J. Plasma Fusion Res. SERIES, Vol.3 (2000) 94-98

Characteristics of Tearing Modes in Steady State High Be
H-mode Discharges in JT-60U
ISAYAMA Akihiko*, KAMADA Yutaka, OZEKI Takahisa, IDE Shunsuke,
IKEDA Yoshitaka and the JT-60 teamNaka Fusion Research Establishment, Japan Atomic Energy Research InstituteNaka-machi, Naka- gun lbaraki-ken 3 I I -0 I 9 3, J apan
(Received: 18 January 2000 / Accepted: 7 April 2000)

Abstract
In the steady state high Bo mode discharges in JT-60U, long sustainment of the beta plasma islimited by the occurrence of tearing modes with low toroidal and poloidal mode numbers. In this paper,
characteristics of the tearing modes are described mainly from the viewpoint of the experimental results.
It is found that the characteristics are similar to those of the neoclassical tearing mode. Tearing modestabilization experiments have been carried out since 1999 by local heating and current drive using thenewly installed electron cyclotron wave injection system. Although decrease in magnetic perturbations
and electron temperature perturbations was observed, complete stabilization was not successful.

Keywords:
high B, mode, edge localized mode, tearing mode, neoclassical tearing mode, electron cyclotron heating,electron cyclotron current drive, tokamak

1. lntroduction

sustained for 9 s [1]. Here, p11is the normalized beta andlong time. It is known that if the pressure profile is too
peaked, the performance is limited by the occurrence ofa beta collapse. On the other hand, ifthe pressure profile
is too broad, the performance is saturated by giant
ELMs [2]. Thus, the pressure profile with medium
peakedness is preferable for the steady state high Bp Hmode discharge. However, under such an optimized
condition, the plasma performance is limited by theocculrence of low-n resistive modes with z = I to 3 inthe higher beta region. Typically tearing modes with
mln = 3/2 andlor 2ll are observed. Here, m and n arepoloidal and toroidal mode numbers, respectively. Froma stability analysis, it was shown that in one of thetypical steady state high B,mode discharges, the tearing

Fl is the confinement enhancement factor for the ITER
89P L-mode power law (H-factor). In a series of themode was destabilized even when A'< 0. Here, A'is thetearing parameter. In the discharge, magnetic islanddischarges, pressure profile and heating profile are
optimized to sustain the high performance plasma for awidth measured by an ECE heterodyne radiometer

In a fusion reactor, impurities including helium ash
must be exhausted from the core plasma since they canreduce the fusion reactivity. An operational scenario
utilizing an edge localized mode (ELM) is considered tobe most promising since the impurities can be exhausted

from the plasma without significant degradation of theplasma performance. In the Intemational Thermonuclear

Experimental Reactor (ITER), ELMy H-mode isconsidered to be a standard operational scenario.

In the steady state high Bo mode discharge in JT60U, an ELMy H-mode state with FN = 2.0 and H = 2.3

is sustained for 4.5 s. And PN = 1.8 and F1 = 1.7 is*Corresponding author's e-mail: isayama@ naka.jae ri. go.jp
94@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchIsayama A. et al., Characteristics of Tearing Modes in Steady State High B" H-mode Discharges in JT-60U

E29M22.5Fa=
6=z(Lo2T8=.E
o3o0.03

o 1.5

6oE.

Eo(t)

1o.2
6.c0o
=0.5

67II

10

timelsl

0Fig. 1 Typical waveform of a steady state high po H-0123456
WdialMJlmode discharge. (a) Plasma current and NBI
power, (b) stored energy. (c) magnetic

Fig. 2 Relationship between neutron emission rate andstored energy.

perturbation with n = 2, (d) magnetic perturbation

with n = 1.

showed that it was comparable to that predicted by theneoclassical tearing mode theory [3].
Similar kinds of tearing modes are observed inatthe957o flux surface ecs= 4.0, triangularity 6= 0.14,elongation r = 1.5, plasma volume Vp= 54 mr, line-

other tokamaks such as ASDEX-U [ ] and DIII-D [5]. It
is said that the tearing modes are caused by bootstrap
current. They are referred to as neoclassical tearing

measured by diamagnetic loops Wd*- 5.6 MJ, poloidal

averaged electron density is 2.1 x l0lem-3, stored energy

beta B, = 1.8, normalized beta B1,' = 2.5, internal
inductance li = 1.2. At t = 6.5 s, an mln = 3/2 mode
grows up, and stored energy decreases. At t = 6.7 s, an
mln = 2/1 mode grows up, and stored energy furtherdecreases. The stored energy after the saturation of the2ll mode is half as large as that before the appearanceof the 312 mode. Relation between the stored energy andthen the neutron emission rate is shown in Fig. 2. Asmodes.

