Plasma and Fusion Research: Regular Articles

Volume 11, 2402101 (2016)

Preliminary Design of Imaging Bolometer Fields of View forWendelstein7-X∗)
Byron J. PETERSON1,2) , Kiyofumi MUKAI1,2) , Ryuichi SANO3) , Haruki KOJIMA1) ,LHD Experiment Group1) , Daihong ZHANG4) , Marcin JAKUBOWSKI4) ,Maciej KRYCHOWIAK4) , Yuhe FENG4) , Ralf KOENIG4) ,Thomas Sunn PEDERSEN4) and the W7-X Experiment Group4)
1)

National Institute for Fusion Science, Toki 509-5292, JapanSOKENDAI (The Graduate University for Advanced Studies), Toki 509-5292, Japan3)
National Institutes for Quantum and Radiological Science and Technology, Naka 311-0193, Japan4)
Max Planck Institute for Plasma Physics, Germany
2)(Received 3 December 2015 / Accepted 9 June 2016)
The InfraRed imaging Video Bolometer (IRVB) provides absolutely calibrated images of the plasma radiation. In this paper we present the design of four imaging bolometers viewing divertor and non divertor regionsof Wendelstein7-X (W7-X). This design assumes a 640 × 480 pixel, 30 fps, 100 mK, IR camera imaging each
IRVB foil. The bolometer cameras consist of a 9 cm × 7 cm × 2 µm thick Pt foils blackened with graphite, which
are located distances ranging 6 cm to 8 cm from a 4.3 mm × 4.3 mm aperture. The 8 cm × 6 cm central sectionof the foil is divided into 24 × 18 IRVB channels each consisting of 504 IR camera pixels. Using a previously
derived expression, the noise equivalent power of the IRVB is calculated as 109 µW/cm2 . Assuming 5 MW ofradiated power uniformly emanating from the 30 m3 plasma, the signal level is estimated as ranging from 4.5 to8.1 mW/cm2 giving signal to noise ratios between 42 and 74.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: bolometer, diagnositcs, imaging, divertor, Wendelstein7-X
DOI: 10.1585/pfr.11.2402101

1. Introduction
The InfraRed imaging Video Bolometer (IRVB) [1]measures plasma radiation images, which can be directlycompared with the results of impurity radiation models toprovide a quantitative measurement of the spatial distribution of the plasma radiation [2]. In a three-dimensional(3D) device such as Wendelstein7-X (W7X) [3,4] the twodimensional (2D) images provided by the IRVB are necessary to understand the complicated divertor and x-point
radiation patterns. The ten-fold W7X divertor is divided
into two sections, designated as the ‘low iota’ divertor andthe ‘high iota’ divertor, referring to the magnetic configuration under which the divertor flux to those sections isdominant [5]. By observing the radiation from these twodivertor sections, as well as non-divertor portions of theplasma, and comparing the bolometric images with synthetic images derived from impurity transport codes weplan to investigate impurity transport and confinement inW7-X.
In this paper we give information on the design of four
IRVBs for W7-X. The ports utilized for this design are theAEM30 and AEA31 ports. The ports and divertor sections
author’s e-mail: peterson@LHD.nifs.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

are shown in a computer aided drawing (CAD) in Fig. 1.
IRVB A views the ‘high iota’ divertor and IRVB B views
the non divertor region, both from the AEM30 port. IRVBC views the ‘low iota’ divertor and IRVB D views the nondivertor region, both from the AEA31 port. FoVs of bothdivertor and non divertor regions are planned in order toincrease the coverage of the 3D W7-X plasma as well as todiﬀerentiate between radiation resulting from the helicalx-points and that resulting from plasma wall interaction at
the divertor plate by comparing the experimental images
with synthetic images from a 3D impurity transport model.
Since the model may not include the radiation from plasma
wall interactions, by comparing the synthetic image andthe experimental image we should be able to diﬀerentiate
between the two.

2. IRVB Design Parameters
The IRVB consist of two parts, the bolometer camera and the infrared (IR) camera, IR vacuum window andits associated optics as shown in Fig. 2. The bolometer
camera is based on a simple pinhole camera design witha 9 cm × 7 cm × 2 micron thick Pt foil mounted in a frameand blackened with graphite viewing the plasma througha 4.3 mm × 4.3 mm square aperture. The distances fromthe foil to the aperture (shown in Fig. 2), lap-f , vary de-2402101-1

c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 11, 2402101 (2016)

Fig. 1 CAD image of divertors and of ports used in W7-X for IRVBs.

Fig. 2 Cartoon showing main components of IRVB.

pending on the IRVB and its field of view (FoV) and are
given in Table 1. With regard to the IR camera and optics, this design assumes a 640 × 480 pixel, 30 fps, 100 mK,microbolometer (8 - 12 µm) type IR camera imaging theIRVB foil. This design does not include any specificationof the IR periscope which may be necessary to bring theIR signal out of the immersion tube to a location wheresuﬃcient space is available to install the IR camera andany shielding that may be necessary. The IR camera noise
equivalent temperature (NET), σIR , is estimated conservatively to be 100 mK compared to the actual value of 50 mK
in order to take into account any losses in the transmission
through the periscope optics.

Table 1 IRVB Design Parameters.

