J. Plasma Fusion Res. SERIES, Vol.5 (2002) 261-264

Advanced Physics and Plasma Control with
Segmented In-vessel Control Coils in the KSTAR Tokamak
LEE G.S., IVANOV D.P.t, YANG H.L., JHANGA Hogun, KIMA J.y., LEE D.K., yOU K.I.,
KIMA H.K., BAK J.S., KWON M., HAN J.H. and LAST J.2
Korea Basic Science Institute, Taejon 305-333, Korea

t Kurchatov Institute, Moscow 123 182, Russia

2

JET Facitity, UKAEA, Oxon, OX14 3EA, UK(Received: ll December 2001 / Accepted: 12 April 2OO2)

Abstract
In-vessel coils are to be used for the fast plasma position control, field error correction (FEC), andresistive wall mode (RWM) feedback stabilization in the Korea Superconducting Tokamak Advanced
Research (KSTAR) device. Recently, a new configuration that incorporates toroidal segmentation
concepl, has been adopted. The new coil system is found to allow a wider range of plasma control
flexibility satisfying the KSTAR advanced physics requirements for the plasma position and FEC/RWM
control capability, in addition to engineering advantages. Here, we report physics and engineering
analyses for the new in-vessel control coil system and explore new physics issues using the in-vessel
control coils.

Keywords:
control-coil, field-error-correction, resistive-wall-mode, KSTAR

realized by using two pairs of inner control (IVC for
vertical position control and IRC for radial position
control) coils while both the error field compensation
and RWM control are to be realized by utilizing thepicture-frame shaped FEC/RWM coils.

1. lntroduction
During the design phase of the Korea Superconducting Tokamak Advanced Research (KSTAR)
device [], the plasma magnetics control has been
considered as one of the important requirements foradvanced tokamak (AT) operations. Here, the magnetics

control implies control of plasma equilibrium

Recently, a significant change has been made to theprevious design. A new configuration of in-vessel coils,

parameters by magnetic means, and it includes controlof plasma position, current and shape, compensation ofin which all in-vessel coils are unified into a single set
by adopting the concept of segmented coil system, has

non-axisymmetric error field due to many sources(including misalignment of superconducting coils), andresistive wall rnodes (RWM) control. In-vessel copper

emerged. Several advantages, particularly in engineering
aspects, have been identified by the adoption of this new

type of in-vessel control coil design. From the physics
point of view, the new coil system has been also foundto allow a wider range of plasma control capability,

coils have been suggested to be used for the fast verticaland radial plasrna position control, field error correction(FEC), and RWM feedback stabilization in the KSTAR
device. In the earlier version ofthe in-vessel coil systemdesign (Fig. l(a)), the plasma position control is to be

including the possibility of helical resonant field
generation for tearing mode and plasma rotation control,02002 by The Japan Society of Plasma

Corre sp onding author' s e - mail : g s le e @ c omp. kb s i. re.kr

Science and Nuclear Fusion Research261

Lee G.S. et al., Advanced Physics and Plasma Control with Segmented In-vessel Control Coils in the KSTAR Tokamak

Fig. 1 Schematic diagrams of the design of (a) theformer KSTAR in-vessel coil (IVCC) system, and(b) the present KSTAR IVCC system.

in addition to the plasma position and FEC/RWM
control capability. In the present paper, the recent design
progress of the KSTAR in-vessel control coil (IVCC)

Fig.2 Configuration of the KSTAR vacuum vessel andrvcc.

system is reported. The design features of IVCC aredescribed in Sec.2. Then, relevant physics
considerations and the results of preliminary engineering
analyses are presented in Secs.3 and 4, respectively. A

summary and some future works are given in Sec. 5.

Radial Control

2. New Design Scheme of KSTAR in-Vessel
Gontrol Coil System

MiddleFEC/RWM-+

UpperFEC/RWM

LowerFEC/RWM

Figure 1(b) shows the present design of the invessel control coil system in KSTAR. Sixteen toroidally
segmented coils, which are located in the same positions
as in the former two IVC and two IRC coils in Fig. l(a)(four toroidally segmented coils at each position),
replace the old IVC, IRC, and FEC/RWM coils. Thus,
the former FEC/RWM coils are eliminated in this newdesign. Each segmented coil can be easily inserted into
the vacuum vessel through three NBItype ports and one

Fig. 3 Cross sectional view of segmented coils and theconnection scheme for field error correction (FEC)
and resistive wall mode (RWM) control.

