J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 46'1470

Interaction between the Helical Goil Current and the Toroidal
Plasma Current in the Large Helical Device
YANAGI Nagato, CHIKARAISHI Hirotaka, IMAGAWA Shinsaku, SEKIGUCHI Haruo,
SAKAKIBARA Satoru, HAMAGUCHI Shinji, MITO Toshiyuki, SATOW Takashi
and MOTOJIMA Osamu

National Institute for Fusion Science, Toki 509-5292, Japan(Received:ll December 2001 / Accepted:20 June 2002)

Abstract
The current of the superconducting helical coils is affected by the change of the toroidal plasma
current in the Large Helical Device due to the tight magnetic coupling between the helical coils and thetoroidal plasma current as well as to the relatively slow response of the present PID control of the DC
power supplies. Since the helical coils are operated in a marginally stable condition, where a finite andtemporal propagation of a normal-zone is allowed, it is important to precisely evaluate the deviation of
the coil current due to the change of the toroidal plasma current. The temporal change of the helical coilcurrent is also examined by incorporating a simple circuit equation.

Keywords:LHD, helical coils, cryogenic stability, toroidal plasma current, PID control

1. lntroduction
During the five experimental cycles of the Largemaximum central toroidal field of 2.9 T 12-41, a sudden
and significant increase of the coil current in this region
may trigger a normal-transition, and in some cases leadto a coil quench which will activate an emergency
shutdown sequence of the system and terminate theHelical Device (LHD), which is steadfastly advancing
reactor-extrapolative plasma experiments with a helicalconfinement approach [], the superconducting coilsystem has been successfully providing the static
magnetic field with a heliotron configuration.

plasma experiment. In this respect, the relationshipbetween the change of the helical coil current and theSince the pair of helical coils (major radius: 3.9 m,

amount of the toroidal plasma current should beminor radius: -1 m, toroidal pitch number: 10 andtemperature: 4.4 K) generate both the toroidal andclarified. This is also important for designing the future
reactor-size machine with a helical approach.

poloidal magnetic fields, they are tightly coupled withthe toroidal plasma current which mainly generates thepoloidal field. Thus, the transport current in the helicalcoils is inherently affected by a fast change of the

2. Cryogenic Stability and Mechanical
Properties of the Helical Goils

plasma current. This is unavoidable when theup to the present time, it has been confirmed that thecharacteristic time of the change of the plasma current ishelical coils allow temporal normal-transition at acurrent and magnetic field slightly lower than theDuring the excitation tests systematically conducted

relatively shofter than the characteristic time of thepower supply control. Since the cryogenic stability ofthe LHD helical coils is presently marginal near thespecified operation point (coil current: 13 kA, magnetic@2002 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author's e-mail: yanagi@ LHD.niJs.ac.jp467

Yanagi N. et al., lnteraction between the Helical Coil Current and the Toroidal Plasma Current in the Large Helical Device

field: 7 T). Eight of the observed transition events wereJul.21, 1999

temporal where the generated nornal-zone shrank afterEa few seconds to result in the recovery of theQ) uDo) lz(uosuperconducting state [3,4]. One event that occurred in

1998, however, did not recover and an emergency

3I

shutdown program was activated as the quench detectors

were triggered [2]. In this event, a large amount of
helium gas was vaporized due to the Joule heating aswell as AC losses during the fast discharge with a time
constant of 20 s. It was estimated from the signals of thetemperature sensors distributed on the helical coil cansFthat the propagation of the generated normal-zone wasfinite in the longitudinal direction, and it seems probablethat the final quench was triggered by the deteriorationof the cooling condition due to accumulation of helium
bubbles at one cross-section in the coil blocks.

