J. Plasma Fusion Res. SERIES, Vol.3 (2000) g7_93

Recent Progress Towards Steady State Tokamak operation
with lmproved Gonfinement in JT-GOU
FUJITA Takaakix and the JT-60 Teamlapan Atomic Energy Research Institute, Naka Fusion Research Establisi;hment,
Naka-machi, Naka- gun, Ibaraki-ken 3 I l -0 I 9 3, JA4AN(Received: 18 January 2000 / Accepted: 13 May 2000)

Abstract
In the JT-60U tokamak, optimization of high B, mode and reversed shear mode plasmas are being
done for establishment of scientific basis for steady state operation of tokamaks. In high po H-mode
plasmas, Fx = 2.9 and .F1se = 2.2-2.4 were sustained stationarily by using high triangularity configurationand pressure profile optimization. Steady state performance was limited by resistive low toroidal mode

number instabilities. Stabilization of resistive modes by using a newly installed ECRF system was
attempted and a decrease of mode amplitude was observed but complete stabilization could not be
achieved. In reversed shear plasmas, high fusion performance with equivalent DT fusion power gain of
0.5 was sustained for 0.8 s or an energy confinement time. The duration was limited by disruptive beta
collapse that was encountered when the minimum value of 4 became 2 even with moderate beta,

PN -1.2.
Stationary sustainment of ITB was demonstrated in a full CD reversed shear plasma with LHCD. The

sustainment of reversed shear current profile by bootstrap current was demonstrated in an ELMy H-modeedge reversed shear plasma with a high triangularity in a high q regime. A confinement enhancement

factor of 3.5 and By of 2 were sustained for 2.1 s with stationary current and pressure profiles. Ar puffing
to H mode plasmas aiming at high confinement with high density and high radiation fraction was
performed and Hsq'I.4 with radiation fraction of 80Vo was obtained at TOVo of Greenwald densitv.

Keywords:
high B, mode, reversed shear, internal transport barrier, tearing modes, ECCD, beta collapse, bootsrrap
current, radiative divertor, impurity injection.

1. Introduction
In the JT-60U tokamak, reactor-relevant plasmaHence experiments for the sustainment of high beta andhigh confinement are strongly related to the steady state
operation of tokamaks.

studies are conducted in support of ITER and steady-state tokamak operation. It is a prerequisite for therealization of steady-state tokamak reactors to sustainhigh energy confinement, high stability and a largebootstrap current fraction with full current drive and

In JT-60U, two approaches towards steady-state
tokamak operation with large bootstrap fraction arebeing pursued. These are high Bo H-mode and reversed
shear (RS) mode. In both modes, internal transportbarriers, ITBs, are formed and improved confinement isobtained [1,2]. The high B, mode has a monotonic 4
profile with weak shear and 4(0)>1. In the RS mode

radiative divertor conditions. Higher beta andconfinement are necessary in steady state tokamak
reactors than in pulsed reactors because of high q
operation and of the necessity for high bootstrap current.
*Corresponding author's@2000 by The Japan Society of Plasmae-mail: fujitat@fusion.naka.jaeri. go.jp
87

Science and Nuclear Fusion ResearchFuiita T. et al., Recent Progress Towards Steady State Tokamak Operation with Improved Confinementin JT-60U

plasmas, the minimum of q (q^) exists and the ITBs
are formed near the location of Q^,n. The density andtemperature profiles in high Bo mode are usually peaked
one or "parabolic type", while those iri RS are often flat

(NTMs) [5]. First, they are observed even whenclassical tearing modes are stable or /'is negative'
Second, there is a minimum value of py (87,1'"*') for
appearance of fluctuations and B1,, has to be reducednear the axis or "box-type" [3]. The confinement
improvement is more enhanced in RS mode and the H
factor larger than 3 is obtained even with an L-mode

lower than FNon'"' to eliminate the existing fluctuations(hysteresis for beta). Furthermore, the island structureedge. This paper reports optimization of high Bo H-mode
and RS mode plasmas for sustainment of high beta andhigh confinement as well as H-mode experiments as anradiometer with 2 cm spatial resolutions and the island
width was consistent with the calculated one based on
NTM [6]. Though we have many features consistent
with NTM, we have not identified the cause of seedisland that is necessary for appearance of NTM. We

was observed in T" profiles obtained using ECE

example of high confinement and large radiation
fraction.

have no sawteeth in high Bo H-mode and fishbone
instabilities are not observed either. The ELMs or error

The rest of the paper is organized as follows. In
section 2, the sustainment of high B, mode is presented.
Section 3 treats sustainment of RS mode. Heat andparticle control in the divertor is described in section 4.
The summary is given in section 5.

