J. Plasma Fusiorr Res. SERIES, Vol. 5 (2002) 29'7-301

High Power ICRF Heating in the 4th Experimental in LHD
YAMAMOTO Taroh, KUMAZAWA Ryuhei', WATARI Tetsuor, MUTOH Takashil,SEKI Tetsuor. SAITO Kenji. TORII yuki, TAKEUTI Norio,sASAo Mamikor, ISOBE Mitsutakal, OSAKABE Masakil and LHD Experiment Groupl
Nagoya University, Nagoya 464-8603, Japan
1National Institute for Fusion Science, Toki 509-5292 Japan(Received: 14 December 2001 / Accepted: 5 July 2002)

Abstract
The utility of ICRF (Ion Cyclotron Range of Frequency) heating in the Large Helical Device (LHD)
was dernonstrated in the third campaign, carried out in 1999. This paper summarizes the investigations in2fi)0 focusing on the optimization of ICRF heating, in which scans of the magnetic field strength and themagnetic axis position were made. In the scan of the magnetic field, the optimized ICRF heating
perfomtance was obtained when the minority cyclotron resonance was located at the saddle point of themod-B surface. Under this condition, the highest electron density of 2.7xl}te m-3 was sustained at apower level of 1.5 MW with ICRF heating only. In the scan of the magnetic axis, better performances ofICRF heating were obtained in the inward shifted configuration than in the outward shifted configuration.
The dilference in performance between the two cases was studied in terms of the difference ofconfinement of the high-energy ions as well as that of the bulk plasma.

Keywords:
ICRF heating, LHD, optimization, magnetic configuration

1. Introduction

The major achievement of ICRF heating in the fourth

ICRF heating is now one of the established heating
schemes in tokamaks. However, its application to helicalcampaign was the injection of 2.7 MW of power,
doubling the previous record. This paper limits its main
scope to the associated optimization of ICRF heating
and describes the performance of ICRF heating, which
depends on the magnetic configuration and its strength:
In Sec. 2, the optimum heating condition for ICRF
heating is identified by sweeping the magnetic fieldstrength. In Sec. 3, two discharges with different

systems is not so straightforward and efforts have been
made to establish it in helical systems [1-4]. The
experimental rosults in the LHD [5] in the 3rd campaign
provide a more advanced demonstration of the ICRF
heating in the helical system than those in previous
experiments, riue to the high plasma stored energy
attained and the overcoming of impurity problems [6,7].
Various rvorking hypotheses were proposed asreasons for the success of the ICRF experiment in thethird campaign and some of them have been introduced
in previous publications [8]. The alignment of the flux
surfaces on the drift orbits of the trapped particles due tothe adopted mergnetic axis shift has been pointed out asa possible candidate of confinement improvement t8,91.

magnetic axes are compared and the differences in thebehavior of high-energy ions are discussed. In Sec. 4,
the results obtained are discussed and summarized.

2. Dependence of the Heating Efficiency on
the Magnetic Field Intensity
In the minority heating regime, a plasma consisting
02002 by The Japan Society of Plasma

C o r r e sp o ndin g aut ho r' s e - mail : ty amamo @ nifs. ac.j p

Science and Nuclear Fusion Research297

Yamamoto T. et al., High Power ICRF Heating in the 4t Experimental in LHDof helium ions and hydrogen ions was heated. Theformer is the majority species and the latter is thepower was 1.5 MW. The line-averaged electron density
was increased gradually with the increase of theminority species. The highest available magnetic field
intensity on the magnetic axis of the LHD is Br-2.9 T.
The locations ofresonance layers and cutoff layers areintroduced helium gas. The radiated power increased
with the electron density up to 500 kW, which was 33 Vo

shown in Fig. 1. The calculation was made with themagnetic axis at R"* = 3.6 m, magnetic field strength B,=2.75T, the applied frequency.f = 38.47 MHz, the lineaveraged electron density n" = lXlOle m-3, and theil0(a){, roominority concentration ratio ngln" = Q.QJ.
The performance of the ICRF heating is considered
to depend on the location of the ion cyclotron resonancelayer, with which the propagation and the absorption of
the fast wave are characterized. Figure 2 shows thedependence of the plasma stored energy and the line-FoB ggaEv

