J. Plasma Fusion Res. SERIES, Vol.5 (2002) 265-269

Real-Time Plasma Current Control Experiment
NISHIMURA Kiyohiko, TAKAMI Shigeyuki, NARUSHIMA Yoshiro, CHIKARAISHI Hiroraka,
TAKAHASHI Chihiro, TAKEIRI yasuhiko and LHD Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japan(Received: 11 December 2OOl I Accepted:4 Apil2012)

Abstract
The plasma current control is useful in plasma confinement studies, because a plasma current gives
a great influence to the plasma confinement and stability even in a helical system. For this reason, thereal-time plasma current control (RCC) system using a standard personal computer, DOS/V machine, wasdesigned and constructed. The coil currents for the plasma current control were calculated using thestandard proportional-integral control algorithm in the digital signal processor and sent to the reflective
memory with 100 msec period.
Before the 4th experimental campaign, the RCC system was modified to isolate from the present

Large Helical Device power supply control system. During the 4th experimental campaign, modified
RCC system was used for the real-time feedback control of the plasma current. The plasma was produced
and sustained by the Neutral Beam Injection at the magnetic field strength of 1.5 Tesla. The ohmic
current was induced by changing the inner vertical coil current that produced the vertical magnetic flux.
The plasma current of about 30kA was reduced by the RCC system. This rate was the upper limit of thepresent coil power supply. The simulation of the plasma current control by ohmic current was carried out
using a single filament model. This result shows a good agreement with the experimental result.

Keywords:
superconducting heliotron device, reflective memory, real-time coil current control, plasma current control, PI control

1. Introduction

Heliotron type device to form the confining magnetic
field, a Heliotron type device is suitable to investigate

The Large Helical Device (LHD) tll is the largest
superconducting heliotron type device with a set of I =the affection of these currents to the plasma

2/m = l0 continuous helical windings. The major andaveraged minor radii are 3.9 m and 0.65 m, respectively.
Key issues of the LHD project are the demonstration ofcurrentless steady-state plasma operation and theachievement of reactor-relevant plasma parameters(confinement of a high beta plasma). To achieve theseissues, real-time control of plasma parameters is

confinement. For this reason, real-time plasma current

control (RCC) is one of the essential issues for theplasma physics research and the RCC system has been
designed and installed in the present power supply (PS)system [3,4] of the LHD.

2. Real-Time Coil Gurrent Control (RCC)
Systemessential [2,3].

There exist some plasma currents like bootstrap
current, beam driven current by neutral beam injection
and so on. Since the plasma current is not essential for a

2.1 Gonstruction of the RCC System
The LHD superconducting coils consist of one pair
@2O02 by The Japan Society of Plasma

C o r r e sp o ndin g aut ho r' s e - mail : ni s imura @ LH D. nifs. ac.j p

265

Science and Nuclear Fusion ResearchNishimura K. er al., Real-Time Plasma Current Control Experimentof helical field (HF) coils and three pairs of poloidal

Figve 2 shows the structure of the modified RCC

induce the plasma current by changing the inter-linkage

flux for plasma. The RCC system was designed tosystem connected to the PS system. Previous system
was linked strongly with the PS system by the reflective
memory (RFM). If the RFM of the previous RCC

control the plasma current of LHD [3]. The RCC system
calculates the necessary coil currents using measured

system stops by the accident, the PS system cannot readthe set currents data for coils and stops to work. For thisplasma current according to a proportional-integral
algorithm, which can be replaced easily by otheralgorithm, and sends them to the PS system. Then the
PS system controls the coil currents using these values.

reason, the RFM of the new RCC system was isolated
from the RFM chain of the PS system. The data for thecoil currents are sent to the RFMs connected to the coil
VII analoe monitor. One RFM of the coil V/[ monitor iscoils (OV, IS, IV) as shown in Fig. l.It is possible to

2.0
1.5
LO

0.,
0,0
"0.5

t.o
-1.5
-2.0

2.0 2.-r 3.0 1.5 4t
R(m)

04m
Fig. 1 Schematics of the LHD coils and plasma.

LHO cortrol LAN

Plasma Current Control System
PS control l-AN

I
I

Poloidal Ooil PS
real-tire oontol

Fig. 2 Structure of the RCC and the PS systems.

