J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 25U254

Strain Monitoring System for Gryogenic Structureof Large Helical Device
NISHIMURA Arata, TAMURA Hitostri, IMAGAWA Shinsaku and SATOW Takashi
National Institute for Fusion Science, Toki 509-5292, Japan(Received: 11 December 2001 / Accepted: 3 June 2002)

Abstract
The Large Helical Device has the world's largest class cryogenic structure to support huge electromagnetic force generated by superconducting coils. The strain monitoring system was designed andinstalled in the Large Helical Device to evaluate the soundness of the large cryogenic structure and themeasurement has been done continuously during the operation. Some experimental results are presented

in the paper and the strain behavior is discussed. No significant difference ofthe strain has been observed
up to now. The fact gives a conclusion that the cryogenic structure is working on well without any faults.
By piling up the data sets, the understanding for the cryogenic structure will get ahead and the certainty
and reliability for the fault or damage will be enhanced.

Keywords:
Large Helical Device, cryogenic structure, strain measurement, superconducting coil, electro-magneticforce

stresses and strains and to evaluate the condition of the1. Introduction
A large superconducting coil system wasconstructed for the Large Helical Device (LHD) tostructure under electro-magnetic force at 4 K. The strain
monitoring was carried out under the same operation

mode and the soundness of the structure was evaluatedcomparing with the data obtained before. Since such a
strain monitoring system was not developed before in

generate a strong magnetic field of up to 3 T at plasma

axis [l]. To support the superconducitng coils and thehuge electro-magnetic force, a cryogenic structure wasdesigned and constructed I2l. The austenitic stainlesssteel (SUS316) was used as the stmctural material andthe maximum thickness was 100 mm [3]. The support

the large plasma experimental devices, it is very
important to develop the monitoring system andinvestigate the status of the support structure from thestructure is cooled down to 4 K together withpoints of device engineering and maintenancesuperconducting coils at each campaign and endures theelectro-magnetic force during coil excitation. The

engineering views for a fusion machine.
In this paper, an outline of the strain monitoring
system is described and some data are presented. The

plasma experiments have been conducted successfully[4] and the fifth campaign began on September, 2001
and is going on now.

data shows that the cryogenic support structure in theLHD is working on well without any problems.

The LHD is the first machine of which coils are allsuperconducting and will be operated for a long time.
So, a strain measurement and monitoring system wasconsidered and installed in the LHD to measure the2. Outline of Strain Measurement
For an application to the LHD strain monitoring@2002by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author's e-mail: nishi-a@ LHD.nifs.ac.jp
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Nishimura A. et al., strain Monitoring system for Cryogenic structure of Large Helical Device

system, a new strain gage was developed to have small
apparent strain at 4 K and had polyimid coated gagelead wires which allowed fine-twist of the wires [5,6].
The gage has zr base diameter of 20 mm, a gage length

cryogenic structure. Two pairs of twisted wires wereagain twisted, and the cable was electrically shieldedand finally covered with a fluoroethylene polymer. One
twisted pair was used for a constant input current andthe other was for an output voltage.
The LHD has a cryogenic support structure whichsustains one pair of helical coils and three pairs ofpoloidal coils. All coils are superconducting and cooled
down to 4 K together with the support structure. The
stnrcture consists of ten sectors and one sector covers 36
degrees at center angle. The top and the cross sectionalviews of Sector I are shown in Fig. 1. On the top view,
inner vertical (IV), inner shaping (IS) and outer vertical(OV) coils are located and fixed to the support structure(shell structure) with coil frames. On this cross section,
helical coils are running at the top and the bottom sides.
The helical coils are set in the coil can and welded toof 5 mm, an electric resistance of 350 C) and a gagefactor of about 2.0.

A Wheatstone bridge was formed near an active
gage using three dummy gages to reduce thetemperature effect on the apparent strain and to cancel
the apparent strain induced by the fluctuating magneticfield. The dummy gages were attached on a so-called
dummy stage, which was a 3 mm thick stainless steel
plate clad with copper, welded near the active gage. An
adhesive of a polyurethane system was used for theactive gage, because the agent solidified at room
temperature and it was hard to heat up the support
structure. For the dummy gage, a phenol system wasapplied and the stage was heat- treated at 353423 K tosolidify the agent. A cable connecting the Wheatstone

the support structure with two shell arms. On theassemble process, each sector was welded in upper half

bridge to a feed through connector was also newly
designed and fabricated taking account of reduction of
the electric or magnetic noise and heat load to theand lower half independently and then both halves werewelded at the inner and outer equators. Therefore, there
are cross joints of welding at the cross point of theequators and toroidal cross section of the sector. The
strain gages were attached on the cross point of the

IS coil OV coilwelding passes [7].
Tri-axial strain gages (three gages were set in onebase) were used, and three tag names were prepared forone strain gage base as shown in Fig. l. HSNE3109 andHSNE3I09

HSNE3l12
HSNE3l13
HSNE3l14(Outer equdor)

HSNE3ll0

HSNE3III(Inner equator)

3l12 show the strains in the toroidal direction,
HSNE3Illand 3114 indicate those in the poloidal
direction, and HSNE3l10 and 3113 are those in 45
degree direction to the torus. HTE3I17 and3219 areilI]832

thermo- sensors (Cernox) located under IV-U coil frameand on the outer equator. In the case of Sector 2, thesecond number from left side in the tag becomes 2. For
example, HSNE3 111 and HSNE3 114 change to(a) Top view of Sector 1.