E0aaveraged electron density on the magnetic field strength.

g\sThe data were taken from the discharges sustained byICRF heating only, in the power range of 1.4-1.6 MW.
Figure 2(a) shows that the maximum plasma storedenergy was obtained &t B, = 2.'15 T [6]. Under thiscondition, the ion cyclotron resonance layer was locatedso as to cross the saddle point area of the mod-Bsurface. The ICRF power may be absorbed effectively
via cyclotron damping because of the gentle gradient of$XIao)\r'IEt(gEE0lo2.6

2.7RIL8Bt [T]Fig. 2 Magnetic field dependences (a) stored energy and(b) line averaged plasma density. The data areataken from the shots sustained by ICRF heating.

the magnetic field strength there. Figure 2(b) shows thatthe electron density attains its maximum value also at B,= 2.7 5 T , reaching 2.7xl0te m-3. The time evolutions of
the plasma stored energy, the ICRF heating power, theline-averaged electron density and the radiated power
are shown in Fig. 3. In this discharge the ICRF heating200*150

lvP2E11 roo-soP

tcn'i €00

tostl!

"?

.FzsErogs!. Eo8i1l:"0*L'NIsJo'
3.O

3.5

4.O

4.5

5.0

Tlmc Isec.]

Major Radius [mJFig. 3 Discharge waveforms: from the top stored energy
and ICRF Power, line averaged electron density,

Fig. 1 The locations of resonance layers and cut-off layers in front of the ICRF antenna and the mod-B

electron temperature, and radiated power are
shown.

surfaces.
298

Yamamoto T. et al., I\gh Power ICRF Heating in the 46 Experimental in LHDof the injected ICRF heating power. At the end of thedischarge (3.5 s) the radiated power was abruptly
increased, indicating the operational density limit ofICRF heated plasma. This density is 83 7o of the limit
density deduced from the density limit scaling of thehelical system [10]. It is noted that the operational
density limit of the NBI heated plasma in the LHD ishigher than the scale [0] giving 4-5xl0re m-3 [ l,12]
for the same heating power. The density limit of ICRF

Figure 5 shows the NBI data taken from theneighboring discharges; the wall conditions were similar
to these in the present experiment and the magnetic axis
was varied in a wider range from 3.6 m to 4.0 m. The

stored energy (Fig. 5(a)) and the normalized energy
confinement time (Fig. 5(b)) in the inward shifted
configuration were confirmed to be larger than those in

the outward-shifted configurations. Thus, thesustained plasmas is thus about one half of the NBI
density limit of the LHD. The difference in the3m

operational density limits between the ICRF- and NBIheated plasmas may be attributed to the difference in theamount of the rmpurity ions between them.
The energy confinement time, i.e., the performanceof the ICRF heated plasma is the same as that of theNBI heated plasma, after the optimizations described inE2oo*t*
2N

this and the nert section are made.

t

3. The Dependence of the Heating Performance on the Magnetic Axis Position
It is known that the drift orbits of trapped ions are.s'{=Gloo>'sG00

improved in the inward shifted magnetic configurationin the LHD. Accordingly, the 5-D Monte Carlo

Time [second]Fig. 4 Time evolutions of the plasma parameters, Themagnetic axis is located at (a) 4, = 3.6 m (b) F", =3.75 m.

simulation preclicts an improvement of the ICRF heating
performance as the magnetic axis is shifted inward [13].
The dependence of the performance of the ICRF

heating on the magnetic axis shift was investigated,applying ICRF on the NBI target plasma. A comparison
was made in the case of the two different magnetic ilxes,
Ru" = 3.6 m and R* = 3.75 m. Time evolutions of theplasma stored energy, the injected ICRF power and theabsorbed NBI power are shown in Fig. 4. The NBIpower 0.9 MW and the line-average electron density
l.0x10te m-3 were the same for these plasma discharges.
The absorbed ICRF heating powers (f = 38.47 MHz)
were 0.84 MW and 0. 45 MW for the two cases of R* =3.6 m and Ru = 3.75 m, respectively. The achieved
plasma storecl energies were 210 kJ and 100 kJ
respectively at their flat tops in the ICRF heating period.