2. Sustainment of High Bo H-mode
The stability of high Bo H-mode has been

5

maximized by optimizing pressure profile and plasma
shape. The major stabilities related to the beta limit in
the high B, H-mode are kink-ballooning mode in the

se
-*e

4

*ffi

core and the high-n ballooning mode at the edge; wherezn is the toroidal mode number. Too peaked profile

3

#tK
'&%

2

results in beta collapse due to the kink-ballooning modein low beta while the beta is limited by ELM (Edge'

Localized Mode) activities caused by high-z ballooning

mode for too broad profile. The plasma shape or
triangularity (fl is related to the high-n ballooning
mode. By increasing 6, the edge pressure gradient canbe kept higher during ELMy H-mode [4]. In JT-60U, 6
can be increased up to -0.5 for -l MA discharges but

oi

3

4

5

6

7

w

(a)

I I

4

the available 6 decreases with the plasma current.

For sustainment of high beta in long pulse

3discharges, magnetic fluctuations appear and limit thebeta. As a result, the beta and confinement in long pulse
discharges are lower than those obtained transiently. In

o)

@
E2

Fig. l, the normalized beta, p* and confinement

KI***" q6enhancement factor over ITER89P L-mode scaling, Flseare plotted against edge safety factor, qs5. Sustainable B1,'(b)and Flrn were about 60-7OVo of transient ones and By <o; 3 4 5 6 7 8

2.7-2.9 and llse < 2.2-2.4 were obtained with high 6
high Br l/-mode for a wide range of 4e5 (3.2-8).
The magnetic fluctuations grow slowly (with -100
ms growth time) and have mln = 3/2,2ll and so on; here9egs

rz is the poloidal mode number. The fluctuations

Fig. 1 (a) Fn and (b) Heg as a function of gru in high Be

observed in JT-60U high B, H-mode plasmas have many
features that are different from classical tearing modes

denote the transient data whose duration was lessmode and high ft H-mode plasmas. Gray circles

than 5r, while black circles denote the sustained
data for longer than Srs.

and consistent with the neoclassical tearing modes
88Fujita T. et al., RecentProgress Towards Steady State Tokamak Operation with Improved Confinement in JT-60U

pulse length was 2 s for I MW of gyrotron output and 5

fields are candidates for the cause of seed islands.
In JT-60U experiments, Byo""t increases with thedensity or electron collisionality. Hence higher density
is favorable to obtain high beta in long pulse discharges.
However, the confinement was degraded when thestrong gas puff was applied to raise the density as
shown in section 4. Thus we have an optimum density
range to sustain high beta, which was about (3-3.5) x
19te --3 for 1.5 MA discharges with 6< 0.3. At thiss for 0.3 MW. The steerable mirror is installed at the

injection port and the wave injection angle can bechanged in the poloidal direction. In the resistive modes
stabilization experiments, the wave injection angle wasadjusted to the center of mln = 3/2 islands which can berecognized by ECE measurements. It was observed thatdensity, 9n -1 .9, Hss -2.2 and Qp7"q -0.16 were

the amplitude of n = 2 magnetic fluctuation and that ofelectron temperature perturbation near the magnetic
island decreased during the EC injection [6]. However,sustained for 4.5 s [4].

complete stabilization has not been attained. In this year,

The stabilization of NTM by ECCD was attemptedusing a newly installed EC system. The frequency is I l0
GHz and the maximum injected power is 0.75 MW (1two more gyrotrons will be installed and clearer
stabilization will be expected by injecting higher power.
Deposition of EC power on the axis of plasma wasalso attempted to increase Z"(0) in high B, plasmas. An

MW at the gyrotron output). To date, the maximum(a)example is shown in Fig. 2. The value of I"(0) was
raised by -1.5 keV and reached 9.5 keV at high density(n"(O)-4 x l0re m-3) with the injection power of -0.75

n=2 appeared
E03s213

MW, which was less than 5Vo of NB power, and

30=

-2

excellent heating performance was confirmed. High B1yof 2.4-2.5 was sustained for 1.4 s during ECH. These
results are promising for the enhancement of current
drive efficiency (4co) of NNB, the record of which is
1.3 x l0re Al\Nlmz achieved in 1.5 MA high BrH-mode
with I"(0) -8.6 keV [7] since 46p increases with the

20?t3

I

3
)r12
-0
)<. I

*9

A+
-o0
'3

electron temperature.

