J. Plasma Fusion Res. SERIES, Vol.l (1998) 52_61

overview of LHD (Large Helical Devicef project
FUJIWARA Masami*, MOTOJIMA osamu, HAMADA yasuji,

WATARI Tetsuo, oKAMoro Masao,sAToH Sadao, yAMAZAKT Kozo, sUDO Sigeru, NODA Nobuaki, oHyABU
Nobuyoshi,
YAMADA Hiroshi, KOMORI Akio, MITO Toshiyuki, KUBO Shin, ToI
Kazuo, ICHIGUCHI Katsuji,
KAWAHATA Kazuo, IMAGAWA Sinsaku, the LHD Group and IIyoSHI
Atsuo
The National Institute for Fusion Science, Toki 509_5292,Japan(Received: 30 September t997/Accepted: 22 October 1997)

Abstract
The t-arge Helical Device (LHD), the large scale superconducting helicaldevice, is under construction at the National Institute for Fusion Science and is now in the last year of the g-year

construction
schedule. The outline of the LHD program, the status of the on-site construction
are described. The
confinement and steady state operation are experimentally studied on hightemperature plasmas relevant for fusion plasmas in a helical device with a size comparable with large
tokamaks. The device will
be completed at the end of 1997 and the initial operation, cooling down
and the machine running will
start from the beginning of 1998.

Keywords:
Large Helical Device, physics design, SC magnet technology, device construction
and operation,experimental schedule

1. Introduction

mined R - 4 m, B - 4 T by the reasonable level of heating power 20-30 MW where the heating power density
is similar to those of existing devices.
(3) The configuration has been selected as an ! : 2 he-

LHD (Large Helical Device), the superconducting

I / m : 2/ l0 helical device of R / a (the major/minorradius) : 3.9 m/0.6 m and B : 3 T, is constructed todevelop a steady state fusion reactor[1]. The mission isaccomplished by the realization of ( T") : < 4 > :3-4 keV, 1n) :liotron,/torsatron configuration because of the existenceof integrated data base and of the experience obtained
from H -EICHS/ATF construction.
(4) Other configuration parameters such as m, the toroidal mode number, 7, the pitch of the helical coils
defined as ma"/ lR, a, the pitch modulation of the helical coils, and the required dipole/quadrupole fields aredetermined by MHD studies on B limit, particle confinement capability, the bootstrap current control etc.
(5) As another important factor to accomplish theLHD mission, steady state operation, the all coils are in
the superconducting state and the continuous helicalwinding is employed to create the divertor structurewith clean helical separatrices for easy particle,/heatl02o m-3, and z.:0.2-0.3 s with

20 MW heating under B : 4 T. LHD has been designed based on the following guideline.

(1) The target plasma parameters are the several-10
keV temperature, l0r3 cm-3-10r4 cm-3 density, and )0.1 s energy confinement time, which cover non-dimensional parameters, the collisionality of ion and electron,* : qR/ )' (the connection length divided by the meanfree path) L0'3-10-4 similar to that in fusion plasmas,<p> - 5o/o, etc.

(2) To realize these parameters based on the LHD
scaling, which is now modified to ISS95 scaling, the de_vice scale and the magnetic field were roughly deter' Corresponding@1998 by The Japan Society of plasma
Science and Nuclear Fusion Researchauthor's e-mail: fuj iwara@ nifs. ac.jp
57

Fujiwara M. et a/., Overview of LHD Project

helical coils consists of 2 independent coils which canbe operated independently for the study of helical axis

control. Of course, the helical configuration is very sensitive to the coil structure and the real machine parameters are compromised with the SC technology require-configurations.

ments (Fig. 1 and Table 1).