RF port from outside of the vacuum vessel for
IVCCI and IVCC4, and the former IRC coils using

installation, as shown in Fig. 2.

The new IVCC system should simultaneously

IVCC2 and IVCC3. One of the FEC/RWM parts of theupper IVCC2 is connected to that of upper IVCCI toform an upper FEC/RWM coil. The other FEC/RWM
part of the upper IVCC2 is connected to that of IVCC3to form a middle FEC/RWM coil. The lower part is acontrol the plasma position and FEC/RWM. Therefore,
the connection method of the segmented coils to
duplicate the functions of the former in-vessel coils is an
important design consideration. Figure 3 represents thecross sectional view of segmented coils and shows theconnection method. Each coil has eight conductor bars
and is divided into an IC part, which functions like theformer IVC or IRC coils, and an FEC/RWM part, whichplays the role of former FEC/RWM coils. The IC partsof the IVCCI and IVCC4 coils consist of six conductorbars while those of IVCC2 and IVCC3 are comprised offour conductor bars. The FEC/RWM parts of all thecoils consist of two conductor bars. The IC parts of thecoils are serially connected to adjacent coils by joining
them extemally to complete the former IVC coils using

mirror configuration of the upper FEC/RWM coilconnection. Figure 4 shows detailed combinations ofcoil connection for position control and FEC/RWMcontrol.

The segmented configuration of IVCC hassignificant advantages in engineering aspects. They
include: (l) enhancement of the coil system reliability
with no welding or brazing points inside the vacuum
vessel, which results in removing the possibilities ofmajor accidents such as cooling water leakage or coildestruction due to weak mechanical strength in weak262

I-ee G.S. et al., Advanced Physics and Plasma Control with Segmented In-vessel Control Coils in the KSTAR TokamakH

UDp.r trEC/RwM

Vcrdelcontrot\\aR-}{--\-i_ j)

500
450

yjq--

400

n.ccr -L--<

350

E.looo 250-.W
ffi

200
150

Fig. 5 Critical gain as a function of feedback coil position(a : plasma minor radius, c : radial position of

feedback coils).
Fig. 4 Combination of KSTAR IVCC for position control
and FEC/RWM control.

irregularities, bus lines, and vacuum vessel welding
have been considered. When the permeability of thepoints, (2) simplification in fabrication and installation

welded vessel joints is 1.10 and the standard deviationof misalignment is 2 mm, the maximum current needed

owing to the coils being able to be fabricated outside thevacuum vessel and installed after device assembly, (3)easy repair and maintenance of the coil system, and (4)substantial saving in in-vessel space by the reduction of

to correct the error fields below the critical value oflocked modes (FEC/RWM parts of all the coils) has
been estimated to be about 13 kA-turns, which is about
half the value obtained for the previous design. In
addition, it is found that the dominant harmonic of error
fields generated from segmented IC connection is (m,n)=(1, 4) component and negligibly small (< 0.07 gauss/kA). Meanwhile, the calculation of required currents for
RWM control has been investigated using a cylindricalmodel [3]. A proportional-integral-differential (PID)
control law has been applied in this study. Figure 5
shows the critical gain over which the RWM is stable,
as a function of feedback coil position. In the present
design of IVCC, it is shown that the RWM can besuppressed with proportional gains larger than 200. The
required feedback coil current is estimated to be about
2.4 kA-turns under the assumption of 5 gauss of RWM
amplitude. There is a reduction of a factor of threecompared with the feedback current requirement fromthe old version of FEC/RWM coils.
Besides the duplication of functions of former invessel coils, there are other interesting physics
possibilities to be explored by using the new IVCC
design. By applying a helical magnetic perturbation,
magnetic islands can be generated on some rationalsurfaces, which will then act to drag locally the plasma
flow. A local flow shear can be thus generated aroundsome rational surfaces, which is well-known to be able
to suppress local turbulent fluctuations. It is, however,two coil sets to one.

