J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 16-21

High performance and Current Drive Study on TRIAM-IM
ZUSHI Hideki*, ITOH Satoshi, SATO Kohnosuke, NAKAMURA Kazuo,
SAKAMOTO Mizuki, HANADA Kazuaki, JOTAKI Eriko, MAKINO Ken-ichi,
PAN Yudong', KAWASAKI Shoji, NAKASHIMA Hisatoshi
Advanced Fusion Research Center, Research Institute for Applied Mechanics

Kyushu University, Kasuga, Fukuoka 816-8580, Japan(Received: 19 January 2000 / Accepted: 11 June 2000)

Abstract
A steady plasma with high performance and high current drive efficiency is reported. A new
favorable regime is obtained in an 8.2 GHz LHCD plasma at high power (130-240 kW). Achieved
plasma parameters are as follows ; n" is 4.3 x l0l3 cm-3 . I rrro is - 70 kA, T" and T; are 0.8 keV and 0.5
keV, respectively, and the stored energy is 1.9 kJ. The energy confinement time tE of - 8-10 ms isachieved and the current drive efficienc! Tlco= nJsDRslPyH reaches - I x lOle A m-2Ail at B =7 T under
the full current drive condition. This regime characterized by improvement of both t76o and t" istriggered by a transition when the power exceeds a certain threshold power.
Current control experiments have been performed by using two opposite traveling waves at 8.2
GHz. The current reduction (NcDllcD < -l|Vo) is clearly seen when the backward traveling LHW (A@ =+90') are added to a target plasma whose current is driven by forward traveling LHW (A@ = -90"). As
the backward LHW power is increased, however, the current reduction becomes small and finally, it
increases by a small amount (-lo7o ) compared with the target plasma level. Mechanisms for thisnonlinear behavior with respect to the backward LHW power will be discussed.

Keywords:
cunent drive. lower hvbrid wave. tokamak

1. Introduction
It is well recognized that the steady state plasma isrequired to realize a nuclear fusion reactor. In the
superconducting tokamak device TRIAM-lM [] the(3) understanding for effects of plasma surrounding
components on steady state operation. It has been

proof-of-principle experiments for steady state tokamak

particle confinement times ?r, ?p for transport study, the(SST) operation have been successfully carried out. The

current diffusion time try1p for current profile control,
and the recycling time r,""r"1in, for plasma-wallinteraction. In order to advance the SST concept the
combination of physical processes with different

recognized that there are dominant timescales among thedifferent physical processes, for example, energy anddischarge duration of more than two hours has beenachieved by lower hybrid current drive (LHCD)
technique at low power and low current level (-20 kW
and -30 kA) t2]. Recent our research activities for SST
are on the following items: (l) realization of highperformance plasma with an internal transport barrier,(2) establishment of the current profile controllability,*Corresponding author's e-mail:IOn leave

timescales should be investigated.

Two topics from recent SST research activities will
be presented in this paper. The first topic relates to a
new discharge mode (Enhanced Current Drive regime)

zushi@ triam.lcyushu-u.acjp

from Southwestern Institute of Physics, 610041, China.

l6@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Zushi H. et al., High Performance and Current Drive Study on TRIAM-lM

characterized by simultaneously achievement of highrange during which ECD is realized. The important time

limiter. The plasma minor radius ao is - O.l2 m and the
major radius R6 is 0.84 m. The working gas is hydrogen.
The direction of the toroidal magnetic field is clockwise
viewing from the top of the machine and the direction of
the driven cunent is also clockwise. In this case we callforward current drive by forward traveling LHW. Whenthe direction of the wave vector is reversed, backward-

scale for which ECD regime is triggered will be

traveling LHW is launched.

current drive efficiency rlco and improved energy
confinement time. Achieved 46.p is by a factor of -2.5
higher than the empirical scaling law based on JT-60[3], and Hfftpts-p is - 1.4 [4]. The power dependence of
ECD regime shows that there exists a threshold power

Two sets of the 8.2 GHz system having 8 klystrons

discussed.

of 25 kW each are separated toroidally by 180'. The

The second topic covers the results of the current
profile control study. In order to achieve a hollow

grill type of antenna is used and the antenna phasing A@
is also scanned by the phase shifter from 70o to 270',but most data are taken at 90o, whose peak parallel
refractive index No,or is - 1.65 corresponding tocurrent density profile j(r) we tested the combination of

two LH waves traveling in the opposite directions.
Especially to study the controllability ofj(r), backward(BW) traveling waves are injected into a LHCD plasma

resonant electron energy of 100 keV.

