22P123-P
Development of dual SXR imaging system
for three-dimensional structural studies in a low-A RFP
低アスペクト比RFPにおける3次元構造研究のための二方向軟X線イメージングシステムの開発
Akio Sanpei1, Kensuke Oki1, Daisuke Fukahori1, Kazuaki Deguchi1, Seiya Nakaki1, Haruhiko
Himura1, Sadao Masamune1, Takumi Onchi2, Satoshi Ohdachi3 and Nobuhiro Nishino4
三瓶明希夫1，大木健輔1，深堀大佑1，出口和明1，中木聖也1，比村治彦1，政宗貞男1，恩地拓己2，大舘暁3，西野信博41)Kyoto Institute of Technology
Goshokaidocho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
京都工芸繊維大学 〒606-8585 京都市左京区松ヶ崎御所海道町
2) University of Saskatchewan
Saskatchewan S7N 5E2, Canada
3) National Institute for Fusion Science322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
4) Hiroshima University
1-4-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8527, Japan
広島大学 〒739-8527 東広島市鏡山一丁目4番1号
In order to study time evolution of the 3-D structure of a high-temperature plasma, soft-X ray (SXR)
imaging systems for simultaneous imaging from tangential and vertical directions have been developed.
Initial experimental have been carried out in the low-aspect-ratio (low-A) reversed field pinch (RFP)
plasmas in RELAX. It is shown that subtraction technique is effective in characterization of the fluctuating
component of the SXR emission, with the results of demonstration of rotating simple helix.

1. Introduction
The measurement of bremsstrahlung soft X-ray(SXR) radiation is one of the useful passive
methods for diagnosing high-temperature plasmas,
because contours of the SXR emissivity
correspond to magnetic surfaces of the plasmas.
Tangential SXR imaging has been applied tohigh-temperature toroidal plasma experiments forthe study of pressure fluctuations either in thecore or at the edge [1].
In the reversed field pinch (RFP), toroidal pitchof the equilibrium field is relatively short andfluctuation component resulting from internallyresonant
globalinstabilities
forms
threedimensional (3-D) helical structure. Equilibrium
analyses have shown that the innermost mode
rational surface can be located away from the axis
in a lower aspect ratio (A = R/a) RFP configuration,
where R (a) is the major (minor) radius of theplasma column. Therefore, growth of a single mode
to a higher amplitude can be expected in a low-A
RFP than in medium- and high-A RFPs, which may
allow easier access to quasi-single helicity (QSH)
state, in which the internally resonant single tearing
mode grows significantly larger than other modes.

In a low-A RFP machine RELAX [2], (R = 0.51m/a = 0.25 m (A = 2)), a quasi-periodic transition toQSH state has been observed [3]. During the QSHstate, the fluctuation power is concentrated to thedominant single m = 1 mode. We have developed
SXR imaging diagnostics using multiple SXRcameras, which are constructed with microchannel
plate (MCP) and fluorescent plate, for theidentification of structures of dominant MHD
instabilities in the QSH state in the RFP [4,5].
Detailed design of the SXR camera has beenreported in ref.[6]. Moreover, we have constructed afast successive SXR imaging system. As apreliminary experiment, we have taken tangential
SXR pin-hole pictures with time resolution of 10
micro sec, to identify time evolution of a simple
helix structure in RELAX plasmas.[7,8] As a nextstep, we have developed a SXR imaging diagnosticsystems, which uses multiple pin-hole SXRcameras together with high-speed cameras to taketime evolution of the SXR images from tangential
and vertical directions simultaneously for the studyof dynamic structures of 3-D SXR emissivity,
through which we expect to discuss 3-D dynamics
of MHD instabilities associated with the QSH state.

the experimental phase velocity.

2. Experimental Set-up and Results
A schematic drawing of the dual imaging system
is illustrated in Fig. 1. One unit is for the tangential
imaging, and the other for the vertical imaging,with two synchronized high-speed cameras. In theinitial experiment described here, the two cameras
are not synchronized, but with different frame rates:
100k frame/s for horizontal measurement, and 150
kfps for vertical measurement.

Figure.3 shows subtracted images of emissivity
structure obtained from vertical port in RELAX
with 150kfps of time resolution. We can recognizethat a zonal structure moves to the upper or left
direction as time goes on. By comparison ofexperimental and simulated images, vertical images
suggest rotating helical SXR emissivity.

Fig.3. Subtracted images of emissivity structure
obtained from vertical port in RELAX. These images are
obtained by subtracting a SXR image from another one
with 6.6 micro sec time interval.

The most recent results using synchronized two
high-speed cameras will also be presented, together
with discussion on possible reconstruction methods
for 3-D imaging.
Fig.1. Top view of dual imaging system.

Raw images of SXR emission contain both theequilibrium and fluctuating component. To clearly
illustrate the fluctuating component, we haveapplied the subtraction technique, which issubtracting a SXR image from another one with 10
micro sec time interval. Figure 2 shows an exampleof the experimental results obtained from tangential
port. A simple helical structure has been observed
clearly in the central part of the tangential viewing
area. An equilibrium reconstruction has shown thatthe minor radial location of the m=1/n=4 resonantsurface is consistent with the experimental
observation.[8]Fig.2. Subtracted images of emissivity structure
obtained from tangential port in RELAX. (a)Experimental result: The image is obtained bysubtracting a SXR image from another one with 10 micro
sec time interval. (b) Simulated image: Finite SXR
emissivity is assumed only inside the m=1/n=4 magnetic
island. The phase change (in 10 micro sec) is based on

Acknowledgments
This work is supported by a Grant-in-Aid for
Scientific Research (No.17360441) from theMinistry of Education, Culture, Sports andTechnology, Japan, and by the National Institute for
Fusion Science (NIFS) Collaborative Research
Program (NIFS10KOAP024).
References[1] S. Ohdachi, et al., Plasma Fusion Res. 2 (2007)
S1016.
[2] S. Masamune et al., J. Phys. Soc. Jpn. 76 (2007)
123501.
[3] R. Ikezoe, et al., Plasma and Fusion Res. 3 (2008)029.
[4] T. Onchi et al., Plasma and Fusion Res. 2 (2007)
S1063.
[5] A. Sanpei, et al., Plasma and Fusion Res. 2 (2007)
S1064.
[6] T. Onchi, et al., Rev. Sci. Inst., 81 (2010) 073502.
[7] A. Sanpei et al., Plasma and Fusion Res., 6 (2011)2406096.
[8] A. Sanpei et al., accepted for publication in the IEEE
Transactions on Plasma Science (2011).