G(E(Edl
5.6

E 5.2-a.=dt!u.9 = 4.8-el-4.4

3000 3500 4000 4500

For the helical windings, a composite-type

5000

Time (s)superconductor is used, which consists of a NbTi/Cu

Rutherford cable, a pure aluminum stabilizer, and acopper sheath with electron beam welds. TheFig. 1 Typical example of the (H-l) balance voltage signals observed during excitation tests.

characteristics of this superconductor have been
intensively studied by carrying out short sample tests aswell as small R&D coil tests. It has been confirmed thatthe conductor becomes transiently unstable even when
the transport cuffent is lower than the steady-state "coldend" recovery current, and an initiated normal-zone
propagates over a finite length along the conductor. It

field is raised up to the original level. As is described in
the following section, it has been observed that thecurrent of the innermost blocks (H-I) of the helical coils
actually experiences a temporal deviation from the setvalue when the toroidal plasma current is induced
especially in neutral beam heated discharges. The
maximum deviation in the positive direction stays lessthan 100 A with the present experimental condition. In
this respect, the H-I coil current is always decreased byshould be noted that the magnetic diffusion time
constant in the pure aluminum stabilizer is relatively

long due to its low resistivity. Therefore, within thischaracteristic time, the longitudinal resistance of thestabilizer remains considerably higher than that in thesteady state, and this might cause a temporal

200 A before starting the plasma experiment.

3. Perturbation of the Helical Goil Current
by the Toroidal Plasma Gurrent

degradation of the cryogenic stability [2-4].
On the other hand, one also needs to pay attention
to the mechanism that initiates a normal-transition. We

During plasma discharges, the toroidal plasma
current is induced mainly by the two mechanisms:

consider that this can be explained by mechanicaldisturbances in the windings due to large

momentum injection by neutral beams (the generatedcurrent is so-called Ohkawa current) and self-induction
by the plasma pressure (so-called boot-strap cunent) [5].

electromagnetic force. During excitation tests, a numberof spike signals are observed on the balance voltage of

It has been observed that the H-I current is mostthe corresponding blocks of the pair of helical coils.
Pulse height analysis (PHA) has been applied to analyze
these spike signals, and it is observed that the spikesignals have clear exponential-like distribution functionssignificantly affected by the generation of the toroidal
plasma current and two typical examples are shown in
Fig. 2. In Fig. 2(a), the toroidal plasma current isinduced in the positive direction (the same direction
with the toroidal magnetic field) with a co-injection ofneutral beams (NB) as the dominant heating source. The

with a high-energy tail component [4]. Thus, the spikesignals are found to be very useful to examine themechanical properties of the coil windings. It should be
noted that when the coil current is slightly decreased,
the spike signals do not appear as is shown in Fig. l. In
this case, a few spike signals are observed when theplasma current reaches up to 80 kA in 10 s and itterminates as the NB is turned-off. Along with theincrease of the toroidal plasma current, the H-I current is468

Yanagi N. et al., lnteraction between the Helical Coil Current and the Toroidal Plasma Current in the Large Helical Device

decreased by 60 A. This can be explained by themagnetic flux conservation. When the plasma current(a)xterminates, the H-I current shows a sharp increase up to+50 A, again to conserve the magnetic flux. It should be
noted that during this process, the terminal voltage ofeach coil is controlled by the DC power supplies,
however, the time constant of the present PID control israther long (typically 10 s), and it is not sufficient tokeep the coil current at the constant value.

Oct. 18,2000
Shot #18827

80

o:60

(!

40

On the other hand, the plasma current is induced in20

the negative direction in Fig. 2(b). This is typicallyobserved in the fifth cycle of LHD experiments after thenew counter-injection NB line is installed. In this case,the H-I current is first increased by 60 A, and shows asharp decrease of -40 A when the toroidal plasma

EanF
010.56x10.6410.62

o'10.6510.58C)

current terminates. We note that during these processes,
the currents in the middle (H-M) and outer (H-O) blocksof the helical coils are almost unaffected. We consider

10.56o()

10.54
64og)
o=o

that this can be explained by the shielding effect of theH-I blocks. On the other hand, the currents in the threesets of poloidal coils show similar waveforms as that of0-2-4

the H-I blocks.