whose current is already driving by forward (FW)traveling LHW, that is, fast electrons are accelerated in
the forward direction by FW waves.
This paper is conducted as follows. The brief

2.2 Characteristics of LHW

For 8.2 GHz LHW there is no lower hybrid
resonance point because of Bo < 8 T. Under thesedescription of the experimental conditions, LHCD

experimental conditions the ratio of the LHW frequency
to the calculated lower hybrid resonance frequency is 4-

systems and characteristics of LHW will be given inSec. 2. The main aspects of the ECD regime will bepresented in Sec. 3. The role of superposition of LHWs
with opposite N11 spectra on controllability of current
profile will be reported in Sec.4. Summary is given in

10.

The launched LHW can penetrate into the whole
region because of high B, and high frequency. The
critical refractive index No.. (bottom line) does notaffect the wave propagation, because Np"o*) N,"" [5].

Sec. 5.

The up-shifting of LHW refractive index N,o (second
thick line from the bottom) is caused by the toroidal
effects, but the maximum value of N,p is limited - 2,
because of high aspect ratio of 7 and high q (=ll;operation. Thus the phase velocity of the up-shifting

2. Experimental Conditions, LHCD Systems
and Characteristics of LHW
2.1 LHCD plasma and system hardware
In these experiments 8.2 GHz LHCD systems havebeen used. The configuration is the D-shaped poloidal

t2
x

.)

€E

l0

"i\

J

d.Dp

€)

HHl

/

wave is about l3 times larger than the electron thermalvelocity r," (Nao p- cll3v,").In this sense there exists alarge 'spectral gap'between them [6].

t2

3. ECD Regime

FiiiiiiJ l0

/3v*

[ii i

iil

A new operation regime, ECD, is found in highpower 8.2 GHz LHCD plasmas. The record values ofplasma performance obtained in the ECD regime are

E

a,

a
il)

I
6

tl

\i

6

4co of 0.9-l x l0re Am-2/W and t6 of 8-10 ms,

c/I3v,

/

c

A

{,

respectively. The driven culrent is 70 kA and the density
is 4.3 x l0r3 cm-3. Total plasma energy including the4

ffi

high energy component reaches - 2 kJ at launched rf"-.$L--0.72 0.76 0.E 0.E4 o.EE 0.92 0.96 P(nil)
RIrJ.(n)power of 230 kW. The ion and electron temperatures are
0.5 keV and 0.9 keV, respectively.
As mentioned in Sec. I, the ECD regime ischaracterized by both improvement of 46'p and te. The
obtained rf5.p is compared with the empirical scaling
law, Tlro = 12 <7">l (5 + Z"fi t31, where <7"> and Z"n are
a volume averaged electron temperature and effective

Fig.1 Characteristic parallel refractive indices, (a) N,*,

N,,, No"-o are plotted as a function ol R-"'. n"(r)and T"(r) are parabolic, and squared parabolic j(r)is used. (b) The launched N,, spectrum is also
shown.
t7

Zushi H. et al., High Performance and Current Drive Study on TRIAM-lM

I

I

F 0.e

,F o.tq- 0.8

5 o'u

2 0.,
3'0.6
3 o.s

t o.r
Fl

-{.2
I

-80.4

F 0.3

F0

10
8

?6

C,

4
-or
t

€)

q)

E

art

2
0

GI

200

i

€)

€

rso

.la

1

0
100

r20 r40 160 180 200
Power(kW)Y€) 100

Fig. 3 (a) 4cD versus P., . In the hatched area plasma has

B

two values of 4", during a pulse width. (b) 16,1",

Ps0-

decreases with increasing P.".

0
120 kW. Although both l7s6p and n" are increased by- 507o, no enhancement of both r1s, and, t, areobserved. After that P." is kept constant in time. The
ECD event occurs with a delay time c7"6n of - 4 sec.
Although the improved factors are not high because ofTime(sec)

4OVo

Fig.2lalnco, (b) ze and (c) rf power are plotted as afunction of time. P.* is increased at 2 sec from 60
kW to 120 kW and then is kept constant. ECD

the lower power, clear jumps of both qro and 16,
denoted by filled arrows in Fig. 2, are observed underoccurredat-6sec.
charge, respectively. In ECD case the numerical factor
is by 2.5 higher than this scaling with an assumption of

the constant power level. We consider that during ra"1o,hidden parameters may grow with a very long time
constant. In this discharge tuR is 0.1-0.2 sec, therefore
the necessary delay time for ECD to be triggered is at

Z"tr= l. The theoretical explanation for this empiricalscaling law has been reported [7,8], which is based onan enhancement of bride from the thermal electrons toleast several tens of TuRatthe lowest threshold power.