3. IRVB Positions, FoVs and Signal toNoise Ratios
The 8 cm × 6 cm central section of each foil is dividedinto 24 × 18 IRVB channels each consisting of 504 IR cam-2402101-2

Plasma and Fusion Research: Regular Articles

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Table 2 IRVB coordinates.

era pixels. The positions of the IRVB foils and apertures
are given in Table 2. The coordinates are defined with respect to the center of the inside (vacuum side) surface ofthe flange with x being in the toroidal direction (clockwise
viewed from the top of the machine), y being in the negative poloidal direction (defined as positive being downward on the outboard side) and z being the distance fromthe inside (vacuum) surface of the flange with positive being towards the plasma. The coordinate systems are defined by the blue axes in Fig. 3. The coordinate system of
the AEA31 port is rotated 22.5◦ clockwise (viewed fromthe atmosphere side towards the vacuum side) around thez axis in order to align the FoV of IRVB C with the low
iota divertor. The cross-section drawing of each port drawn
through the vertical center of the foils is shown in Fig. 3and the FoVs of each IRVB channel are shown by the blackgrids in Fig. 4.
The noise equivalent power density (NEPD), S IRVB ,of the IRVB is calculated using Equation 1
ηIRVB Nbol
Af√32fbol Nbol fbol
10ktf σIR Nbol
= +,
5κ2A2ffIR NIRS IRVB =(1)where ηIRVB is the IRVB noise equivalent power density,
Nbol = 432 is the number of IRVB channels, Af = 48 cm2is the utilized area of the foil, k = 0.716 W/cmK is thethermal conductivity of the foil, tf = 2 µm is the thicknessof the foil, fIR = 30 fps is the frame rate of the IR camera,
NIR is the number of IR camera pixels used, fbol = 30 fps isthe frame rate of the IRVB and κ = 0.251 cm2 /s is the foil
thermal diﬀusivity. This equation is derived from Eq. 10in Ref. [6] by dividing ηIRVB by the area of each bolometer channel, Abol = Af /Nbol = 11.1 mm2 and neglecting
the contribution of the black body radiation term. The firstterm under the radical is also small and can also be neglected, but we retain it in the calculations. Applying thisequation to our design results in an S IRVB of 109 µW/cm2 .
The signal level, S signal , is simply calculated by assuming a uniformly radiating plasma using Eq. 2S signal =Psignal
Abol Aap cos4 ∂Prad lplasma=,2Abol
Abol 4πlop-fVplasma

Fig. 3 CAD drawings of cross-sections of Ports AEM30 (a) andAEA31 (b) showing foil and aperture locations and thefields of view (red) of each IRVB.

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where Psignal is the radiated power to each detector fromthe plasma, Aap = 18.8 mm2 is the area of the aperture,
Prad = 5 MW is the total power radiated by the plasma,Vplasma = 30 m3 is the volume of the plasma. The following three quantities are shown in Fig. 2: the distancefrom the foil to the aperture, lap-f , the average path lengthof the sightline through the plasma, lplasma = 150 cm, andthe average angle of each detector’s line of sight with respect to the aperture normal vector, θ = 20◦ . Assumingthese values, the signal level estimates and signal to noise
ratios (SNR) given by the ratio of Eq. 1 to Eq. 2 are shownin Table 1. Using Equations 1 and 2 the bolometer pixels
sizes and corresponding aperture sizes are chosen to givethe maximum number of channels (and therefore best spatial resolution) while maintaining a reasonable SNR.

4. Summary and Future Work
This work has shown that IRVBs with reasonable
FoVs, numbers of channels and signal levels can be designed for W7-X. The main bolometer camera parameter,lap-f , and the locations of the foils and apertures given in
Table 2 are chosen to give an adequate view of the respective divertor region for IRVBs A and C without being too
close to the plasma. In the case of IRVBs B and D the locations of the apertures and foils and the selection of lap-f
are somewhat arbitrary and will be optimized in the nextstep (see below).
The next step in the design process is to calculate theprojection matrix for each IRVB and then multiply it by theradiated power density (resulting from an impurity transport code such as EMC3-EIRENE) to produce a synthetic
image. These synthetic images will then provide a detailed
estimate of the signal level for each channel that then canbe used to provide a more accurate estimate of the SNR.
Also the synthetic images can be used to fine tune the FoVs
for IRVBs B and D which are viewing the non divertor regions.

Acknowledgement

This work was supported by NIFS budget code
NIFS15ULHH026 and the NIFS International Collaboration budget.
[1] B.J. Peterson, Rev. Sci. Instrum. 71, 3696 (2000).
[2] K. Mukai et al., Nucl. Fusion 55, 083016 (2015).
[3] T. Klinger et al, Fusion Eng. Des. 88, 461 (2013).
[4] H.S. Bosch et al., IEEE Trans. Plasma Sci. 42, 432 (2014).
[5] H. Renner et al., J. Plasma Fusion Res. SERIES 1, 143(1998).
[6] B.J. Peterson et al., Rev. Sci. Instrum. 74, 2040 (2003).

Fig. 4 CAD images of FoVs for IRVBs A (a), B (b), C (c) andD (d) in W7-X showing high (green), low (purple) iota
divertor and baﬄe (yellow).

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