J. Plasma Fusion Res. SERIES, Vol.5 (2002) 324-327

Effect of ECH/ECCD on Sawtooth Oscillations
in NB-heated Plasmas in JT-60U
ISAYAMA AKihiKo, IDE ShunsuKe, IKEDA YoshitaKa, SUZUKI TaKahiro,
KAJIWARA Ken and JT-60 teamNaka Fusion Research Establishment, Japan Atomic Energy Research Institute, Ibaraki 3lI-0193, Japan(Received: 11 December 2001 / Accepted: 4 June 2OO2)

Abstract
Sawtooth period and amplitude during electron cyclotron (EC) wave injection to a neutral beamheated plasma are investigated. Significant decrease in sawtooth period is observed for EC injection
inside the inversion radius in co-direction: the normalized sawtooth period 251/7"(0)l'5 reaches 1/10 ofthat in an ohmic heating phase, which shows a strong destabilization effect (here, f51 and 7"(0) aresawtooth period and the central electron temperature, respectively). While time scale for the change inthe sawtooth amplitude is comparable to that in the inversion radius, time scale for the change in thesawtooth period is much shorter than that in the inversion radius. This suggests that the change in thesawtooth amplitude is related to the current profile, and that the strong destabilization effect of thesawtooth period occurs independently of the change in the current profile.

Keywords:
sawtooth oscillation, magnetohydrodynamic instability, electron cyclotron heating, electron cyclotron
current drive, neutral beam, JT-60U, tokamak

1. Introduction

zatron-ls an lmportant rssue.

Effects of additional heating and/or current drive
tools on sawtooth oscillations have been investigated inmany devices: For example, in JT-60/JT-60U, behaviorof sawtooth oscillations during neutral beam (NB)

Sawtooth oscillation, which is a kind of magneto-hydrodynamic instabilities in the central region of aplasma, has been observed in many devices since 1970s[1]. Although a number of experimental and theoreticalresearch have been done, underlying physics of theinjection, lower hybrid heating/current drive, ion

sawtooth oscillations has not been fully understood yet.

cyclotron heating, and electron cyclotron heating (ECHyIn application to a fusion reactor, sawtooth

electron cyclotron current drive (ECCD) have been
investigated [2-5]. In most cases, however, theseexperiment have been done from the viewpoint ofoscillations are attractive since heat and particle in thecore region can be expelled through a sawtooth crash.
On the other hand, sawtooth oscillations can becomestabilization.

undesirable since they can (a) limit the plasma

In this paper, effects of ECH and ECCD on
sawtooth oscillations in NB-heated plasmas in JT-60U
are described. The difference between the results in thispaper and those in ref. 5 is that EC wave was injected inohmic heating (OH) plasmas in the latter case. We

performance through the flatting of the pressure profile,(b) form seed islands for neoclassical tearing modes, and(c) increase heat load at divertor plates to an intolerable

level if a sawtooth crash is too large. Thus, to control
the sawtooth oscillations
stabilization and destabili--@2OO2 by The Japan Society of Plasma

Corresponding author's e-mail: isayama@ naka.jaeri. go.jp
324

Science and Nuclear Fusion Researchlsayama A. et al., Effect of ECH/ECCD on Sawtooth Oscillations in NB-heated Plasmas in JT-60U= 0.83 m, safety factor at the 95Vo flux surface ess = 5.6,
plasma volume Vp = 60 m3, and line-averaged electron

concentrate orr the EC injection inside the inversion
radius and investigate the destabilization effect by EC
injection. After this introduction, experimental condition
and results are described in Sec. 2, and summary anddensity fr" = l.l x 10le m 3. Plasma configuration andthe layout of heating and diagnostic systems are shownin Fig. 1. Two units of neutral beams, whose injection

discussion are described in Sec. 3.

power and beam energy are respectively 2 MW and 80

2. Experiment

keV, are tangentially injected to the hydrogen plasma inboth co- and counter-directions. Electron cyclotron
wave, whose frequency is 110 GHz, is injected from thelow-field side. Electron cyclotron emission (ECE)

