22P170-B
Two-dimensional radiation profile measurements with AXUVD in LHD
LHDにおけるAXUVDを用いた二次元放射強度分布計測
Kimiharu Oyama1, Tomohiro Morisaki2, Naoki Tamura2, Suguru Masuzaki2, Yukio Nkamura2,and LHD Experimental Group2
大山公治1，森崎友宏2，田村直樹2，増崎 貴2，中村幸男2，LHD実験グループ2
1

Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
2
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan1
名古屋大学 〒464-8601 名古屋市千種区不老町
2
核融合科学研究所 〒509-5292 土岐市下石町322-6

Two 20-channnel AXUV photodiode arrays are used to measure the radiated power distribution in LHD.
From the one-dimensional radiation profiles measured with each array, two-dimensional radiation
distributions are reconstructed using the tomographic technique, which can be mapped to any poloidal
plane using the three-dimensional equilibrium database. To investigate the accuracy of the inversion, theradiation distributions during the super dense core (SDC) discharge with highly peaked density profile arecompared with pressure profiles measured with the Thomson scattering system. In the discharges withadditionally puffed neon or nitrogen, the enhanced radiation in the peripheral region is clearly observed.

1. Introduction
It is worthwhile to investigate the radiation
property emitted from the hot plasma in thefusion experiment devices, since it determines theenergy balance in the plasma. In addition, it is
known that the radiated power distribution wellreflects the impurity behavior which plays animportant role in energy transport.
For the detection of the radiated power
distribution in the plasma, the one-dimensional
detector array has been utilized in many tokamaks
which have the axisymmetric toroidal geometry.
However, in helical systems like LHD without
axisymmetry, the poloidal cross section varies
toroidally and sometimes has a complicated shape.
Therefore in order to identify the correct radiation
distribution in the complicated three-dimensional
plasma, it is necessary to observe radiation
profiles from different directions with severaldetectors.
In LHD two detector arrays are utilized toreconstructatwo-dimensionalradiation
distribution on a cross section of the plasma
column, using the computed tomography
technique. For the tomography, two different
methods are used and compared [1]. In theconference, observation results from the superdense core (SDC) discharge and the impurity
injection discharge are presented.

2. AXUV photodiode array detector
In the bolometric measurements for hightemperature plasmas, the absolute extreme
ultraviolet photodiodes (AXUVD) are used for theirhigh time resolution (~ 0.1 ms) and a flat spectrum
variation of their photon efficiency from 200 eV up
through the soft x-ray region [2].
For the tomographic reconstruction for theradiated power distribution on the cross section of
the plasma column, two 20-channnel AXUVD
arrays with pinhole are installed in the top and thehorizontal ports, as shown in Fig. 1. In thisconfiguration, the cross section is almost tangential
to the toroidal direction, and the spatial resolution isabout 5cm.

Fig. 1. Schematic view of observing geometry.

3. Tomographic method
To get the two-dimensional radiated power
distribution image on the semi-tangential
cross-section viewed with two AXUVD arrays, theplane is divided by 32 × 32 = 1023 pixels in thecomputed tomography. The Phillips-Thikhonov
regularization method and the Hopfield neuralnetwork method are used in the procedure [3, 4].
4. Experimental results
4.1. SDC discharge
Recently the SDC mode was discovered in LHD,
which is characterized by its high central electron
density more than 5 x 1020 m-3. In such a highdensity discharge, the relatively high radiation, i.e.
bremsstrahlung radiation, takes place. The central
position of this radiation generally corresponds tothe plasma center where the plasma pressure ishighest.
In this experiment, positions of the radiation peakobserved with the AXUVD bolometer and thepressure peak observed with the Thomson
scattering system are compared to confirm theaccuracy of the alignment of detectors and thetomographic method. Fig. 2 shows thereconstructed radiation distribution measured with
the AXUVD bolometer, superimposed withpressure profiles based on the Thomson scattering
data and the three-dimensional equilibrium analyses.
It is found that the plasma centers measured with
the AXUVD bolometer and the Thomson scatteringsystem agree well, although the shape of theradiation contour is relatively different from thepressure contour.

4.2. Impurity injection discharge
The impurity gas injection is tried to enhance theedge radiation and to cool the divertor plasma
connecting to the divertor plates or wall materials.
On the other hand, there exists a risk for the coreplasma to be cooled, if impurities penetrate into thecore region.
In order to see the behavior of the injected
impurities, radiation distributions are compared
between before and after the impurity gas injection,
as shown in Fig. 3. It can be seen that the radiationfrom the core region is not so significant after theimpurity injection, and the impurities stay in theedge region, which is a favorable situation.
(a)(b)

Fig. 3. Radiation distributions (a) before and (b) after
impurity gas puffing.

Fig. 2. Radiation distribution during SDC discharge
measured with AXUVD bolometer. Pressure
contour measured with Thomson scattering is alsosuperimposed.

References[1] B.J.Peterson, et al., Plasma Phys.Controlled
Fusion 45, 1167 (2003).
[2] D.S. Gray, et al., Rev. Sci. instrum 75 376 (2004).
[3] Y.Liu et al., Rev. sci. Instrum. 79 10F501 (2006)[4] Naofumi IWAMA, et al., J. Plasma Fusion Res.
SERIES, Vol. 8 (2009).