Plasma and Fusion Research: Regular Articles

Volume 14, 3405080 (2019)

Mechanical Design and Structural Analysis for Lower Port Opticsof ITER Divertor Impurity Monitor∗)
Toshiyuki MARUYAMA, Toshihiro OIKAWA1) , Sin-iti KITAZAWA1) , Kimihiro IOKI, Saori MERA,Ikuma NOMURA, Hiroaki OGAWA1) , Suguru TANAKA1) and Takaki HATAE1)
1)

TOYAMA Co. Ltd., Yamakita-machi, Ashigarakami-gun, Kanagawa 258-0112, Japan
Naka Fusion Institute, National Institutes for Quantum and Radiological Science and Technology, Naka, Ibaraki
311-0193, Japan(Received 4 January 2019 / Accepted 17 March 2019)

The Divertor Impurity Monitor (DIM) for ITER is a spectroscopic diagnostic that measures the parametersof impurities and isotopes of hydrogen in the divertor plasmas. The DIM systems are installed in upper port,equatorial port, lower port (LP) and the divertor cassette. The LP systems consist of the side-upper/lower opticalsystems and the central optical system. In the central system, the front-end optics (optical mirrors) are locatedunder the divertor-dome and this optics is required to have a structure that can withstand extremely high thermalloads in terms of establishing a sensitive optical structure. This paper presents a mechanical design of the optics
with taking manufacturability into account and structural integrity assessment according to ITER load conditions(thermal and electro-magnetic loads, etc.) and design criteria specified in the RCC-MR code. The mechanicaldesign of the optics forms a box-shaped optics and the structural analysis results indicate the mirror box satisfies
design criteria in the RCC-MR code, except for the inner support part interfacing with the divertor cassette.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: ITER, plasma diagnostics, lower port, divertor cassette, structural analysis
DOI: 10.1585/pfr.14.3405080

1. Introduction
The main function of the ITER Divertor Impurity
Monitor (DIM) is to measure the parameters of impurities
and isotopes of hydrogen (tritium, deuterium and hydrogen) in the divertor plasmas by using spectroscopic techniques in the wavelength range of 200 - 1000 nm [1, 2].
Japan Domestic Agency is responsible for procurement of
the DIM system. The DIM consists of five optical systems,
one system in upper port, one system in equatorial port andthree systems in lower port (LP). The DIM in LP consistsof the side-upper and side-lower optical systems, whichview through the gap of the divertor cassettes respectively
from the upper side and the lower side of the LP, and thecentral optical system, which directly measures the innerand outer vertical targets of the divertor cassette as shownin Fig. 1. In the central optical system, the front-end optics(optical mirrors) are located under the divertor-dome andthis optics is required to have a structure that can withstandextremely high thermal loads, in terms of establishing asensitive optical structure such that precise optical mirrors
are installed in the limited space, and the loads in the divertor area are much higher than in the vacuum vessel port
(upper, equatorial and lower ports) area where other optical diagnostic systems are normally located. In the previauthor’s e-mail: t-maruyama@toyama-jp.com∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

Fig. 1 DIM optical systems in LP.

ous work [3], the thermal structure of a box-shaped optics,
named mirror box, on the divertor cassette was developedand its structural feasibility against high thermal loads was3405080-1

c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

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Volume 14, 3405080 (2019)

Fig. 2 Mechanical design of mirror box.

investigated. However, other mechanical loads (electromagnetic and seismic loads, etc.) and manufacturabilityof the mirror box were not taken into account. In this paper, a mechanical design of the optics with taking manufacturability into account was developed and its structural integrity assessment according to ITER load conditions anddesign criteria specified in the RCC-MR code [4], which
was developed for sodium-cooled fast reactors and ITER
Vacuum Vessel, was carried out.