266

Nishimura K. et al., Real-Time Plasma Current Control Experimentconnected to the RFMs for the coil PS control and theother RFM to the RFM for the RCC system. Since two
RFMs of the V/I monitor are working individually, any
troubles on the RFM of the RCC system do not give aserious influence to the PS system. The part surroundedby broken lines is the RCC system and the other part isThese components are expressed as follows,

fo= alov + bI6 + clly ,(1)

fq= dlou + el6 + fly ,(2)Iov, 4s and Irv are the coil currents of the OV, IS and IV

the present PS system. As shown in Fig. 2, the PS
system consists of one PS total control computer andthree real-time VME controllers which control poloidal

poloidal coils, respectively. The inter-linkage flux Q

coil power supplies, helical coil power supplies and theanalog output voltage and current monitor for all coils.
The data communications between the total control(3)

produced by the poloidal coils are expressed as follows;(Dr=glov+hls+iIN,

where coefficients a-l are constants. The poloidal coilcurrents are derived from eqs. (1)-(3)computer and VME controllers are done through the PS

control LAN and the data communications among VME
controllers are done using RFMs.
The coil currents are controlled using the values[ii] lt:il

written on the RFMs by the PS system or the RCC
system. When the interlock signal and/or any abnormal

'[l:]

(4)

The inter-linkage flux S is expressed as follows by using the plasma current 1p, plasma self-inductance Z" andsignal are detected, the control priority of the power
supply is switched to the PS total control computer
immediately. The designed value of the control period
for the RCC system is 20-100 msec because the time
constant of the LHD structure is much larser than 20

plasma resistance Rp.

-@p = Lplp + nrJlat.

(5)

Programs for the plasma current control system arecoded using eqs. (4) and (5).

msec.

The main computer of the RCC system is astandard DOS/V machine (MMX Pentium 200 MHz)
which operating system is Windows-NT. The measured

3. Experimental Results

plasma current is inputted to the digital signal processor

The real-time plasma current control experimentusing the RCC system was carried out at 1.5 T during

through an analog digital converter (A/DC). The

the 4th cycle experimental campaign. Figure 3 showsnecessary coil currents are calculated according to thestandard proportional-integral control algorithm
programmed in the digital signal processor and sent to*26205250

the RFM. The digital input and output (DIO) are used

200

for the communication of the timing signal and the

a iEo

p roo

statements of the RCC system between the RCC system

50

and the central control system computer.

040

i ' -i"1. '' -i,---,

Eeo ."'.

2.2 Gontrol Method of RGG System
The magnetic configuration is characterized bytoroidal field Bn, magnetic axis position R*, ellipticity of

30cross-sectional shape r and inter-linkage flux for thetoroidal plasma @0. To control the plasma current with

2.O .'
E.....i .. 1.0 bs/: i i i i+ i

3

6800

^3 6600

keeping Bs, R,* and r constant, three poloidal coils
should be controlled simultaneously. The necessary coilcurrents are calculated from Be, Ru^, K and 4 + 44,
where AQ is the necessary flux calculated from theactual and reference plasma currents. The helical coil

E 6400...........j..............1.............;.............i ...

... i

r

........i.....'-4:-.

;.............i......H-....i ....

J

6200
6000

:4i.............i... . i .

;,,...........i...........

o2468101214
time(sc)currents calculated from Bs is kept constant. To keep Ril

Fig. 3 Time evolution of plasma parameters and the lV

and r constant, the dipole component ffi) and thequadrupole component Cfq) must be kept constant.

coil current in real-time plasma current control
discharge.
267

Nishimura K. et al., Real-Time Plasma Current Control Experiment:6t25 A

ry

rI

ir:

1.0

0.5

5 0.0
N

-0.5

-1.0
4.0

J.J

2.5

t {dl
t.0

3 0.0
N

-0.5

2.s

4.0

3.0

R(m)

4.5

R(m)

Fig. 4 Magnetic flux surfaces in /,u = 6125 A and /,u = 6550 A.

the time evolution of a plasma stored energy (IVo),neutral beam injection (NBI), gas puffing (gas puff), aline averaged electron density (<2">) and a plasma