The power dependence of ECD phenomenon iscarefully studied to obtain the range of the variation of

the resonant electrons. Although the spectral gap is very

wide, the Z" increment from 0.5 keV to 0.9 keV in ECD
contributes to the enhancement of tlro.The confinement
time is also compared with the ITER-89-P scaling [4]Pthreshotd and also to get a power dependence of t6"1or, ?s

shown in Fig. 3. The target plasma is sustained by LHW

and H-factor of 1.4 is obtained. Phenomenologicalcomparison with H-mode transition [9] will be given

at fixed power of 85 kW. Additional LHW is injected 2

seconds after the plasma is initiated. Thus the target
plasma parameters for additional LHW injection are
kept constant. The power level of the additional LHW isvaried shot by shot and the power dependence of T4"6risobtained. We observe that ECD occurs within a range oflatter.

It is found that one of the necessary conditions for
ECD to be realized is the LHCD power, that is, there
exists a threshold power. Figure 2 shows one example
obtained near the lower limit of the threshold power
Pthreshotd. At t = 2 sec Py11 is increased from 60 kW to

Pthreshotd (120 kW-170 kW), which means A,P,1,,",1,o17I

1Ps,,"r1,o6) is 30Vo. Here, LP,1r,""1"o,, is the power window,18

Zushi H. et al., High Performance and Current Drive Study on TRIAM-lM

hatched region in Fig. 3, &rrd 1P,1,,",1,o17> is the meanvalue of the threshold power. It is also found that ?4,7o,
becomes short and finally it approaches to 0.18 sec (tuR ) as P1s increases up to - 170 kW, which is theupper limit of Pth,",hotd. Above this value it is expected

4. Gurrent profile control experimentsthat the transition for ECD may occur with a shortertime constant. These experiments are left for future.
This threshold power level is almost l/3 compared with
the ITER H-mode power scaling tl0l. This reduced
value may depend on T4"1o, which is not taken into
account for the H-mode power threshold database. In
order to reduce the threshold power above which an

whose N;1 spectra are opposite, as shown in Fig. 4.

It is widely recognized that a hollow current profile
is necessary to improve the transport [3]. In order to
get a hollow 7(r) we carried out the current profile
control experiment by combining LHW (40 = +90o)

If both power spectra are the same, no driven
current is expected in ideal case without the toroidal
electric field because of the symmetric velocity
distribution function along the parallel velocity. Since
plasma current is already driven by forward LHW inthis experiment, the change in the already flowing
current is studied by applying backward LHW. The
direction of the forward current is clockwise. The BW
traveling LHW are injected into this target plasma. The
antenna phasing are mainly -90" (FW) and +90' (BW),respectively, but the effect of BW A0 on Iusp is alsostudied from +70o to +180'. The power of BW-LHW isvaried from 20 kW to 60 kW step by step and is kept
constant for the injection duration (usually 2 sec).
The plasma parameters in this experiment are as
follows: n"-1.5 x l0r3 cm-3, Te(0) - 0.3 keV and Zr(0) 0.2 keV. The bulk plasma parameters are slightly
changed when the BW-LHW are injected. The line

improved plasma state can be attained we must
investigate the background physics for a relation
between T1"1o, a;nd Pthreshotd.

A hysteresis of 4., with respect to Py1 is found. In
this experiment the power of one LHCD system is kept
constant to sustain the performance of the target plasma
and the power of the another one is varied in time for 6
sec from 0 kW to 85 kW and 85 kW to 0 kW. A
relation, similar to one shown in Fig. 3, is observed
around Prhreshotd in a single shot. For case of power rampup 4cD jumps from 0.4 to 0.6 x lore Am-2/W at around155 kW and for case of power ramp-down it drops from
0.6 to 0.35 lgte 4--2197 ar 130 kW. We consider that"
this hysteresis is attributed to T4"1o, for the ECD regime

density is reduced by -5Vo, but the power dependence ofAn" is not significant. I is not changed or slightly (1020 eV) decreased. Although Z" shows a large scattering,
it is not concluded that electrons are heated by BW
LHW within error bars. The change in the stored energy
is from 0.3 to 0.4 kJ but rs is reduced from 3.5 ms to
2.2-2.7 ms. It is considered that the increment of thestored energy is attributed to the energetic electrons
produced by BW LHW.

depending on the power I l].