2.1 Experimental setup
Since ther period and amplitude of sawtoothoscillations are affected by discharge conditions, weperform this series of discharges under the following

diagnostics and motional Stark effect (MSE) diagnostics

fixed conditions: plasma current 1p = I MA, toroidal

are used for measurement of electron temperature andcurrent profiles, respectively. Profiles of electron

field B, = 3.7 "l', major radius R = 3.4 m, minor radius atemperature and safety factor during EC injection are

shown in Fig. 2(a), and the change in the electron
EO3A2B7

temperature profile during a sawtooth crash is shown in

Fig. 2(b). In this series of discharges, inversion radius ofa sawtooth oscillation is located at p > 0.15. According
to a Fokker-Planck code combined with a ray-tracing
code, the peak position of the EC-driven current profile
is located at p - 0.1. Thus, most of the injected EC
wave is deposited inside the inversion radius. In order todiscriminate between the effects of ECH and ECCD,
behavior of sawtooth oscillations for co-ECCD andAg-"////ll"// t

J-ll Iln ll tn::il tnu\\
ll- \\
l trn\,

counter-ECCD cases are compared each other.

\ \!lur

2.2 Experimental Results

2345

Time evolution of the central electron temperature
is shown in Fig. 3. Sawtooth oscillations appear before
the NB injection, and sawtooth period is about 0.04 s.
After the NB injection from / = 8.0 s, the sawtooth
period is extended to about 0.1-0.15 s. EC wave of 1.5Rlm]
Fig. 1 Plasma shape and the layout of heating and diagnostic systems.

(a)66555fz

Os4c-Fr

2805E038286o^=4(b) f,0'2.!a !5;0F

29

NBI rc

01

18=oz

3o$e

1o=CoECCD

=4$.s

4-4

EO3A2a7

9time [s]CounterECCD

0.1

po.2

Ft1

0.3

0Fig.2 (a) Profiles of electron temperature and safety
factor; (b) Change in electron temperature duringa sawtooth crash.

OL

108=oo4 o-t6zt-0.3

1022

NBI ec8910

O(Ltime [s]Fig. 3 Time evolution of the central electron tempera-ture.
325

Isayama A. et al., Effect of ECH/ECCD on Sawtooth Oscillations in NB-heated Plasmas in JT-60U

MW is injected from / = 8.5 s. While the central
electron temperature for the co-ECCD case iscomparable to that for the counter-EccD case,the co-ECCD case is about twice as larse as that for thecounter-ECCD case.
Behavior of sawtooth oscillations can be affectedby a current profile. Time evolution of inversion radius,
which is a measure of the 4 = 1 surface, is shown in Fig.
7. The inversion radius increases by -0.05 in volume-

significant difference is observed in the behavior of thesawtooth oscillations: the sawtooth period decreases toabout 0.02 s for the co-ECCD case, and it increases to
0.16 s for the counter-ECCD case. After the EC
injection is quitted, the sawtooth period increases toabout 0.12-0.13 s, which is comparable to that before

averaged minor radius for the co-ECCD case, while nosignificant change in the inversion radius is observed forthe counter-ECCD case. Similar result is also obtainedin current profile: change in MSE angle is observed onlyfor the co-ECCD case. While time scale for the chanse

the EC injection.

According to a theoretical analysis, sawtooth period
increases with Z"(O)tr [6]. It was reported that sawtooth

period follows the scaling in NB-heated or EC-heatedplasmas in JT-60/JT-60U t3,51. In Fig. 4, time evolutionof the sawtooth period is plotted as a function of theCounter-ECCD
Transienl Dhase

central electron temperature. For the co-ECCD case, thesawtooth period is much smaller than that from thescaling: the sawtooth period of 0.02 s obtained in the coECCD case is about 1/6 of that in the NB phase. Bytaking into account the increase in electron temperatureduring the EC phase, this shows that the sawtoothoscillations are strongly destabilized. For the counter-0123451

2345

r"(0) [kev]r"(0) tkevl

Fig. 4 Dependence of saMooth period on the central

ECCD case, on the other hand, the experimental data areelectron temperature.