600*{400FOma 3.&nV 3.75m
I 3.9rtr
X 4Omoao. ' tif'YI15g\.3v$Iv

V*osThe energy confinement times normalized to ISS95 n4l
scaling are 1.46 and 0.95 for the two cases of R* = l.fm and R* = 3.75 m, respectively. This indicates that theperformance of ICRF heating is better with an inward
shifted configuration. However, it has to be rememberedoaaaara)

$ ,-.

Iv

a 3.6nV 17&rtj'

0(a)':(b)01234567
4ude^3tthat the energ)'confinement time is improved with aninward shift of the axis even with NBI [15], whereenergetic particles are passing particles and would beaffected less by the change of the orbit of trapped

Fig.5 The dependences of W, and the normalizedenergy confinement timeon the magnetic axis
"Eposition. The magnetic fieldstrength was B, / F",=2.75T 13.6 m, 2.64 T 13.75 m,2.54 T/ 3.9 m andparticles.

2.48 T I 4.0 m on the axis.

299

Yamamoto T. et al., High Power ICRF Heating in the 4fr Experimental in LHD

improvement of the performance of the ICRF heating
should be attributed predominantly to the improvedconfinement of the bulk plasma.
In order to distinguish the effect of improved drift
orbits ofhigh energy ions on the overall performance inconfinement time of the high-energy ions u6 is estimatedthe inward-shifted configuration from that of bulk

The 15 is estimated from Eq. (2) to be 0.04lsec andplasma, the energy distribution was measured with theNDD (Natural Diamond Detector) [16] during the first
ICRF pulse shown in Fig. 6. The second ICRF pulse is0.026 sec for R = 3.6 m and 3.75 m, respectively, as theof a different frequency (f = 4O.47 MHz) and is not

performance of the ICRF heating is comparable to thatof NBI, 16 has to be a few times larger than that of theassuming the following equation:

1= 1-1-2P*"

specific volume Vn= 0.14 Vlgp is quoted. However,slowing down time which is of the order of 0.1 sec. If
the plasma volume is taken as Vn = 0.3 VLHD, the r5 isestimated tobe 0.21 sec and 0.13 sec for R = 3.6 m and3.'75 m, respectively. This analysis suggests that the confinement of high-energy particles is better with R = 3.6m than with 3.75 m. However, a more detailed analysis
is required in order to determine the effect of the magnetic axis shift on the performance of ICRF heating via
the improvement of orbits of the high-energy particle,
because of the possible difference in the V1, between theand Ru* = 3.75 m, resPectivelY.

On the other hand, the effective temperature Z"s iscalculated using the Stix formula [6,17]::",?t;l()\\-)

these values are unacceptably small; since the overall

analyzed in this paper. The high-energy ions are found
to extend up to 200 keV. The tail temperature T,u;r wasdetermined in the energy range from 50 keV to 150 keV
to be 2,61 = 25 keV and 2161 = 22 keY for Ru* = 3.6 mt
T*, zT" lr *\ JnHvhIeJI

Tn- 7"*0. Xs - 3n"Vn T"iip'-1trs '(1)lJerc,7", Pub", Tu ns and V6 are the electron temperature,

two cases.

the RF power absorbed by high-energy ions, the ion en-ergy slowing-down time, the minority proton density
and the heating volume, respectively. The followingplasma parameters are used in the calculation; T.s - 2.0
keV and nsln"=9 7o for Ru* = 3.6 m and Z.s = 1.6 keV
and nsln" = 4 Vo for R^ = 3.'75 m. The heating volume
Vr, is 14 Vo of the total plasma volume Vr-no (= 30 m3; ina theoretical estimation [18], which may be taken as theminimal estimate.
The effective temperature is calculated as 38 keV
and 32 keV for Ru* = 3.6 m and Ru* = 3.75 m,respectively. The ratios of the tail temperature to theeffective temperature were 0.66 and 0.69 for Ru* = l.f p

