Proceedings of ITC/ISHW2007

Development of Imaging Bolometers for
3-D Tomography of Radiation from LHD
Byron J. Peterson1, Masahiro Kobayashi1, Artem Yu. Kostryukov2, Homaira Parchamy Araghy1,Naoko. Ashikawa1, Yi Liu3, Naofumi Iwama4, Masahiko Emoto1, Hisato Kawashima5, Shigeru
Konoshima5, Yuehe Feng6, Hiromi Hayashi1, Shinji Yoshimura1, and DongCheol. Seo7
1

National Institute for Fusion Science, Toki-shi, Gifu-ken 509-5292, Japan2
St. Petersburg State Technical Universit y, St. Petersburg 195251 Russia
3Southwestern Institute of Physics, Chengdu, 610041 China4Daido Institute of Technology, Nagoya-shi, Aichi-ken, 457-8530 Japan
5Japan Atomic Energy Agency Naka-machi, Ibaraki-ken 311-0193 Japan
6Max-Planck Institute for Plasma Physics, Greifswald, D-17491 Germany
7National Fusion Research Center, Daejeon 305-806 Rep. of Korea

Imaging bolometers are a powerful tool for diagnosing plasma radiation in a reactor relevant environment. Ina tokamak, an imaging bolometer with a tangential view has demonstrated the ability to provide two-dimensional
poloidal profiles of radiation through tomographic inversion under the assumption of toroidal symmetry. In
helical devices the imaging bolometers have shown their usefulness in the interpretation of 3-dimensionl radiation
structures. In the Large Helical Device, modeling of transport in the stochasitic edge region predicts a very
three-dimensional structure in the flow velocities and in the associated accumulation of carbon impurity ions. In
this paper we make a proposal for the diagnosis of these three-dimensional radiation structures. This is to be done
through the measurement of the radiation using multiple imaging bolometers and the tomographic inversion of theirsignals to produce a three-dimensional image of the plasma radiation for comparison with the three-dimensional
structure of the carbon radiation from the transport model. We present and discuss the recent results of thisongoing research including imaging bolometer development, possible symmetry assumptions and prospective
research targets.
Keywords: bolometer, imaging, three dimensional, tomography, helical, infrared

1. Inroduction
Bolometers play an important role in fusion devices inthe diagnosis of radiative losses from the plasma [1]. The
conventional bolometer detector is a gold or platinum
resistive bolometer which consists of a thin radiation
absorbing foil backed by an insulating layer of either
kapton or mica which is in turn backed by a resistive gridof the same material as the absorbing layer [2]. Thechange in temperature of the foil due to the absorption ofradiation is sensed by the change in the resistance of thegrid as it heats up.
These resistive bolometers have several drawbacks,
especially regarding application to a radiation rich, steadystate fusion reactor including: signal drift, restrictions on
absorber thickness, the risk of numerous wire vacuum
feedthroughs, radiation induced electronic noise and theauthor’s e-mail:peterson@LHD.nifs.ac.jp

lack of neutron resistant materials. Therefore, researchand development has been carried out on an alternative
detector known as the InfraRed imaging Video Bolometer(IRVB) which addresses these concerns through theapplication of infrared technology to measure thetemperature of a thin absorber foil [3,4].
The use of infrared imaging of the absorber foiladvances bolometer diagnostics from the conventional one
dimensional arrays of resistive bolometers to twodimensional images of the plasma radiation from imaging
bolometers. These imaging bolometers have been usedin LHD to investigate radiative phenomena [5] and a single
imaging bolometer on the JT-60U tokamak has provided
two-dimensional radiation profiles through tomographic
inversion under the assumption of toroidal symmetry [6].
Helical devices differ from tokamaks in their three
dimensional nature, compared to the two dimensional

Proceedings of ITC/ISHW2007
nature of tokamaks resulting from their toroidal symmetry.
Therefore, while a single imaging bolometer can provide
the two dimensional (minor radial and poloidal) radiation
profiles needed to fully diagnose the radiation from atokamak plasma, in helical devices multiple imaging
bolometers can be used to perform the three dimensional
tomography necessary to fully diagnose the radiation. Thisis analagous to multiple one dimensional arrays of resistive
bolometers being used to perform two-dimensional
tomography in a tokamak. In this paper we discuss
preparations for such three dimensional tomography ofradiation on the Large Helical Device (LHD), including
imaging bolometer development on LHD, symmetry
assumptions and prospective subjects of research.