2. Construction Status of LHD Device, Heating Devices and Diagnostics

These design procedures are quite different fromthe W7X design where the magrretic configuration isoptimized for core plasma stability and confinement

LHD device construction had started from 1990 as
the 8 years schedule and various R & D research forSC technologies, gyrotron development, NBI ion

and then the coil structure is determined [2]'(6) The other important point is the flexibility of theconfiguration which is a powerful tool for study thesource, plasma facing components and others'(1) Almost all of main components of LHD device
were completed and are under installation into thecryostat. The key elements of LHD, helical coils (13kA conductor using NbTi stabilized Al, pool boilingtype cooling) and poloidal coils IVIIS/OV (21-31 kANbTi conductor of cable in conduit type, forced flow

optimum configuration for the plasma confinement'
LHD coil system has various functions to satisfy theserequirements. The change in y by the independent
operation of three blocks of helical winding can control
L(r), %, and shear. The three pairs of poloidal coils cancontrol the position of the mapetic axis and the elongation of the plasma cross section. T\e ! / m : 2/ l0

cooling by SHe) have been already completed and in-stalled in the cryostat fixed by the supporting shellstructure. The mechanical accuracies, which are key
parameters for obtaining clean nested magnetic surfaces, for helical coils and poloidal coils are listed inTable 2. The overall accuracies for helical coils are

Table 1 The mechanical accuracy of LHD coils<Errors of Fabrication of Helical Coils (3 o)>

Bobbin Winding Welding(HCMinorcan)

radius t1 .43

Overturning 11 .50
Major radius {ave.) -0.6

Total

to Shellwithin 2 mm. Main components of the inaccuracy is
considered tobe m: 10 and lower mode number com-.07 < +1 .O < +2.1 mm< 10.75 4 +'1 .0 < 12.0 mm-0.6 -1 .5 mm-0.3
11ponents are much lower. The thermal contraction ( - 10mm) by the cooling down to 4.4 K is almost uniform in
the major radial direction, and the expansion by theelectromagnetic force under 3 T operation is 1'6 mm in<Errors of Fabrication & Installation of Poloidal Coils>

Each

Coil Installation Total

the minor radial direction. These deformations do notcontribute to low mode error fields, and the clean magnetic surface is obtained in LHD device.
The vacuum vessel (discharge chamber) is made by

Pancake (ave.1 into ShellMajor radius
Height<12.0 < 10.5<10.5-< t1 .0< +1.5 mm<t1 .0< t1.5 mm

Ouler 80 K
Radiation ShieldSupportingStructuro
Helical Coils
Heical Coil Cans
Shell Arms

Major radius
Coil minor radius
Plasma radius
Plasma volumel/mr (0) /c (a)Helical ripple € h(a)%/Bmax

Poloical Coil Supports
Poloidal CoilslV CoilslS Coils
OV Coils
lnnor 80 K
Radiation Shield
Plasma VaccumVessel

Helical coil current
LHe temp.
Poloidal coil (IV/IS/cV)
Magnetic en€rgy
Plasma duration
Repetition time
Heating pof,erECRH
NBI
lcRF

Fig,lTheCross-sectiona|viewofLHDdeviceandmachineparameters)6

3.9 m
0.975m
0.5-0.65m

zo-30m32t10<0.5/1o.2
3 /6.6T(4 /9.2T)

5.85MAT(7.8MAT)
4.4K (1.8R
5.0/-4.5/-4.5MAT
0.9GJ (1.6GJ)

10s-CW
5 min.
1OMW

15MW (20Mw)

3MW(12MWIFujiwara M. el a/,, Overview of LHD proiect

welding many stainless steel plates of which thickness is
15 mm. The vacuum vessel is covered with the first wall

main heating method for ECH plasmas, are now underconstruction. The results using a test ion source arelisted in Table 2 and the extracted negative ion beam
current reaches 21 A, which is a half of the target valueof L0 mm stainless plate which is cooled by water inorder to operate plasmas in steady state heating.

(2) The coils, the supporting structure, and the dis-of. 40 A/ion source, with 10 mrad. divergence andbeam size. The ICRF heating system has been also developed and two kinds of launcher are prepared for thecharge chamber are installed in the large cryostat. Thecoils and the supporting structure are fixed at the bottom plate of the cryostat through 10 cryogenic support
columns. The discharge chamber is supported by theother structure and the port is connected to the outer

initial experiments using a half turn antenna for fast
wave heating (1 MW level) and the folded wave guidefor Ion Bernstein Wave heating (0.5 MW level).
(6) Various diagnostic instruments are also under con-cryostat port flange through the bellows to keep thedischarge chamber free mechanically from the crytostat

struction, 12 channels FIR interferometer with 119
pmlCHTOH Laser, YAG Thomson scattering systemsystem.