?

s

z2
-1

3. Sustainment of Reversed Shear
3.1 Sustainment of high fusion performance

.z

E

0

5

678

9

IJ

o

In JT-60U RS plasmas, very high confinement (/1setime [s](b)>3) with an L-mode edge is obtained due tosignificantly reduced transport in the ITB layer that is
located at a large radius. The equivalent fusion power
gain, Qor"o of 1.25 was achieved transiently at I, = ).$10

t 8 ?i,?:dq
3.0
o

4

0L
2.5

MA and Bt = 4.4 T [9]. In these plasmas, the duration of^.d ftb#hhigh performance was limited by beta collapse that was
encountered when 4r1n became 2. Efforts to sustain highfusion performance as long as possible were attempted.
To extend the high performance period, one can either
raise the performance as early as possible or delay/
suppress the beta collapse at q^in= 2.

33.54

through in ELMy H mode edge RS plasmas at 1.5 MA
and 3.5 T [9]. Hence we tried to make an H-mode edgein high current (>2 MA) high field (4.3 T) RS plasmas

It was observed that 4r1n = 2 could be passed

R [m]Fig.2lal Waveforms and (b) electron temperatureusing a similar technique used to obtain H-mode atmedium current plasmas or changing NB toroidal
momentum input from balanced one to co-directional

profiles of a high po H-mode plasma in which on-

axis ECH was applied. A peaked electron

temperature profile was formed by ECH; n"(0)-4 x

dominant one. Although we certainly obtained H-mode
transition, collapses were encountered during current

101s m-3 for f = 6.1-6.4 s. Modes with n = 2
appeared from f= 7.9 s and Brywas degraded.
89

Fujita T. et al., Recent Progress Towards Steady State Tokamak Operation with Improved Confinement in JT-60U

ramp when we continued the co-directional NB

started at r = 4.0 s and continued to the end of discharge.

injection. These collapses are supposed to be related tolocked modes since the toroidal rotation at the plasmaedge became nearly zero for co-directional injection
case (the edge rotates in the counter direction forbalanced injection case in JT-60U, which is mainly due
to formation of negative radial electric field throughripple loss of beam ions). When we retumed to balanced
injection after the H-mode transition, the edge pedestal
continued to weaken and the edge returned to L-mode
ultimately. It is not yet possible to sustain the H-modeedge in high current, high field RS plasmas in JT-60U.
It was also attempted to reduce the beta or tochange the toroidal rotation just before q-inbecame 2.
For the balanced NB injection case, collapses wereobserved even with low B1,' of 0.8-1. When the counter
injection was employed with same B1y, plasma survived
even though a mini collapse occurred at Q^i, = 2. This
may suggest that q^i, = 2 collapses can be suppressed by
the toroidal rotation shear, but the optimization of thisIn the second box of Fig. 3, the reference value of W7,oby a dotted line and the measured value of Wl1oby asolid line are shown. It is found thatW1lo was controlled
precisely according to the reference value. W6io wasramped up from 4 s to 4.8 s to form an ITB with a large
radius and was kept constant to suppress collapses at
e.i,-3. After r = 5.6 s, Wlio started to increase andreached 5.3 MJ at 6.2 s and was kept constant after that.

As the W7,o increased, I13e also increased and Qrr"t = 9.5
was reached. The timing of ramp up of Waio $rom 2.7
MJ to 5.3 MJ) was adjusted to reach high performance(5.3 MJ) state as early as possible without sufferingcollapses. As a result, Qor'e = 0.5 was reached at6.25 s
and was sustained until 7.05 s or the beta collapse ate^tn-2.The duration was 0.8 s or equal to the energy

confinement time. During the same period,
FN= l.l-1.2, Hss = 2.5-2.7, fi(0) = 12-14 keV were
sustained.