In order to find triggering parameters, which
dominate ECD event, several signals (l mco, Ho, density,
charge exchange flux @-, ion saturation current 4d, inthe scrape off layer) at the onset time of ECD are
studied in detail. The reference time is defined as thetime at which 12ry6.2 starts to increase. Within l-2 ms Hostarts to decrease. The density rise is delayed by - l0
ms. The enhancement of n"lHo ratio suggests the
improvement of the particle confinement in ECD. The

f = E.2cHz, htl=cot

reduction of 1,o, also supports this. On the contrary @",enhances in spite of reduced Ho, suggesting Ti increase
and better ion confinement. The change in @", precedesby - 30 ms with respect to the ECD event. Thus it ist;
E

BW trevelliu

fW-tr

Ao=+90 !ill

1.5

Ao=-90

=
t

o

t.0

CL

found that the fastest signal is @",. Althougho 0.5

perpendicular @", measured along the major radius issensitive to the formation of radial electric field E, undersome conditions [2], direct E, measurement has been
not yet performed. The order of these timescales isobserved to be in the range of a few to several times ?E.
Thus within the fastest timescale (l-10) x z"
performance change of plasma is finished.

o
o

]

r
o, 0.0 -6N,-1-202.10
500 50 25---l--4105l0
2

E (keV)T.tto {kev)Fig. 4 The spectra for forward and backward traveling
LHW whose antenna phasing are t90o. The peakparallel refractive index corresponds to theresonant energy of - 100 keV.

19

Zushi H. et aI., High Performance and Current Drive Study on TRIAM-lM
990629(#706r6-694)

Fig.6.
Thus it should be noted that there are two phases,

current compensation phase and enhanced forward
current drive phase. Data points in this figure were taken
at the middle of the pulse width as the typical meanxvalues. However, in the early phase within 0.2 sec justafter BW LHW was injected [s6.2 dropped by almost
the same value and then it varied depending on its BW
power during the rest of the pulse width. Thus current
compensation by BW-LHW shows a highly nonlinear
behavior with respect to the BW power.
In order to investigate controllability ofj(r) by BW
LHW second experiment has been performed. The

-o30
(J
F1

-

20

012345current profile change is monitored by Shafranov A(= 0,time(sec)+ lilz - 1) as a function of the BW power. Here Bp is
poloidal beta and li is the internal inductance. In thisexperiment the BW power is varied smoothly in timefrom 5 to - 70 kW for 3 s. The similar results withrespect to N711sp - Pp,y relation (shown in Fig. 6) are

Fig.5 lrrro is plotted as a function of time. The power
sequence is shown in the figure. BW LHW are
injected from 2 sec to 4 sec.

obtained in a single shot. During the current isis monotonicallycompensated by BW LHW,--"t'-'

4

io--i-.s90increased and A,zl is - 0.8. From ^this measurement anddiamagnetism measurement the current profile becomes

o

,L

E0
-t

-t

'2

It
rt:

-4

!

peaking during the current compensation phase. A
drastic reduction is observed in .,1 when the power isincreased above the certain value (in this case - 40 kW).

Since the hard X ray measurement does not change at
this critical power level, the contribution of the energetic
electrons to B" is not so large. Thus at that power levelj(r) is changed from a peaked one to a broad one andthen the broad profile is kept during the enhanced
forward current drive phase. Finally the total current is---,t---

BW LHW Power(kW)Fig. 6 The relative change in /.r", is plotted as a functionof BW LHW.

increased higher than that in the initial FW LHCD
plasma.