(3) The helium refrigerator/liquefier has the coolingcapability of 20.6 kW at 80 K, 5.67 kW at 4.4 K and(200 channels / > 300 Hz repetition rate), and a normal
incident VUV spectrometer. The bolometer systems isof 650 l,/hr. simultaneously. These are enough for therequirement of 13.75 kW at 80 K,2.05 kW at 4.4 K
and 650 ! /hr in order to cool LHD coils, support

time and high spatial resolution. The Heavy lon BeamProbe (6 MeV) is developed for the direct measurement of 8,. A multi-layer pellet is developed as the650 l/hr. liquefaction. It also can deliver liquid helium

for local/global measurement of radiation with faststructure, SC bus lines, 80 K shield and other components [3].
The superconducting bus lines are used to connect

new diagnostic instrument.

3. Machine Build Up and Experimental Schedule of LHD

coils with power supplies which are placed 50-60 m
away from the device in order to reduce large ohmic
loss with conventional water cooled bus lines. The SC
bus lines have been also installed

After the construction, the LHD is in operation by
the following steps.

(1) Simultaneous pumping down of the cryostat andthe discharge chamber by the use of pumping sysrem
which are TMP (5500 !/sx2)/cryopump (25000 !,/sx2) for the cryostat (volume 900 m3, 10-5 Ton.) andTMP (5000 4/sx2)/cryopump (43000 t/sx2) for thedischarge chamber (volume of 200 m3,/surface area of730 m2,3 x 10-8 Ton).
(2) Cool-down procedure is as follows. First, 300 K to
80 K precooling will be accomplished by mixing ofwarm helium gas with LN, cooled helium gas. Further
cool-down to - 6 K will be conducted with low temperature helium gas from the refrigerator. Finally, twophase flow helium will be supplied to helical coils, supporting structures and SC bus{ines while poloidal coils
utilize forced flow SHe (0.97 Mpa at a.5 K)[6].
(3) The measurement of the magnetic surface using
electron beam tracing is carried out by the use of the[4].

(a) As the divertor experiments in LHD, the closed
helical divertor fully around the torus is the final one tobe studied. In the early experimental phase, however,
LID (Local Island Divertor) and a local helical divertor
are studied. The LID coil, which is used to control islands (m / n : L / | or 2/ l), is mounted on the cryostat.
The LID coil can be also used to correct the low (m, n)enor fields [5].
(5) The heating devices, ECH, NBI, and ICRF are prepared for LHD experiments. ECH is used for theplasma production and electron heating using two kindsof gyrotron 84 GHz and 168 GHz. The R&D results
are 400 KW/10.5 s-500 kW/2 s for 84 GHz and 450
KW/0.1 s for 168 GHz. Two NBI injectors, which isTable 2 Test results of NBI!o. oll0tgs
154

(H-)

lsmA)

keV)

t.5

104(Mw)to.l(13')\o.2

770(13C )1Targel70

t0

1E07.2'70)Ox2

120

t.5

andryclescanning fluorescent rod.

)urationt@m)iverge

team:ocuss)mrad.)

cm):nergy10t0

1.22101010(a) As the first step of the discharge cleaning, 2.45
GHz/20 kW/CW ECH plasma is used to reduce thel4cm
10.4m
?4cm

10.4m

1/5 siz6

impurities such as oxygen sticking on the stainless steel
first wall of the discharge chamber and { gettering system is prepared as the back up. In the later stage, theboronization is possibly applied. The effective conditioning by LID (Local Island Divertor) is now under

1/1 size;2cm13m

Injdr

i2cmx2 ources

13m

Injdr

study.
59

Fujiwara M. et al., Ovemiew of LHD Project

ECH is planned up to 3-9 MW using 168 GHz gyrotron and the power of NBI (180 kev) is raised up to 15(5) The first cycle experiment is using two 84 GHz gy-rotrons (500 kW x 2/t0 s) and possible use of 168GHz gyrotrons (400 kW x 2/2-3 s) for B : 1'5 T
operation. The simple estimation under the following
conditions is shown in Fig' 2.

MW. ICRF power is also increased 3-9 MW.