3.2 High confinement high bootstrap reversed

technique has not been investigated.
To raise the performance as early as possible, we
employed the stored energy feedback control techniquethat has been developed [8] and is shown in Fig. 3. In
this discharge, the stored energy (W72) feedback control

shear
For long sustainment of RS plasmas, the necessityof current profile control to suppress the change of q
profile or decrease of q^i,is highlighted in the fact thatthe passing e.i, = 2 was difficult as shown in the above
section. Also current profile control is necessary tosustain the location of ITB; in the ELMy H-mode RS,
e.i, = 2 could be passed but the location of ITB moved
inward according to the inward movement of location of

E34292.4.3I

3

20

0

0

IMA]

IMW]
6

q^1, which was caused by ohmic current penetration into''" [1oto
o /sl

tMJ]0

1.5

the core [9]. In JT-60U, application of LHCD forsustainment of reversed shear configuration has been3

Has

ll r.r0

intensively performed. First, sustainment of 4 profile
with low beta (without ITB) was demonstrated [0] andsustainment of higher beta plasma with ITB was
achieved later [11]. In the later case, FN-O.9, Fo-0.7
were sustained with full noninductive CD conditions

0

5[A.U ]
00.5

including bootstrap current (lBsllp-0.23) at 0.85 MA and

0
4

2T.

A high bootstrap current fraction (7O-807o) is
required for steady state operation. So sustainment of
RS with higher bootstrap current fraction is importantand is directly related to steady state tokamak operation.
We can expect a hollow current profile is naturally
formed and sustained under high bootstrap current
fraction. However, it is not known whether the pressure
and current profile are sustained stationarily under highbootstrap fraction because the current profile is strongly
affected by the pressure profile through bootstrap

2
1

34567

time lsl

Fig. 3 Waveforms of a RS plasma in which Oor"q = 0.5

was sustained for 0.8 s. From the top, plasma
current (/r) and NBI power (Prua), reference value(dotted line) and experimental value (solid line) ofstored energy (Wa,"l and neutron emission rate
l9,l, BN and Hrr, deuterium recycling emission at
the divertor, Qor'q and q^,,.
90

Fujita T. er al., Recent Progress Towards Steady State Tokamak Operation with Improved Confinement in JT-60U-2.0 (Wdi, - 2.4MJ), Pp -2.6-3 were sustained (8,= 3.4

current while the pressure profile may be changed by
the current profile through the effect of magnetic shear

T, Ip = 0.8 MA, Qss -9,6 -0.4). The profiles oftemperature, density and q are shown in Fig. 4 (b). The
density and 4 profiles are shown for 3 timings (t = 7.5,
8.5,9.5 s). From this figure, it is found that density and
4 profiles were sustained almost stationarily. The
location of q^in was -65Vo of plasma minor radius andthe value of Q*in was -3.5. Using these profiles, thebootstrap current fraction was estimated to be -807o
while the NBCD current was estimated to be 200 kA or
25Vo of plasma current. Hence full noninducitve CD is
supposed to be maintained during t = 7.3 to l0 s. Thison the transport. Hence experiments to demonstrate highbootstrap RS and to study the evolution of pressure andcurrent profiles are planned. Since high B, is required toachieve high bootstrap current fraction, an ELMy Hmode RS with high d configuration and high {e5 wasselected; higher stability (9i is expected in the high 6

configuration as shown in the high B, H-mode
experiments while higher B, with same B1y is obtainedfor higher qe5.

Figure 4 shows a typical discharge in which

can be confirmed from the fact that loop voltage wasnearly zero as shown in Fig. 4 (a). A distinctive featureof this discharge is that very high confinement or l1se
-3.5 (tt -0.4-0.5 s) was sustained. This is attributed to
the large ITB radius as shown in Fig. 4 (b) and highsustainment of RS with high bootstrap current fraction
was demonstrated. The initial current ramp up to I MA
was employed to establish a strong ITB with moderatebeta 1B" <1.5) without suffering collapses. The plasma
current was ramped down during t = 6 to 6.8 s and Bo
was increased from 2.0 to 2.6. During t = J.3 to l0 s, B1yedge stability due to high 6. Although l/se>3 was