2. Mechanical Design of Mirror Box
on the Divertor Cassette
The detailed mechanical design of the mirror box on
the divertor cassette was developed with taking into account manufacturability and actual assembly. The designand optical paths and mirrors in the mirror box are shownin Figs. 2 and 3. The mirror box consists of two-boxes
structure, inner and outer boxes, for assembly and adjustment of optical mirrors. The main function of the inner box
is to support the mirrors and of the outer box is to protect
the mirrors against radiative plasma heat loads and dusts.
A cooling pipe is embedded inside the plate of the mirrorbox.
Materials of the mirror box are stainless steel,
SS316L, and copper. Copper is partially used to reduce
temperature and thermal stress due to high thermal loads.
The mirrors are attached on the plate with cooling circuit,cooling paths are not embedded in the mirrors themselves.
A high temperature gradient occurs on the 1st and 4th mirrors due to high thermal loads for 1st mirrors and mirror
size for 4th mirror, so that these mirrors consist of mirror surface made of SS316L, 3 mm thickness, and mirror
base made of copper in order to improve thermal conduction as shown in Figs. 4 (a) and (b). The SS316L mirror
surface and the copper mirror base are joined by explosive

Fig. 3 Optical paths and mirrors in the mirror box.

bonding. An eﬀective material for reflectivity in ultraviolet region (molybdenum, rhodium etc. to be determined) iscoated on all mirror surfaces, 1st to 4th mirrors. An additional improvement of thermal conduction for 1st mirrors
is required due to high thermal loads, so that a copper alloy
block is embedded in the part of the inner box plate whichcontacts with 1st mirror as shown in Fig. 4 (c).
A concept of manufacturing and mechanical design ofa plate with a cooling pipe is shown in Fig. 5. The platewith a cooling pipe is manufactured so that a bent pipe
is sandwiched between plates channeled along the piping
and joined by HIP (Hot Isostatic Pressing). The outer box
forms a cooling water circuit with narrow gaps between
cooling paths as possible in order to withstand extremely
high radiative plasma heating, which are much higher thanin the vacuum vessel port area where other optical diagnos-

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Fig. 4 Parts with improved thermal conduction; (a) 1st mirror,(b) 4th mirror and (c) inner box plate in contact with 1stmirror.
Fig. 6 Outer box structure.

The assembly procedure of the mirror box is shownin Fig. 7. The side plates are separated into lower and upper ones because the upper side plates are assembled at thefinal step in order to enable the access and position adjustment of the 1st to 3rd mirrors before completely closing
the mirror box. The configuration of pipes is designed withtaking into account the space for inserting welder.

Fig. 5 Box plate with cooling pipes; (a) concept of manufacturing and (b) mechanical design.

tic systems are normally located. Since the plasma facing
sides directly receive radiative plasma heating, the plates
on the plasma sides require the cooling circuit with the narrowest gap. Distance between cooling paths in the plasmaside plates is required about 20 mm, but a pipe whose outer
diameter is 8 to 10 mm cannot be bent at such a short distance and the plasma side plates cannot be manufactured
in an integrated plate by the concept shown in Fig. 5. On
the other hand, direct manufacturing of cooling circuit intoa single plate by milling or drilling needs many welding
parts for forming vacuum boundary on the plate and a highrisk of vacuum leakage from welding parts. The cooling
circuit in the mirror box should be formed by pipes to minimize the risk of vacuum leakage. Therefore, the outer box
has a structure that consists of multiple cooling layers combined with bolt screws as shown in Fig. 6, in order to form
cooling paths with about 20 mm interval by pipes with taking manufacturability into account.

3. Structural Analysis for Mirror Box
on the Divertor CassetteA structural integrity assessment for the mirror box
shown in the previous section was carried out according toITER load conditions, which specify some kinds of loadcases such as thermal and electro-magnetic loads, and loadcase combinations to be taken into account. In particular, dead weight, thermal load, electro-magnetic load, acceleration from divertor, seismic load and pressure loads,
as shown in Table 1, should be taken into account as representative load cases. As a first step, each single loadcase was analyzed in order to investigate significant loads
and design drivers for the mechanical design of the mirror box. Table 1 summarizes the maximum stress value
obtained due to each single load case. These results indicate that the thermal load during plasma operation andthe electro-magnetic load during a plasma disruption event(VDE: Vertical Displacement Event) are significant loads
for the mirror box. The analysis results for the both loads
are described below.
Analysis conditions for the thermal load during
plasma operation are shown in Table 2. Nuclear heating and radiative plasma heating are considered as thermal loads. Nuclear heating values are 2 MW/m3 applied

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Fig. 7 Assembly procedure of the mirror box; (a) Inner box is attached inside outer box, (b) Lower side plate is attached, (c) Both upperside plates are attached after final adjustment and (d) Assembly completed.