Phoenix camera is operating on the 6-T port IRVB. We
plan to install another SC500 camera at port 1-O in thenext year. In the future we plan to move another Phoenix
camera from JT-60U to LHD port 6.5-L in time for the DD
experiments planned for LHD. This system includes aneutron/gamma/magnetic shield, a 3.7 meter IR periscopewith 4 CaF2 lenses, an Al mirror and a sapphire vacuum
window with a 5 micron thick Ta foil and is shown inFigure 2. The other foils installed on LHD are 1 micron
Au or Al. Other possible IRVB installations would

2. Imaging bolometer development on LHD
In addition to IR camera development by industry,
imaging bolometer detectors are advancing through ourresearch into absorbing foil materials, suitable IR optics,
camera shielding materials and calibration techniques.
This work has been summarized recently [7,8]. A typical
IRVB system consists of a foil mounted in a frame in apinhole camera in the vacuum vessel, a vacuum IR
window an IR optical system consisting of lenses andmirrors and the IR camera. In LHD two types of cameras
are currently being used. The first is an FLIR SC500 longwave (7.5 - 13 µm) camera with 320 x 240 pixels and aframe rate of 60 Hz. The second is an Indigo Phoenix
mid wave (3 - 5 µm) camera with 320 x 256 pixels and aframe rate of 345 Hz. Foils have been installed at four
locations in LHD as shown in Figure 1. Currently anSC500 camera is operating at the 6.5-U port IRVB and a
1-O
10-O

Fig.2
3-O
3.5
U5.
64-O

L5.
68-O
6.5

5-O

Side view of LHD Port 6.5-L showing
position of foil, lenses, vacuum window,mirror, and IR camera in shield.

include moving the SC500 camera from 6.5-U port to the8-O port and replacing it with an Omega IR camera with aperiscope. Another possible installation would be with asecond Omega IR camera at the 8-I port. This would givea total of 6 IRVBs with over 1000 channels. Except for
the IRVB moved from JT-60U to Port 6.5-L, all the otherIR cameras would need to have shielding against neutronsand gammas added before the first DD campaign on LHD.

6-TFig.1

Top view of LHD vacuum vessel (black)showing existing (cyan and green) andprospective (yellow) IRVB foils.

3. Plans for three dimensional tomography onLHD
In general, in order to carry out computed
tomography the target volume is divided into cells with theProceedings of ITC/ISHW2007
centers of each cell representing the points at which one
would like to know the measured quantity. The numberand spacing of the cells determine the spatial resolution.
In general for a well defined solution to the tomographic
inversion the number of cells should not exceed thenumber of channels and each cell should be viewed by as
many detectors from as many angles as possible. In orderto reduce the number of unique cells and/or improve thespatial resolution certain assumptions regarding symmetry
can be invoked. For instance in a tokamak by assuming
toroidal symmetry, one of three spatial dimensions can beignored greatly simplifying the problem.
In the same way in a helical device several different
assumptions may be made to reduce the number ofunique cells and simplify the tomographic inversion.
For example let us start by dividing each half field period(18 degrees toroidally) into 10 radial sectors (8 inside thelast closed flux surface (LCFS) and 2 in the ergodicregion), 6 toroidal sectors and 8 poloidal sectors for a
total of 480 cells/ half field period or 9600 cells
throughout the entire toroidal volume. Since we do nothave enough IRVBs to see every part of the plasma, wemust make symmetry assumptions to reduce this numberof cells.
The first assumption that can be made is what iscalled helical symmetry. If zero toroidal angle is defined
to be at either the horizontally or the vertically elongatedcrossection, then this can be expressed as S(r, θ, φ) = S(r,-θ, −φ) in toroidal coordinates minor radius, r, poloidal
angle, θ, and toroidal angle, φ, where S is the local plasma
emissivity. This essentially means that the plasma
reproduces itself every half field period or 18 degrees
toroidally in LHD. This reduces the number of uniquecells to 480 in the previously given example.
Another assumption which is less strict is calledtoroidally periodic symmetry. This can be expressed asS(r, θ, φ) = S(r, θ, φ+2π/m), where m is the number oftoroidal field periods, or 10 in the case of LHD. Thismeans that the plasma reproduces itself every field period
or 36 degrees toroidally in LHD. This would result in 960
unique cells in our example.
Another much more simplifying assumption iscalled flux surface symmetry which requires that theradiation is constant on the flux surface. In the region ofwell formed flux surfaces inside the LCFS this removes
the two coordinates of poloidal and toroidal angle fromconsideration, leaving only the minor radius. In ourexample this would reduce the number of cells inside theLCFS to 8 from 384, while the ergodic region where such
symmetry can usually not be assumed would be divided
into 96 or 192 depending on if helical symmetry or
toroidally periodic symmetry where chosen, respectively.
The choice of which of these symmetries to assume

