linerepresent the limits of the visual ﬁeld and the line sightof the center of the visual ﬁeld, respectively.ponent, we have applied the subtraction technique; wehave obtained diﬀerential signals of two consecutive im-ages. Figure 2 shows an example of the results. A simplehelical structure has been observed clearly in the centralpart of the tangential viewing area. An equilibrium re-construction has shown that the minor radial locationof the m=1/n=4 resonant surface is consistent with theexperimental observation; The simple helix is located at0.10-0.14 m in a poloidal cross-section.We successfully obtained time evolution of SXR im-ages from tangential port.Fig. 2: Above two images are raw data. Below one is aresult of subtraction.1) S. Ohdachi, et al., Plasma Fusion Res. 2, S1016 (2007)2) S. Masamune et al., J. Phys. Soc. Jpn. 76 (No.12)(2007) 123501.3) R. Ikezoe, et al., Plasma and Fusion Res. 3, 029(2008)211§17. Physics Study on 3-D Helical Equilibrium Plasmas with 2-D Imaging DiagnosticsSanpei, A., Masamune, S., Himura, H., Oki, K., Nakamura, M., Higashi, A., Motoi, H., Fukahori, D. (Kyoto Inst. Tech.),  Watanabe, K.Y., Suzuki, Y., Ming, T.F., Ohdachi, S.

Physics Study on 3-D Helical Equilibrium Plasmas with 2-D Imaging Diagnostics

Physics Study on 3-D Helical Equilibrium Plasmas with 2-D Imaging Diagnostics

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