-oFigure 3 summarizes the relationship between the5101520

deviation of the H-I current and the amount of thetoroidal plasma current. In Fig. 3, the closed squares
correspond to the maximum change of the H-I current

Time (s)(b)J

Sep. 25, 2001
Shot #26514

observed in the phase that the toroidal plasma current
develops as the plasma is heated, while open squares
correspond to the sharp change of the H-I current afterthe plasma current terminates. As is seen in Fig. 3, when
the plasma current (10) is limited within the range of-100 < 1o < +100 kA, the change of the helical coilcurrent (A1g) remains within the level of -100 < A1H <0c{,(Jo -40(a -60[-ro11.08

1.
5 11.06
E 11,04

I rr.oz311'10.98=$ ro.eoo

9so:4
g2$u;-2

Looo0=oT

f-505101520

ooo)Time (s)-50Fig. 2 Waveforms of the toroidal plasma current, the coilcurrent and the terminal voltages of the innermost blocks (H-l) when the toroidal plasma cur--100

rent is induced in (a) the positive and (b) negative
directions. The two helical coils (H1 and H2) areconnected in series and the current is common.
The terminal voltages for the two blocks (H1-l andH2-l) are separately measured.

-50050100

Plasma Current (kA)
Fig. 3 Relationship between the change of the H-l coilcurrent and the amount of the toroidal plasma
current.
469

Yanagi N. et al., Interaction between the Helical Coil Current and the Toroidal Plasma Current in the Large Helical Device+100 A, and there is no concern about the stability
deterioration of the helical windings under the present

show fairly good agreement with the measured H-I current. The small deviation from the actual waveforms
might be explained by the skin effect in the stainlesssteel plates used for the helical coil case, the plasma

experimental condition.

4. Discussion

vacuum vessel and the 80 K radiation shield panels.

It should be noted that by reducing theHere we examine whether the change of the helicalcoil current can be simulated by the temporal change ofcharacteristic time constant of the DC power supplies by
having a higher gain in the present PID control or by

the toroidal plasma current and the terminal voltage

which is controlled by the DC power supply, We

incorporating a more sophisticated control logic, such as
the so-called H-infinity control [6], it might be possible
to reduce the deviation of the coil current.

consider a simple equivalent circuit formed by the H-I
blocks, the poloidal coils and the toroidal plasma
current. The circuit equation gives-- d/" V",=0, il^,, ^, d1rc* Mut_pdt"rff+M"r_rc
di5. Summary
The helical coil current is affected by the magnetic
coupling with the toroidal plasma current in LHD. The
current in the innermost blocks changes with thedevelopment of the toroidal plasma current in order to(1)where Ls1 denotes the self-inductance ofthe H-I blocks,

Mut-pc is the mutual inductance between H-I and thepoloidal coils, Mgy-o is the mutual inductance between
H-I and the toroidal plasma current, I and V are the current and voltage of the corresponding circuits. We
should note that the waveforms of all the poloidal coilcurrents are found to have very similar shape as that ofsatisfy the magnetic flux conservation under the present
condition with a relatively slow response of the PID
control in the DC power supplies. It has been confirmedthat the deviation of the coil current is within the range
of +100 A when the amount of the toroidal plasma

currenr is limited within +100 kA. This condition is
acceptable from the standpoint of the cryogenic stabilityof the helical coil superconductors. The coil current canbe well simulated by a simple circuit equation

the H-I current, and the following equation holds:

r {I! , t.t"*dt, dl*
6, -- "ett 6, .

dlPc

where Z"1i means an effective self-inductance of theoverall coils. Thus, by integrating eq. (l) in time, weReferencesA.
Iiyoshi
Fusion 39,1245 (2000).
etal.,Nucl.
t1lN.
Trans.
Appl. Supercond. 10,
Yanagietal.,IEEE[2]obtain
1,, =*(Jv* dt - tttrr-ot,).

(2)

610 (2000).
[3] S. Imagawa et al.,IEEE Trans. Appl. Supercond.

By conducting numerical calculations regarding themagnetic field generated by the coils and the toroidal

11, 1889 (2000).

t4l N. Yanagi et al.,IEEE Trans. Appl. Supercond 12,662 (2002\.

plasma current, Ls11 and Mst_, are evaluated as 1.57 H[5] K.Y. Watanabe et al., this conference.

and 1.86 mH, respectively. In Fig. 2, the waveforms of
the H-I current obtained by Eq. (2) is plotted withdashed lines for the two cases. The simulated curves

t6l H. Chikaraishi et al., this conference.

4'70