(8) As for the divertor experiments, LID is tested in
IggS-tggg and helical divertor is examined in the onen"(r\ : n"o(l - p2 )0's + 0.I' in the unit of 101e m-3

pitch of helical structure [5]'T"(r\: 7"0(1 - p')' + 0'05 in the unit of keV
Ti:0.27", and Z.o: 1.6 (1'"/" OxYgen)
Here, the impurity transport D : L m2ls is assumed.

first and gradually increased up to 3 MW depending on
the development of the heat removal method in
1ee8-2000 [7].
The basic physics operation (the first phase ofLHD project) is planned as described above [8]. The
one of the most important issues to be studied is theconfinement improvement to demonstrate the targetparameters /-7") : <Tr> :3 keV, 1n) : l02o
rrt-3, tp.:0.2 s with 20 MW heating under B:3 T'
The operation regimes in the physics phase are plottedin the nz T-Top plane as shown in Fig. 3.

(9) The steady state operation of 1 MW level is tried at

The fraction of the radiation loss determines the rangeof the operation plasma density and for 1MW ECHcase as an example 2.5 x l01e m-3 is expected to be apossible plasma density based on the experience ofCHS experiments. The central electron temperature is
1.5 keV.

Diagnostics prepared for the 1st cycle experiment

In the upgrade phase (2nd phase), the magnetic

are 2 mm interferometer for nl, ECE radiometer forQ(r), X-ray PHA for ?. and impurity contents, I2/2O
channels bolometer using thin metal films for P,,o(r),Visible spectrometer (200-700 nm) for no(H) and f,'field is increased to 4 T by 1.8 K operation of the helical coil, the tull helical divertor equipped with baffle
plates, by which closed divertor operation is possible,
will be installed and the D beam will be injected into D

and magnetic measurements such as magnetic probes,plasmas.

Rogowskii coils, and diamagnetic loops'(6) The second cycle experiment is carried out in thelater of 1998 by the use of ECH and NBI. The energyof NBI is 120 keV because of the lower field (1.5 T)
operation and the power is 5 MW in total (2 injectors).

102The diagnostics available are FIR interferometer,
Thomson scattering for T"(r)/ n.(r) and VUV spectrometer.

(7) At the end of FY1998, the machine is tested for 3
T operation and heating power is increased more. The

1noTioTE(1020m-3. keV.s)(1)B-r.5T,P=1MW

rc-2(2) B-1.sr, P=sMw(3a) B-3T, P=l oMwto:(3b) B'3T, P-l MWP;=1 MW
2.o1o -4

F
1.0

1o -6
0.5

1.01.5

2.O 2.5
4 (101em-3)

3.0

3.5

rc-2 'lfiz 104 106 1OB 1010
Torl.s (s)Fig. 2 The density and electron temperature estimated for
ECH plasmas in the initial operation.

Fig.3 The progress of LHD plasmas in nz T-Top (theplasma duration).

60

IFujiwara M. et al., Overview of LHD proiect

References

t1l

A. Iiyoshi et al., Fusion Technology 17, 169(1990); O. Motojima et al., I3th International
Conf. on Plasma physics and Controlled Nuclear
Fusion Research, Washington, 1990, yol.2, p.709(1991); K. Yamazaki et al., I3th International
Conf. on Plasma physics and Controlled Nuclear

Fwion Research, Washington, 1990, Vol.3, p.513(1991); M. Fujiwara et al., The proceeding of l0th
International Conf. on Stellarators, Madrid, Spain,p.175 (1995); M. Fujiwara er a/., plasma phvs.
Control. Fusion 39, A26I (1997).

I2l Wendelstein Group; WENDELSTEIN VII_X,
Application for preferential Support, Ipp-EU_RATOM Association. 1990.

61

t3l S. Satoh er a/., Fusion Eng. Design 20, I2g (1.gg3).
I4l S. Yamada et al., IEEE Trans. on Magnetics 32,2422 (1e96).

t5lA. Komori et al., in these proceedings, p.39g(1998); N. Ohyabu et al., in these proceedines.
p.13s (1ee8)
t6l T. Mito et al., proc. CEC/ICMC (1997), portland,
CPH3.
17l N. Noda et al., in these proceedings, p.130 (1993).

l8l

S. Sudo et al.,in these proceedings, p.126

liela;.