depends on the characteristics of the phenomena to bestudied as is discussed next.

4. Physical targets of three dimensional
tomography in LHD
Several phenomena which have appeared in LHD
present excellent subjects for the application ofthree-dimensional tomography. The first of these isasymmetric radiative collapse, which occurs at theradiative density limit in LHD [9]. This has been
observed by two IRVBs and diode arrays at two toroidal
locations to be roughly toroidally symmetric. With
three-dimensional tomography the degree of toroidal
symmetry could be evaluated and the evolution of thisphenomena’s three-dimensional structure could be studied
in detail. In this case toroidally periodic symmetry
would be the appropriate assumption since the toroidalsymmetry breaks the helical symmetry and theasymmetry is observed inside the LCFS.
A second interesting phenomena is the change inradiation structure predicted by the EMC3/EIRENE edgetransport code in the ergodic region as the density
increases. In the horizontally elongated cross-sections
shown in Figure 3, the carbon radiation is localized
polodally and radially at the inboard x-point at low
density and moves inward towards the LCFS as density
increases.
With three-dimensional tomography thisstructure and its density dependence could be
experimentally confirmed. The appropriate symmetry
assumption for this study would be helical symmetry in
the ergodic edge and flux surface symmetry inside theLCFS.
A third phenomenon of interest might be theserpens mode that has been observed under partiallydetached plasmas in LHD. This phenomenon has been
described as a rotating belt of cold dense radiating plasma
beneath the LCFS [10]. Since it rotates poloidally andtoroidally none of the assumptions above may be
applicable, but by observation with the multiple imaging
bolometers this rotation could be observed in detail andconfirmed.

4. Summary
Developemnt of imaging bolometers for LHD has
been reviewed with a view towards the development of
three-dimensional tomography of radiation.
Thepresent status of imaging bolometers on LHD and futureplans have been presented. Possible simplifying
symmetry assumptions have been reviewed and explained.
Prospective subjects of study for three-dimensioanal
tomography have been presented from phenomenapreviously observed on LHD. However, much workProceedings of ITC/ISHW2007
remains in order to realize this plan.

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B.J. Peterson et al., Rev. Sci. Instrum. 74, 2040 (2003).
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(2003).
[6] Y. Liu et al., to be published in Plasma Fusion Res.
(2007).
[7] B. Peterson et al., to be published in Plasma Fusion Res.
(2007).
[8] H. Parchamy et al., to be published in Plasma Fusion Res.
(2007).
[9] B. J. Peterson, et al., Plasma Fusion Res. 1, 45 (2006).
[10] J. Miyazawa et al., J. Plasma Fusion Res. 81, 331 (2005)

Fig.3

Two-dimesional profiles of carbon
radiation intensity from the ergoic edge ofLHD calculated by the EMC3/EIRENE
code for nLCFS = (a) 3 x 1019/m3, (b) 6 x1019/m3 and (c) 6.8 x 1019/m3.