Helical coilCenter ofLHD

HSNE321 I and HSNE32I4 in Sector 2.

Since the helical coils are pool boiling typesuperconducting coils, the coil can is filled with liquid
helium and the temperature is kept at the boiling

HSNE3I12
HSNE3l13
HSNE3l14

I'u

HTE3IIT(Outer equaror)
109

HSNE3lIO

HSNE3III

temperature of the helium. The poloidal coils are forced

TITE3I 19flow type superconducting coils. So, supercritical

HTE32I9

helium is forced to flow into the conduit of the coil. The(Inner equator)
Crvosenic

inlet temperature is controlled at 4.5 K under the gage

subdningposr

pressure of 0.9 MPa. The cryogenic support structure is-cooled by two-phase helium at around 4.5 K using 40
parallel cooling channels. Depending on the quality ofthe two-phase helium, the inlet temperature changes
slightly during the cooling.

(b) Cross sectional view of Sector 1.
Fig. 1 Cryogenic structure of LHD and location of strain
gages.

251

Nishimura A. et aI., S1ain Monitoring System for Cryogenic Structure of Large Helical Device

the relation becomes straight line when the structure
responds to the electro-magnetic force elastically. The

3. Results and Discussion
The LHD plasma experiment program has several

strains on the inner equator are smaller than those on theouter equator, for the outer vertical coils generate huge

operation modes to create different magnetic surface. In
this paper, the results obtained under #1-d mode withgamma of 1.258 will be presented. Naturally, the strain

tensile electro-magnetic force to make an inward shift ofthe plasma axis.
The results of HSNE3I I I shows a hysteresis curvecondition of the structure depends on the electromagnetic force generated by the superconducting coils.
So, the operation mode was fixed to compare the data

and this would be caused by the different deformation
process of the helical superconductors on the ramp-up
and ramp-down processes [7]. On the other hand,
HSNE3 1 14 presents linear relationship. Thesesets easily.

The helical coil has 450 turns of superconductors
which were divided into three parts, i.e., inner part,
middle part and outer part designated as H-I, H-M andH-O, respectively. The inner part is the nearest to theplasma. The current of each part can be operated
independently. Under #1-d mode, a radius of themagnetic axis is 3.6 m and the currents of each part of
the helical coil are 10.749, 11.628 and 11.823 kA for HI, H-M and H-O respectively at 2.85 T at the plasma
axis. It results in gamma of 1.258. This current

tendencies are observed both on October 4,2000,January 19, 2001 and November 29, 20O1.

To investigate the difference and the scatter ofthese hysteresis curves, the measured strains werecompared at the same magnetic field. The results are

H-I/M/O: 10.749ll 1.628/l L823kAo.2

distribution gives the current center outward shift in the33miner radius direction.
The LHD has no electrical break in the torus. So,
an eddy current runs around and heats up the structure
when the magnetic field increases or decreases soz.) @0.15

33! o.l
g 0.05quickly. This temperature rise makes the strain;measurement difficult, because the additional apparent
strain appears. Therefore, the lower ramp rate of 0.02 TlF"'3AUE-0.05

min was adopted.
Figure 2 shows one example of the slower ramp-up
and ramp-down operation conducted on November 29,
2001. The operation mode was #1-d with 1of 1.258 ast0-0.1

9:00

10:00 1 1:00

12:00 13:00 14:00

Time (hour:min.)Fig. 2 Change in temperature and magnetic field under#1-d mode with 1of 1.258 measured on Novem-explained above. The temperatures near the inner
equator and on the outer equator vary within about 0.02

ber 29,2001.

K. The temperature change on the inner equator
becomes larger than that on the outer equator asreported before [5], and this fluctuation is considered tobe caused by the change of the quality of the flowing
helium.
Such slow ramp rate operations were carried out
three times to measure the strains under coil excitation.
The first was carried out on October 4, 2000 at theF
L6

beginning of the forth campaign, and the second wasdone on January 19, 20Ol at the ending of the campaign.
The last was performed on November 29, 2001 during

'5o4

2

the fifth campaign.
The examples of the measured strains are shown inFig. 3. In the figure, the results of HSNE3 I I I and 3l 140

are described and those are plotted against the square of

the magnetic field at the plasma axis which isooo1t;-Strain (pe )Fig. 3 Comparison of hysteresis curves between squareof magnetic field and measured strain.

corresponding to the electro-magnetic force. Therefore,
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Nishimura A. et al., Strain Monitoring System for Cryogenic Structure of Large Helical Device

shown in Fig. 4. Figure 4 (a) indicates the results ofIn both figures, the total error will become +/- three