(b)
1.2

lp o.g

15 PNe10lMAl o.+

5 tMW]

003Fru, Fp 2
1

10

I

vo 6
o
F 4
F 2

0
(f)
I

4

Hes g

E

0
4

3

CD

o

2
5

co 0
1

4

9min

3
2

Vsurf
tv1

8
7

0.3

u 6
5

-0.3

Dodiv
[1020]

4

1

35
o

10

12

-e7.5s--o-- 8.5s
+9.5s

0.2 0.4 0.6 0.8

1ilatime [s]Fig. 4 (a) Waveforms and (b) profiles of a RS plasma in which high confinement and high bootstrap current fraction
were sustainedlor 2.7 s. In (a) from the top, plasma current (/o) and NBI power lPr"), F, (dotted line) and Pry (solidline), Hss, g-,n, surface loop voltage V",a, deuterium recycling emission at the divertor. In (b), from the top, ion
temperature 7,r and electron temperature (7") profiles at 8.5 s, electron density (n") profiles at7.5,8.5,9.5 s and q
profiles at 7.5, 8.5, 9.5 s.

9l

Fujita T. et al., Recent Progress Towards Steady State Tokamak Operation with Improved Confinement in JT-60U

obtained in JT-60U RS transiently, the confinement in

However, the core confinement tends to degrade as thelong pulse (>5c6) was restricted to llsecl.T in thedensity increases as shown in Fig. 5 (a). This isprevious ELMy H-mode RS with lower Bo (<1.3) [9]. In

supposed to be caused by shrinkage ofthe edge pedestal
width at the high density with strong gas puffing. In

addition to the increase of triangularity, the outward
displacement of ITB location caused this improvementof confinement. Smaller ITB radius (r/a < 0.4-0.5) was
obtained in the long-pulse lower 0p plasmas due to thecurrent penetration. Hence these results indicate thathigh confinement can be sustained in ELMy H RS if theappropriate current profile is sustained stationarily.
Since the confinement of long pulse high B, H-mode is
H8s< 2.4 (including high 4e5 regime) as shown in Fig. I(b), the confinement of the discharge shown in Fig. 4 isthe highest one sustained in JT-60U plasmas.

deuterium puffing cases, Ilse quickly decreased with thedensity and became -l-l.l at the density of 70Vo of
Greenwald density (ncw). On the other hand, when Ar
was puffed, the degradation of confinement with thedensity was reduced significantly and the confinement at
the high density regime was improved. One of thecauses of this improvement of confinement is supposed

to be a reduction of deuterium gas puff in the Ar
injection cases. The radiation fraction was also increased
by -lO-2UVo in the Ar injection cases. As a result, Flse
-1.4 with radiation fraction of -8OVo was obtained at

4. Heat and Particle Control in the W-

n"lncw = 0.7 or until the detachment occurred.
When the radiation fraction increased too high, thecore plasma temperature decreased, Ar penetrated into
the core, and the confinement was lost. To sustain the

shaped Divertor
A large radiation fraction is required for long pulse
operation, especially in the next step device, to reduce
the heat load to the divertor plates and to suppress thedamage to the divertor materials. One of the major
issues is to integration of high confinement in the core

ELMy H-mode,

2.5T,1.2 MA,

and the large radiation fraction in the edge, SOL ordivertor plasmas. To enhance the radiation power in thedivertor plasmas, high density, low temperature andhigh impurity content are favorable. To keep highconfinement and high fuel purity in the core plasma, wehave to suppress backflow of neutrals and impurities," ., P*t = 16-20 MW

4.v'

frfrfffi

1.8
1.6
O)

f r.+

from the divertor to the core. To achieve theseobjectives, W-shaped divertor with pump was employedin JT-60U in 1997 [12]. In 1997-1998, the pumping slot
was located at the inner (high field side) divertor whileboth inner and outer sides are pumped in 1999.
In the W-shaped divertor, the capability of heat
handling was improved and sustainment of ELMy H-

1.2

1.0

(a)