In this paper, characteristics of the tearing modes,
mainly from the viewpoint of experimental results, aredescribed comparing with those of the neoclassicaltearing modes. After this introduction, overview of thesteady state high p, H-mode discharge is described insection 2. The characteristics of the tearing modes aredescribed in section 3. In section 4, initial results of thetearing mode stabilization experiment by local heating/curent drive using electron cyclotron wave injection aredescribed. Summary of this paper is described in section

shown in this figure, if the instability occurs,degradation of the neutron emission rate, which means
the degradation of the fusion reactivity, is more serious.

3. Characteristics of The Tearing Modes
The neoclassical tearing mode is different from the'classical' tearing mode in that a) the mode is

2. Steady State High Bo H'Mode Discharge

destabilized even when A'<0, b) magnetic island width
increases with the beta value, c) onset B1y has positive

The high B, H-mode plasma is characterized by aq-profile with q(0) > 1, and the thermal transport barrier
appears at a weak positive magnetic shear. A typical
waveform of the steady state high p, H-mode discharges
is shown in Fig. 1. At t = 4.85 s, NB power of 23 MW
is injected and the beta value increases. The plasma

dependence on density and collisionality, d) hysteresis

exists in the beta value at which the tearing modeappears and that at which the mode disappears, e) somekind of MHD event, such as sawtooth and ELM, isneeded for the seed island formation [6]. We willenters an ELMy H-mode state. At t = 5.7 s in thiscomp,ue the above characteristics with the experimental

discharges, plasma parameters are as follows: plasma
current Ip = 1.5 MA, toroidal field B' = 3.7 T, major
radius R = 3.2 m, minor radius a = 0.8 m, safety factor

results.

From a sequence of the steady state high Bo mode
discharges, it was found that the onset BN has positive95

Isayama A. et al., Characteristics of Tearing Modes in Steady State High Bo H-mode Discharges in JT-60U

32
6.8-9.2s

J 1.5

ocooTzco-,Eu?1otG'90.52

rrJ|["no mode00

1

o2-^-q[t ot'm-',

00.5

31

1.5 2

Fru?t R:3.55m]

Fig. 5 Relation between {lBl /f )o5 and Br.
Fig. 3 Density dependence of onset p, for an mln = 312
mode.
dependence is confirmed more clearly. The curve in Fig.

3 was obtained in a shot where no mode was observed.
In this shot, the attained beta was slightly lower because

injected NB power was lower. This result shows theexistence of the threshold of the mode onset, and also
shows that the plasma becomes more unstable in higher

>1_a0
32zo1

beta region. This trend is the same as that of theneoclassical tearing mode.

0-6=>
XolE'o-=gAnother distinctive characteristic of theneoclassical tearing mode is that saturated island width

1increases with the beta value. If the neoclassical effect isF10T

not included, the saturated island width decreases withincreasing the beta value because of the Glasser, Greene+o:oand Johnson effect [7]. In order to investigate thetimelsl

dependence, NB injection power was slightly decreased

after the amplitude of the tearing mode is saturated, The
waveform is shown in Fig. 4. At / = 6.1 s, an mln = 312
mode grows up. At I = 6.5 s, the beta value is slightly

Fig. 4 Waveform of (a) plasma current and NBI power,(b) normalized beta, (c) amplitude of magnetic
perturbation with n = 2, (dl mode frequency
during NB power-down.

decreased by reducing the NB injection power. Relation
between the normalized beta and {(dBldt)lf)os is shownin Fig. 5. Here,/is frequency of the tearing mode. Thus,[(dBldt)|fl05 is a measure of the magnetic island width.
The value of {(dBldt)lf}05 linearly decreases as thedependence on electron density [3]. In order toinvestigate the density dependence of the onset more
precisely, the onset B1y is investigated with a fixed
plasma configuration. The density dependence of theonset By for an mln = 312 mode is shown in Fig. 3. In
this systematic scan, electron density was changed bychanging the gas puff rate and NB power. Other
conditions are fixed as follows: Ip = 1.5 MA, B, =3.7 T,
R = 3.2 m, a = 0.8 trr, egs= 4.O, 6= 0.14,

normalized beta decreases. Note that change in current

profile and pressure profile which must be included fordetailed analysis is not considered in this evaluation.