located near the scaling, which suggests that the increasein the sawtooth period during the EC injection isattributed to the increase in electron temperature.
The dependence of the normalized sawtooth period

0.04u?

zsr[s]/(2"(0)[keV])l5 on EC injection power Ps. isshown in Fig. 5. For the co-ECCD case, the normalizedIsawtooth period reaches 0.005 even for one-unit EC
injection (Psc = 0.5 MW;25Vo of NB power). When Pp6= 1.5 MW, the value reaches about 0.002, which isabout l/10 of that in the OH phase. For the counter
ECCD case, the normalized sawtooth period gradually
decreases with EC power, and reaches about 0.15 when
Pnc = 1.5 MW.
In addition to the sawtooth period, sawtooth
amplitude is another important parameter. Timeevolution of the sawtooth amplitude is shown in Fig. 6.
The sawtooth amplitude A51 is defined as follows:oFor

PNa =2MW

0.03

Counter-ECCD

0.02
0.01

CJ)

00

0.5 1

1.5

Pec [MW]Fig. 5 Dependence of the normalized sawtooth period
rsrl7"(0)15 on EC injection power.

0.3

ar',. = (r"10;hto'" - r"(o)*-)/ {(r"(o)*t* + r"(o)"rt"')/ 2}

Fat)

Here, Z"(0)bro* and Z"(0)d"'are the central electron temperature before and after a sawtooth crash, respectively.

o.20.1The value of A51 is 0.04 in the OH phase and increases
to 0.08 in the NB phase. While A51 continuously in-Icreases to 0.26 for the co-ECCD case, it saturates at
0.12 for the counter-ECCD case. The value of As1 for

8.5 99.5 10 10.5

timelslFig. 6 Time evolution of savvtooth amplitude.

326

Isayama A. et al., Effect of ECFVECCD on Sawtooth Oscillations in NB-heated Plasmas in JT-60U

found that sawtooth period can be significantly

0.25

decreased by injecting EC wave in co-direction. In thiso.2

case, the normalized sawtooth period rs1/2"(0)l s reaches

o o.tsl/10 ofthat in the OH phase. The strong destabilization
effect has not been observed for NB-heated or ECheated plasmas in JT-60/JT-60U. Interaction between

0.1

fast ions and EC wave may play an role, but theu.5

mechanism has not been clarified yet.

0.25

Unlike the sawtooth behavior, no significant
difference in stored energy has been observed between
the co- and counter-ECCD cases. One of the reasons isthat energy released by a sawtooth crash is small since
inversion radius is small in this series of discharges.
While the reachable central electron temperature for theco-ECCD case is similar to that for the counter-ECCD
case, the central electron temperature averaged over one
sawtooth period for the co-ECCD case is lower than thatfor the counter-ECCD by several percents. This suggestso.2

o o.ts
c0.'t

8.5

99.5

10time [s]Fig. 7 Time evolution of sawtooth inversion radius.

that heat and particles in the core region can bein the sawtooth amplitude is comparable to that in theinversion radius, time scale for the change in thesawtooth period is much smaller than that in theinversion radius. This suggests that the change in theeffectively expelled by co-EC injection without serious

sawtooth amplitude is more closely related to the current

tll S. von Goeler et al., Phys. Rev. Lett. 33, l20l

profile, and that the strong destabilization effect by theco-ECCD case occurs independently of the change inthe cument profile.

[2] S. Ishida et al., Nucl. Fusion 30, 552 (1990).

confinement degradation.

References(r974\.

[3] Y. Kamada et al., Nucl. Fusion 33,225 (1993).
t4l H. Kimura et al., Radio Frequency Power in Plas-

3. Summary and Discussionmas (AIP Conference Proceedings 289), (AmericanEffects of ECH/ECCD on sawtooth oscillations in
NB-heated plasmas have been investigated. In thisInstitute of Physics, New York, 1994) p. 52.

t5l A. Isayama er a/., Fusion Eng. Des. 53,129 (2001).
t6l W.Parket al., Nucl. Fusion 30,2413 (1990).

paper, we have investigated the destabilization effect byEC injection inside the inversion radius. It has been

327