Table 1 Summary of maximum stress due to each single loadcase.

Table 2 Analysis conditions for the thermal load during plasma
operation.

cooling path diameter is applied on cooling path area as aboundary condition for cooling. Thermal conductance on
the contact surfaces between the mirrors and the inner box
plate are also considered and the conductance values werecalculated by Tachibana’s formula [7]. Analysis results ofthe temperature and thermal stress distributions are shownin Figs. 8 and 9. The highest temperature is 321◦ C generated on the 3rd mirror close to 1st mirror and the highest
thermal stress is 215 MPa generated on the inner box plateclose to 1st and 3rd mirrors, the highest temperature area.
Stresses obtained by elastic analysis must satisfy the following criteria;
P + Q < 3S m ,where P is primary stress which means stress due to mechanical loads (seismic and electro-magnetic loads, etc.),
Q is secondary stress which means thermal stress and S m
is the allowable stress specified in the RCC-MR [4]. S mof SS316L at 150◦ C is 127 MPa. Regarding the high thermal load area at the upper side of the outer box close toplasma and around 1st mirrors, the primary stress due tomechanical loads other than thermal loads does not concentrate on the area and the value of P is suﬃcient lower
than the maximum value of Q = 215 MPa. Therefore,
P + Q < 3S m = 318 MPa,for the outer box and 1 MW/m3 applied for the inner box
and mirrors [5]. Radiative plasma heating values are frommaximum 150 kW/m2 applied on the plasma facing area ofthe outer box to minimum 50 kW/m2 applied on the lower
area of the outer box and the reflecting surface of the 1stmirrors [6]. Heat transfer coeﬃcient which depends on

is satisfied on the high thermal load area and the structuraldesign of the mirror box is feasible for the thermal loads inthe current design. However, a new requirement of radiative heating load due to reflected lights in the divertor area
has been suggested [8, 9]. This eﬀect needs to be investigated in the future study and fed back to the mirror box
design.
The time change rate of magnetic fields in the mirror box due to the worst transient plasma disruption eventin the divertor area is shown in Fig. 10. They are specified in the ITER load conditions, where Bdot(r), Bdot(t)3405080-4

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Fig. 8 Temperature distribution on (a) outer box and (b) inner box during plasma operation.

Fig. 9 Thermal stress distribution on (a) outer box and (b) inner box during plasma operation.

Fig. 10 Time change rate of magnetic fields in the mirror box due
to a transient plasma disruption event.

and Bdot(z) are the time change rates respectively in theradial, toroidal and vertical directions defined in the ITER
coordinate system. The highest time change rate is 280 to178 T/s in the radial direction. An electro-magnetic analysis was carried out by using the time change of magnetic
fields. The electro-magnetic force induced for the mirror
box by the magnetic field change was calculated. The results of the electro-magnetic analysis are shown in Figs. 11and 12, time changes of electro-magnetic force and torquegenerated for the mirror box, where F(r), F(t) and F(z) areFig. 11 Time change of electro-magnetic forces in the mirror box.

the time change of electro-magnetic force respectively inthe radial, toroidal and vertical directions, T (r), T (t) andT (z) are the time change of electro-magnetic torque in thesame manner. The highest electro-magnetic force occurs
at 46 ms. The electro-magnetic force vector distribution at
46 ms is shown in Fig. 13. A structural analysis was carried out by using the electro-magnetic force distribution
shown in Fig. 13 as the static load. The analysis result ofstress distribution is shown in Fig. 14. The highest stressis 250 MPa generated on an inner support part, which theanalysis modeling is simplified, interfacing with the diver-3405080-5

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Fig. 12 Time change of electro-magnetic torques in the mirror
box.