January 19,2001 against October 4,20OO and (b) is theresults of November 29,2001 against October 4,2OO0.

digits. Therefore, +/- three digits boundaries, whichcorrespond to about +l- 7.5 pr, are displayed by thedotted lines. It is very clear that all the experimental
points ofHSNE3lll and 3l14 on January 19, 2001 andNovember 29, 2OOl are plotted within the +/- three

The data acquisition system has an error of +/- onedigit fundamentally. So, the error of +/- two digits isinvolved in the results shown in Fig. 4. In addition,
another error of +/- one digit will be generated when thedigits scatter bands.
From these results, it can be concluded that thecryogenic support structuie is not damaged at all after
the plasma experiments in the forth campaign and thedata are compared each other at the same magnetic field.

a250

warming up and cooling down processes before the fifthtc.l

200#l-d mode

campaign.

B.ou* = 2'857

The change in the measured strain on each sector issummarized in Fig. 5. The number in the horizontal axis

1=. 1.258>|indicates the sector number and measured strains ofHSNE3XI I and 3X14 are plotted in the correspondingbox, where X is Sector number. Round, triangle andsquare symbols show the result of each monitoring on
October 4, 2000, January 19,2001 and November 29,
2001. Depending on the sector, the measured strainchanges very much, for the horizontal port shape anddimensions are not the same. In addition, the location of
the strain gage is not entirely in the symmetric position.
However, when each three data sets are compared, thedifference is within three digits, and mostly two digits.

.''-.-'-,:.,

150

HSNE3I1000)

50C)

00

100 150 200

502so
Measured strain on October 4, 2000 (pe)Taking account the error of two digits into
consideration, it is possible to make a conclusion that(a) Comparis;on of strains measured on October 4,2000 and January 19,2001.

the strain does not change and that the cryogenic
structure is not damaged at all up to now.
These results described above reveal that the strain

6I 250(.r 200#l -d mode
BInax = 2.857

o{^l '= 1.258measurement system installed in the LHD is very
effective to evaluate the status of the cryogenic support
structure during the coil excitation. The continuous dataot rso; SN E 3 t4IIC)

zE 100.F'..

!

i1t.= r50

I50'oo

350

100

150

200iiorrlii

iiior.
oArii

OOctober4,2000

aJanuarylg,2oolo November 29' 2001

,a.,

ill

EiiIiloiio^rio^rlI

Eie 100'rr i sotia mark: HSNE3l 14liiiiOA!.
ioali ,,Jnn I"^. !iioa_iOA!
iono iontr
io^tr] tOA;iio -iI i "io0Ii::t:iiii250;

Measured strain on October 4, 2000 (pe)Open mark: HSNE3I I 1o 2 4 6 810121416182022242628303234363840(b) Comparison of strains measured on October 4,2000 and November 29,2001.

Sector number

Fig. 5 Measured strains at 2.85 T under #1-d (^1 = 1.2531
during the forth and the fifth campaign.

Fig. 4 Comparison of scatter of data set.

253

Nishimura A. et al., Strain Monitoring System for Cryogenic Structure of Large Helical Device

acquisition and piling up the data will enhance thecertainty of the monitoring system and the reliability ofthe evaluation of the cryogenic support structure. It is(2) The hysteresis curve between the square of the magnetic field and the strain does not change on October 4,
2000, January 19,2O0l and November 29,2001.
(3) The total strain under the change of the magnetic
field from zero to 2.85 T shows no significant difference

expected that the system will give a special waming ona certain fault of the structure before the trouble occurs.

among three monitored data sets.

4. Summary(4) From these results, it should be concluded that theThe LHD has the large cryogenic support structure
which sustains one pair of helical coils and three pairs

cryogenic support structure in the LHD works wellwithout any problems up to now.
(5) The strain monitoring system is working effectively
and expected to give a warning before unexpectedof poloidal coils. To measure the strain during coil
excitation and to evaluate the soundness of the structure,
the strain monitoring system has been considered andengineering. During the consideration, new gages andtrouble.
(6) The continuous data acquisition will make the certainty and reliability ofthe system higher and the systemwires were designed and developed. The strain

work more effectively.

installed in the LHD as part of the fusion device

measurement has been performed continuously and theinvestigation on the structure has been carried out under

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l2l T. Satow et al.,Proc. MT-15, 377 (1998).
t3l A. Nishimura er al.,Fusion Technol., 873 (1998).
t4l M. Fujiwara et al.,Nu'cl. Fusion 41, 1355 (2001).
t5l A.Nishimura et al.,Proc. MT-15, 1275 (1998).
t6l A.Nishimura et al., Adv. in Cryo.Bng.45,745

purpose.

In this paper, the outline of the strain monitoring
system is described and some results are presented. The

main results are summarized as follows:(1) The slower ramp rate operation condition, (#1-dmode, y = 1.258, maximum magnetic field; 2.85T) gives
a clear data set of the strain monitoring for the cryo-(2000).

t7l A.Nishimura et al.,2lst SOFT, F-42 (2000).

senic structure.

254