Although the observed counter current drive

We observe that the total current is clearly reduced
when the BW-LHW are injected at low power, as shown

scenario shows a nonlinear behavior depending on thecurrent compensating BW power, the following

in Fig. 5.
In this case BW LHW (-27 kW) are injected from
2 to 4 s. First the cunent dropped at the very beginningphase of BW injection and then it saturated within 0.2
sec. The toroidal electric field (< 100 mV) is produced
during this transient phase. With increasing power the
total current is reduced further, and the reduced value

explanations ate considered for the current
compensation process. In order to interact for BW LHW

with resonant electrons moving in the clockwise
direction, energetic trapped electrons produced via pitch
angle scattering process may be necessary. There are
two situations with respect to the confinement time ?yo"rof the energetic electrons. In order to satisfy thecondition Tsos ) Tsc (scattering time) the order of 0.2 secof go", is necessary. If the large angle scattering process
occurs, these trapped electrons are directly created.
These electrons are drifting in back and forth directions
along their fat banana orbits, that is, when they are
drifting in the BW-direction, resonant interaction occurs

and the relative reduction factor reach-2 kA and -l0%o, respectively. When the BW power isincreased above 27 kW. the further current reduction issuppressed, and then above - 34 kW the direction of thetotal driven current is completely reversed, that is, thedirection becomes the same as that driven by FW-LHW.
This power dependence of Nmsp is summarized in20

Zushi H. et al., High Performance and Current Drive Study on TRIAM-lM

between BW-LHW and these trapped electrons. This

References

results in detrapping the barely trapped energetic
electrons drifting in the backward direction, and[]

S. Itoh et al., in Plasma Phys. Control. Nucl. Fusion
Research 1986 (Proc. I I'h Int. Conf.. Kyoto, 1986)

therefore, the backward current is produced [4]. Thus
the total current I,o,= Irw- 16p is reduced. Since trapped

Vol. 3, IAEA Vienna,32l (1987).

[2] S. Itoh et al.,in Fusion Energy 1996 (Proc. 16fr Int.

particles are located in the outer board, the in-out
asymmetry in hard X ray profile is responsible for thismechanism. Since backward current tends to flow in theConf., Montreal, 1996) Vol. 3, IAEA, Vienna, 351(ree7).
Y.
t3l lked,a et a/., in 15th IAEA Conf. Plasma Phys.
and Contr. N. F. Res. Vol. 1, 415 (1994).
t4l P.N. Yushmanov er al., Nucl. Fusion 30, 1999

outer region and current cancellation is expected there,
the final current density profile will become peaked.
This is supported by enhancement of /;. The plausible
theoretical explanations for the second enhanced FW
current drive phase are given in ref. [5,16] based on thelarge angle scattering process and small toroidal electric(1990).

t5l T.H. Stix, 1992 Chapter 4 in 'Waves in Plasmas',
American Institute of Physics New York.

t6l N.J. Fisch, Rev. of Mod. Phys. 59, 175-234 (1987).
[7] K. Ushigusa, Plasma Phys. and Control. Fusion 38,

field effects. Quantitative comparison with these
theories is left in the future.

1825-1830 0996).

t8l E. Barbato, Plasma Phys. and Control. Fusion 40,
5. Summary
In summary the high performance and current
profile control experiments in LHCD plasmas are

463-A76 (1998).

[9] F. Wagner et al., Phys. Rev. Lett.49,l408 (1982).
tl0l K. Thomsen et al., in lTth IAEA Int. Conf. on
Fusion Energy 1998, Yokohama, IAEA-CN-69/

described.

The ECD regime is obtained in high power 8.2
GHz LHCD plasmas and it is found that there exists the

ITERPI/07.
tl ll H. Zushi et al., Nuclear Fusion 40, I183 (2000).
[l2] J.A. Heikkinen, T. Kurki-Suonio and W. Herrmann,
Plasma Phys. Control. Fusion 40,679 (1998).
[3] D. Moreau and I. Voitsekhovitch, Nucl. Fusion 39,

threshold power level for the ECD regime to be
triggered. This regime is characterized by simultaneous
improvement of r16p and t6.
In order to aim at compensating the current and685-693 (1999).

[4] T. Ohkawa, General Atomic Company Report GA-

obtaining a hollow profile the current profile control
experiment is carried out by using two opposite
traveling LHWs. The backward traveling waves are
injected into target plasma whose current is alreadydriven by forward traveling waves. The results show anon-linear response in the change of the driven current

A 13847 (1976).

ll5l R. Jayakumar et al., Phys. Lett. A 172, 447-451(1993).

[6] V.S. Chan et al., J. Plasma Fusion Res., rn press.

as the backward wave power is increased.

2l