4. Conclusion

and Ru" = 3.75 m, respectively, values which are close tomagnetic axis, it was found that the global performanceeach other. In a more systematic analysis, theof ICRF heating was improved in the inward shifted

The performance of the ICRF heating was studiedin various magnetic configurations. The location of theion cyclotron layer was one of the key parameters for
optimizing the ICRF heating. The best performance ofthe ICRF heating was observed when the cyclotron
resonance layer locates crossing the saddle point area ofthe mod-B surface in the LHD magnetic configuration.
In this optimized configuration, the electron density of
2.7x10re m-3 was achieved attaining 83 Vo of the density
limit predicted by the scaling obtained from the medium
size helical devices. In the experiment shifting theconfiguration. The improvement is attributed to theimproved energy confinement of bulk ions to a large
degree. The confinement time of high-energy particles

tBB
hwas also evaluated from the energy distribution function
measured with NDD. The experimental result suggestedb-sts*Ss\oSB

that the volume of wave-particle interaction is larger
than that estimated from a wave propagation code. Withan assumed larger effective volume, the analysis gives
reasonable values for the confinement times of highenergy particles and indicates its improvement in theinward-shifted case. However, there still remains anambiguity due to the possible difference in the RF(b) R--3.7sm

PR'tdDoo09lml$n8
EtaglkWostml$NAEqglkvl

Fig. 6 Energy distributions of the high energy ions measured by NDD on the different magnetic axes: (a)F", = 3.6 m (b) &, = 3.75 m.

interaction volumes.
300

Yamamoto T. et al., High Power ICRF Heating in the 4,h Experimental in LHD

Acknowledgements
The authors thank all the scientists and thetechnical staff at the National Institute for Fusion

Lett. 85, 4530 (2000).

t8l T. Watari, T. Mutoh et al.,Ntclear Fusion 41,325(2001).

t9l T. Watari, Plasma Phys. Control. Fusion 40, Al30998).
[10] S. Sudo, Y. Takeiri, H. Zushi ef a/., Nuclear Fusion
30, 11 (1990).
[11] M. Fujiwara, K. Kawahata, N. Ohyabu er c/.,Nuclear Fusion 41,1355 (2001).
tl2l K. Nishimura, Annual Report of NIFS Apr. 2000 Mar.2001 p14.
ll3l S. Murakami et al., Nucl. Fusion 39, 1165 (1999).
ll4l U. Stroth er al., Nuclear Fusion 36, 1063 (1996).
t15l H. Yamada, A. Komori et al., in proc. of 28th
European Conference on Controlled Fusion andScience for opt:ration of the LHD device.

Reference[1] T. Mutoh e/ a/., Nuclear Fusion 24, 1003 (1984).
t2l R. Kumazawa et al., (proc.2lst European Confer-ence on (lontrolled Fusion and Plasma Phvsics.
199 4), part-Il, p. 1 000- 1 003.
R.
t3l Kumaz awa et al., proc. 13th Topical Conference

on Applir:ations of Radio Frequency Power toPlamas, AIP Conference Proceedings 485, p.160-1631999).

t4l T. Mutoh et al., Plasma Physics and ControlledFusion 42,265 (2000).
t5l M. Fujiwara et al.,Plasma Phys. Controlled Fusion
41, rs1 B (1999).
t6l R. Kumazawa, T. Mutoh et al.,Physics of Plasmas
E,2139 (2001).

Plasma Physics, Madeira, 2001.

l16l A.V. Krasilnilov, Nuclear Fusion 42, l-9 (2OO2).

llTl T.H. STIX, Nuclear Fusion 15,137 (1975).
[18] K. Saito, R. Kumazawa and T. Mutoh, Nuclear Fusion 41, l02l (2001).

t7l T. Mutoh, R. Kumazawa, T. Seki et a/., Phys. Rev.

301