.r' ^a detachment

Or

O Itr:O€^ "5ir%8.?'..o0.8

0.4 0.5 0.6 0.7 0.8 0.9 1

mode with heating power of -20 MW for 9 s wasdemonstrated; the total heating energy reached 200 MJ[7]. This is a significant improvement from the open
divertor, in which a carbon burst happened and degradedthe confinement in several seconds. As for the particle
handling, efficient helium exhaust was demonstrated in

1.0

(b)

E o.a

o

IL

; E 0.6
a 0.4

ELMy H-mode plasmas [3].
For the integration of high core confinement andlarge radiation fraction, argon (Ar) injection into ELMy
H-mode plasmas was performed. The results are shown

0.2
0.4

detachment

0.s 0.6 0.7 0.8 0.9

14l ncwin Fig. 5 in comparison with deuterium gas puff
(without impurity seeding) plasmas. The typicalconditions are Ir- 1.2 MA, Bt=2.5 T, 6-0.35 and P1,6
-20 MW. To enhance the radiation fraction, higher(edge) density is favorable as shown in Fig. 5 (b).

Fig. 5 (a) Hrr and (b) radiation fraction versus the density

normalized to the Greenwald density. Opensymbols denote deuterium gas puff cases whileclosed symbols denote Ar injection cases.
92

Fujita T. et al., Recent Progress Towards Steady State Tokamak Operation with Improved Confinement in JT-60U

radiation fraction in a suitable range, feedback control ofradiation from the edge plasma adjusting the Ar puffing

obtained at707o of Greenwald densitv.

rate was employed in this series of experiments.

Acknowledgment

Typically, radiation fraction of -65Vo and F18e - I .4 wassustained for 2.6 s at n"lncw -0.65.

The authors would like to thank to all researchers
who have contributed to JT-60U experiments under awide range of collaborations for their valuable

5. Summary

discussions.

In JT-60U, experiments towards steady state
tokamak operation are being executed mainly in high Bp

Referencesmode and RS mode plasmas. In high Bo H-mode

tll Y. Koide et al., Phys. Rev. Lett. 72,3662 (1994).

plasmas, PN < 2.7-2.9 and l13e < 2.2-2.4 were sustained[2] T. Fujita et al.,Phys. Rev. Lett. 78,2377 (1997).
t3l H. Shirai et a/., Nucl. Fusion 39, 1713 (1999).
t4l Y. Kamada et al., Nucl. Fusion 39, 1845 (1999).
t5l A. Isayama et al., "Characteristics of tearing modes

in high 6 (<0.5) configurations. The steady-state
performance was limited by low n resistive modes. A
newly installed EC system was employed to deposit EC
power at the islands and a slight decrease of mode
amplitude was observed but complete stabilization is notin steady state high Bo mode discharges in JT-60U",
this conference.

yet obtained. In a RS mode plasma with an L-modeedge, Qp7"q = 0.5 was sustained for 0.8 s, which was

t6l A. Isayama et al.,Plasma Phys. Control. Fusion 41,

equal to the energy confinement time, at 2.4 MA and 4.4

t7l Y. Kamada and the JT-60 Team, Plasma Phys.

T. The stored energy feedback control was useful toControl. Fusion 41,877 (1999).
[8] T. Oikawa, JAERI-Research 99-048, l5 (1999).
[9] T. Fujita er a/., Nucl. Fusion 39, 1627 (1999).
[0] S. Ide et al.,Plasma Phys. Control. Fusion 3E, 1645
(1ee6).
ll ll S. Ide er a/., Nucl. Fusion 40,445 (2000).

35 (1999)'.

extend the high performance period but the duration was

limited by beta collapses which occurred when q^;nbecame 2. Sustainment of 4 profile and ITB structures
was successfully done both by LHCD and by bootstrap
current. By LHCD, FN -0.9 was sustained at 0.85 MA, 2
T. In the bootstrap current scenario, Fr -2, 0, -2.6-3,
bootstrap current fraction -807o and F/se -3.5 were
sustained in an ELMy H-mode RS with 0.8 MA, 3.4 T,
qs5 -9 and 6 -0.4. Argon injection into ELMy H-mode
plasmas improved the confinement at the high density[12] N. Hosogane et al., J. Nucl. Mater. 266-269,296(leee).

tl3l A. Sakasai et al., J. Nucl. Mater. 266-269, 312
fl999).

and f/se -1.4 with radiation fraction of -807o was93