As shown in Fig. I and Fig. 4, once the tearing
mode appears, it persists until the beta becomes lowvalue. Typically, the beta value at which the tearing
mode appears is at least twice as high as that at which it
disappears. This suggest that hysteresis exists in the beta

r= 1.5 and Vo= 55 m3. As shown in this figure, the density
96Isayama A. et al., Characteristics of Tearing Modes in Steady State High po H-mode Discharges in JT-60U

value at the mode onset and that at the modedisappearance. This kind of hysteresis is observed inmany of the steady state high Fp mode discharges.

the heterodyne radiometer is 24, and typical spatialof the neoclassical tearing modes. However, it is not

resolution is about 2 cm in major radius, and themaximum temporal resolution is I ps. As shown in Fig.
6, the injection angle of the EC wave was adjusted sothat the wave can be deposited at the center of theclarified what forms the seed island. According to themagnetic island.

neoclassical tearing mode theory, seed island is needed
for the neoclassical tearing mode to be destabilized. Theseed island is formed some kind of MHD event such as

Typical waveform of magnetic perturbations andelectron temperature perturbations are shown in Fig. 7.

a sawtooth oscillation or a fishbone instability or anELM. Error field can also form the seed island. In JT60U, neither sawtooth oscillation nor fishbone instability
is observed in this series of discharges. Thus, one of thecandidates is an ELM. Further investigation for thiscontinuously before the EC wave injection. The center

problem is needed.

6 kHz. As shown in this Fig. 7(b), amplitude ofSo far, most of the results show the characteristics

In this discharge, a tearing mode with mln = 312 existsof the magnetic island locates around R = 3.63 m. The
operational region was chosen so as to reach thethreshold of the mode onset shown in Fig. 3. In thisdischarge, the frequency of the tearing mode was aboutmagnetic perturbations slightly decreases during the EC
wave injection. Amplitude of electron temperature
perturbation near the magnetic island also decreases asshown in Fig. 7(d). These results suggest a sign of thetearing moCe stabilization by the EC wave injection.
However, complete stabilization has not been achieved

4. Stabilization of The Tearing Mode
It is necessary to stabilize the tearing modes torecover the plasma performance. ln 1999, a new ECH/
ECCD system was installed, which can inject radio

frequency wave of ll0 GHz, and the maximum
injection power is about 750 kW (l MW at the gyrotron
output). Toroidal injection angle is fixed at about 20o,and poloidal injection angle can be changed by rotating

so far. In 2000, the tearing mode stabilization
experiment will be performed with higher EC wave
injection power (=2.3 MW).

the steerable minor [8].

In order to stabilize the tearing mode, it is also
necessary to identify the mode location. It was shown

5. Summary
Characteristics of tearing modes in the steady statehigh B, mode discharges are described mainly from thethat the location can be identified by measuring electron
temperature perturbation measured by an ECE
heterodyne radiometer [3]. The number of channels ofae

2-o

J.5-oFFt'6-=9-48rfIFU-4d<-6'IFrF0-4s,FrF- O234Rlml

time[s]Fig.5 Plasma configuration in a tearing modestabilization experiment. Ray of EC wave and10Fig. 7 (a) Plasma current, NB injection power and EC
wave injection power. (b) Amplitude of magnetic

perturbation with n = 2. (c)-(e) Amplitude ofmeasurement range of the heterodyne radiometerare also shown.

electron temperature perturbation.
97

Isayama A. et al., Characteristics of Tearing Modes in Steady State High B" H-mode Discharges in JT-60U

viewpoint of the experimental results. TheAcknowledgements
Authors wish thank Dr. A. Funahashi, Dr. H.
Ninomiya and Dr. R. Yoshino for their continuing
support and encouragements. Authors thank Dr. Y.
Neyatani for his help on the evaluation of magnetic

characteristics are similar to those of the neoclassicaltearing mode:a) Onset B1u, increases with electron density.

b) The mode becomes more unstable withperturbations. Authors also thank Dr. N. Isei for his help

increasing beta.

c) Amplitude of saturated magnetic perturbations

on ECE diagnostics.

decreases when the beta value was decreased

References

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