Fig. 15 Positions where the primary stress concentrates.
Table 3 Stress categorization for the worst load case, DW +PLO + SL-1 + EM + EM Acc. + THO.

Fig. 13 Electro-magnetic force vector distribution in the mirror
box.

Fig. 14 Stress distribution due to electro-magnetic force in themirror box.

tor cassette. The primary stress due to mechanical loads isprone to concentrate on the support parts.
In the ITER load conditions, specific combined loadcases must be assessed for structural integrity. The analysis results indicate that the worst combined load casefor the mirror box is DW (dead weight) + PLO (pressureload during plasma operation) + SL-1 (Seismic load during a aSL-1 event) + EM (electro-magnetic load duringa VDE) + EM Acc. (acceleration from divertor duringa VDE) + THO (thermal load during plasma operation).
Primary stresses due to mechanical loads at the points
where the stress concentrates were categorized to obtainthe membrane, local membrane and bending stresses andtheir stress criteria were assessed according to the RCCMR [4]. The positions where the primary stress concentrates are shown in Fig. 15 and the categorized stresses andcriteria for the worst load case at these points, where Pm
is membrane stress, PL is local membrane stress and PB
is bending stress, are summarized in Table 3. Stresses onmost points satisfy the allowable ones, but the stress on thesupport part is higher than the allowable one. Since thestress on the support part is localized due to the simplified
analysis modelling, a more detail structure of the support
parts will be considered and a more detailed analysis will
be carried out. Since the electro-magnetic load due to atransient plasma disruption event is significant as a mechanical load, the rigidity of the support parts will need

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to be enhanced. On the other hand, if a radiative heatingeﬀect due to reflected lights in the divertor area is significant, it is necessary to consider downsizing the mirror box
and optics to reduce thermal and electro-magnetic loads.

4. Summary
In the central optical system for the DIM in the lowerport, the optical mirrors are located under the divertordome to measure the inner and the outer vertical targets ofthe divertor cassette. A mechanical design of a box-shaped
optics, mirror box, was developed so that it can withstandextremely high thermal loads in terms of establishing asensitive optical structure. Its manufacturability was taken
into account on the design. Its structural integrity assessment was carried out. Thermal and electro-magnetic loads
are significant for the mirror box. The structural analysis results indicate the mirror box satisfies design criteria
in the RCC-MR code, except for the inner support parts
interfacing with the divertor cassette. Primary stress due
to mechanical loads concentrates on the support part andelectro-magnetic loads are dominant for the stress. A moredetail structure of the support parts and enhanced rigidity of the support parts will be considered. On the other
hand, if a radiative heating eﬀect due to reflected lights inthe divertor area is significant, it is necessary to consider
downsizing the mirror box and optics to reduce thermaland electro-magnetic loads.
[1] S. Kitazawa et al., Fusion Eng. Des. 101, 209 (2015).
[2] T. Oikawa et al., “Preliminary Design of Divertor Visible Spectroscopy in ITER Radiation Environment”, Rengo
kouenkai, Shiga, June (2018).
[3] T. Maruyama et al., Plasma Fusion Res. 11, 2405083 (2016).
[4] RCC-MR, Design and Construction Rules for Mechanical
Components of Nuclear Installations, AFCEN (2007).
[5] R. Villari et al., Fusion Eng. Des. 88, 2006 (2013).
[6] A. Encheva et al., Fusion Eng. Des. 86, 1323 (2011).
[7] F. Tachibana et al., J. Jpn. Soc. Mech. Engrs. 5, 102 (1952)(in Japanese).
[8] S. Kajita et al., Plasma Phys. Control. Fusion 55, 085020(2013).
[9] S. Kajita et al., Contrib. Plasma Phys. 56, 837 (2016).

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