80
70

current (10). Plasma was produced and sustained by the
NBI only. The control of the plasma current by the RCC
system was started at2 sec and terminated at the end of
the plasma discharge. After 4 second, the plasma current
1o decreases gradually.
The control of the plasma current was carried out
by changing the IV coil current that produced thevertical inter-linkage flux. Other coil currents (/sn,1ov,[5) were controlled to keep them constant to make clear
the influence of changing coil current to the other coils.
Figure 4 shows the displacement of the magnetic axis in
Iw = 6125 A and !y = 6550 A. These currents were thecurrent at the start and the end of the control. The
displacement of the magnetic axis was about I cm anddoes not give serious influence to the plasma confinement.

OU

:40
*30
20

't0
0

6800

^3 6600
E 64006200
6000

o246.?101214
trme{sec )Fig. 5 Comparison between an experimental result andsimulation result at 1.5 T.
l"l#17 1 06l : without current control,l"(#262051 : with current control,/"(sim) : simulation result,/"(ref) : target current for control.

4. Simulation Results
Figure 5 shows the comparison of the plasma
current without control (#17106) and with control(#26205).10 (ref.) is the target current. The RCC system
268

Nishimura K. et al., Real-Time Plasma Current Control Experiment

the real-time feedback control of the plasma current.
The ohmic current was induced by changing the IV coilcurrent that produced the vertical magnetic flux. Thisohmic current controlled by the RCC system reduced
the plasma current of about 30 kA. The displacement ofcompares the plasma current (10) and this target current
as a reference. and calculates the needed coil current(1rv-"o1). The change of the IV coil current (I1y-.o1) was

also shown irr Fig. 5 bottom. The IV coil current
increases continuously at the rate of 40 A/sec. This rate

the magnetic axis was small to affect the plasma

is the upper limit of the present coil PS. The plasma
current of about 30 kA was reduced by the RCC system

confinement during discharge. The simulation of theplasma current control by ohmic current was carried out

during dischare,e. The simulation was carried out using asingle filament model. Used plasma parameters are T,0.7 keV, 1n") = 1.7 x 10re m-3, Rp = 3.6 m and ao = Q.S
m. The plasma resistance and inductance are rr= J.Q) v

using a single filament model. This simulation resultshows the similar tendency to the experimental result.
The RCC system showed a good performance to control

10-6 f,) and Lp= 9.6

x 10-6 H, respectively. The loop
voltage is 0.0096 V when the coil current (1rv-"o1)the coil currents.

changes at the rate of 40 A/sec. The time constant (Zol

Acknowledgements
The authors are grateful to the people in thero) is 1.91 sec. This simulation result shows the similartendency to the experimental result. A difference
between simulation and the experimental result ismainly caused by change of the electron temperature

research operation division and the control technology

division in NIFS for useful discussions and helps.

References
O.Motojima,H.
Yamada, A. Komori, N. Ohyabu,
t1l
K. Kawahata et al., Phys. Plasmas 6.5, 1843

during discharge. The electron temperature decreases
gradually in the latter half of the dischage. The plasma
resistance has a strong dependence on the electron
temperature of T"-u3.
Since the changing rate of the IV coil current islimited to the 40 A/sec and the increasing rate of the(1999).

t2l K. Nishimura, K. Yamazaki, M. Shoji, H.
Chikaraishi, H. Yamada, K. Watanabe and O.
Motojima, Fusion Eng. Des. ISFNT-4, 169 (1998).
[3] S. Takami, K. Nishimura, H. Chikaraishi, C.
Takahashi, T. Haga et al., J. Plasma Fusion Res.

plasma current is about 6 kA./sec., plasma duration of 10

sec. is too short to reduce the plasma current to zero.

The changing rate of 40 A/sec in the IV coil current
corresponds to about 4 kA/sec in the plasma current.

SERTES 3, 513 (2000).

[4] S. Tanahashi, T. Satoh, S. Morimoto, S. Yamada,
5. Summary

S. Kitagawa et al., Flsion Eng. Des. 20, 107

The RCC system was modified to isolate from thepresent LHD power supply control system and used for0993).

269