3. Physics Requirements
The relevant physics issue of KSTAR IVCC is thatit should be capable of controlling (l) unstable verticalmovements inherent to elongated plasmas, (2) rapid
radial position adjustment to provide effective antennaplasma coupling, (3) resistive wall modes, and (4) thecompensation of non-axisymmetric error fields.
The power supply requirements for the control ofvertical and radial motions are not changed significantly
from the previous design values because the locations ofsegmented control coils coincide with the former IVC
and IRC locations. The maximum feedback currents andvoltages for vertical position control (IC parts of IVCCIand IVCC4) and for radial position control (IC parts ofIVCC2 and IVCC3) are 42 kA-turns andl23 V/turn. and22 kA-turns and 52 V/turn, respectively. These values
are the same as those reported in ref .2.

There is a considerable reduction of required
currents for both error field correction and RWM
control by the adoption of the new design concept
owing to the change of coil positions (closer to plasmaboundary). In the assessment of possible error fields inKSTAR, various possible sources of error fields, such as

misalignment during coil installation, coil winding
263

Lee G.S. et aI., Adyanced Physics and Plasma Control with Segmented In-vessel Control Coils in the KSTAR Tokamak

As in the case of the IC connection, the maximum load

noted that the islands can act negatively by providing aseed for the neoclassical tearing modes or by

distribution is located at the circular-to-straightdestabilizing the RWM by the reduction of plasmatransition part, whose absolute maximum value reaches
tp to 12 kN/m in radial direction, 59 kN/m in toroidal
direction, and 99 kN/m in z-direction. On the basis of
this calculation, a detailed three-dimensional analysis isrotation. A more careful study is thus necessary to see
whether this scheme can be used effectively for thecontrol of local flow shear and related turbulent

now in progress and will be reported in a futuretransport. Other possibilities are the control (suppressionor generation) of tearing modes, and initiation of static
magnetic islands at the plasma edge for impurity control

publication.

5. Summary and Future Works
A new scheme based on toroidal segmentation of

4. Preliminary Engineering Analysis
Mechanical stress analyses for several possible

coils has been adopted in the design of KSTAR IVCC
system for the control of vertical and radial plasma
position, compensation of error fields, and control ofRWM. It has been found that the new design scheme

worst case scenarios have been carried out in the case ofIC connection to design support structures. The numberof vertical supports used in the analyses for IVCCl andIVCC4 is twelve (three per segment) and that of IVCC2and IVCC3 is sixteen (four per segment). One inter-has many outstanding advantages not only inengineering aspects but also in physics point of view.
Combining distinct plasma control activities withdifferent time scales in a single coil system is notsegment joint is located at every circular-to-straighttransition part where two adjacent segments meet eachother. The allowable peak equivalent stresses for theconductor and case material are 218 and 207 MPa,respectively. The calculated maximum peak stress on
the conductor material of IVCC2 and IVCC1 are 123
and 147 MPa, respectively. But those on case material
reach up to 249 MPa for IYCC2 and 340 MPa forryCcl. These results show that the stress on casestraightforward and requires extensive research works.
From physics viewpoint, the development of optimal
IVCC operation scenario under the consideration ofrelationship between error fields and RWM control is inprogress. The possibility of using IVCC for the control
of tearing modes and the plasma rotation will beinvestigated in the future. Detailed engineering design of
the IVCC system, including mechanical and thermal
stability analyses of the coils, design and analysis ofmaterial of the circular part of IVCCI exceeds forseveral worst cases. Although increasing the number ofsupport structures, coil feed-through, and development

the IVC vertical supports is considered to resolve thisproblem, there is little choice to increase the number ofsupports due to the limited in-vessel space. Thus, it isof coil baking scenario, is in progress and will bereported in the near future.

necessary to compromise the case thickness and materialselection. From the analyses of time delay andAcknowledgement

amplitude reduction of feedback currents based on thesolutions of a set of circuit equations, the maximum
allowable case thickness has been found to be 4 mm.
Another important design issue comes from theThis work was supported by the Korea Ministry of
Science and Technology under the KSTAR projectcontract.

References

dual function configuration, meaning non-uniformity

tll G.S. Lee et al., Nucl. Fusion 12,l5l5 (2001).

and non-axisymmetry of currents in the coil system.
Thus, mechanical and thermal effects of theconfiguration should be carefully investigated. The[2] Hogun Jhang and J.Y. Kim, Fus. Eng. Des., to bepublished.
[3] Hogun Jhang, S.H. Ku and J.Y. Kim, Phys. Plasmas 8,3107 (2001).

electromagnetic load calculation has been completed forthe mechanical stress analysis of the segmented coils.

264