Joint Conference of 17th Int. Toki Conf. & 16th Int. S/H Workshop 2007
Ceratopia Toki, Gifu, Japan, October 15-19, 2007

ECCD Experiments
in Heliotron J, TJ-II, CHS and LHDK. Nagasakia), G. Motojimab), A. Fernándezc), Á. Cappac), J. M. Fontdecabac),Y. Yoshimurad), T. Notaked), S. Kubod), T. Shimozumad), H. Igamid), Heliotron
J Teama)b), TJ-II Teamc), CHS Teamd) and LHD Teamd)a) Institute of Advanced Energy, Kyoto University, Japanb) Graduate School of Energy Science, Kyoto University, Japanc) Laboratorio Nacional de Fusión, EURATOM-CIEMAT, Spaind) National Institute for Fusion Science, Japan

Motivation• Study of non-inductive current is one of key issues to control plasma
confinement in tokamaks and S/H systems• In S/H systems, no Ohmic current is required for plasma equilibrium• However, the bootstrap current is inherently driven due to plasmapressure, affecting MHD equilibrium and stability due to the change inrotational transform• Electron cyclotron current drive (ECCD) is recognized as a usefulscheme for the suppression of MHD modes, high performance and/or
full non-inductive operations• ECCD is considered to suppress the bootstrap current in order toavoid the MHD instabilities or to make local strong magnetic shear

Degraded confinement due to low orderresonances at low β, no shear: TJ-II
ECH discharge with induced OH current•time evolution of effective Xe (obtained from power balance calculations). Te profiles fromECE diagnostic•estimated time evolution of iota (assuming total current due to OH transformer): it isforced to flatten so that 3/2 occupies part of the plasma with no shear.

3/2 does not deteriorate transport until 1160-80 msTransient flattening of the pressure profilePurposes of ECCD in S/H Systems
1. Control of rotational transform
Avoidance of magnetic island at rational surfaces
Suppression of bootstrap current
2. Understanding of ECCD physics
Accurate measurement of 0.1 kA orderNo synergetic effect of E||Effect of trapped electrons due to magnetic ripples
Ohkawa effectPhysical Mechanism of ECCD
• Simply considering, electron cyclotron waves accelerate electrons only
perpendicularly, resulting that they do not give toroidal momentum• However, the anisotropy in velocity space due to the EC waves withfinite N|| produces electron parallel momentum
Fisch-Boozer EffectOhkawa Effect

V⊥V⊥

ν１

ν1
ν1< ν2

Heating

Isotropization bybounce motion

Heatingν2

ν2V||V||ECCD Experiment in W7-AS• ECCD was investigated with an ECRH power of up to 1.3 MW at 140 GHz• Highly localized EC current up to 20 kA were estimated• The linear prediction is in reasonably agreement with the current balance
except for low-density discharges with highly peaked on-axis deposition• EBW current drive was also demonstrated

Erckmann NF (2003), Maassberg PPCF (2005)

Schematic Views of Plasma Devices• The device parameters are similar• The plasma parameters are similar, ne=0.2-2x1019 m-3, Te=0.3-2 keV

Heliotron J
R= 1 m, a= 0.2 m, L=1, M=4TJ-II
R= 1.5 m, a= 0.2 m, L=1, M=4CHS
R= 1 m, a= 0.2 m, L=2, M=8LHD
R= 3.75 m, a= 0.6 m, L=2, M=10

Magnetic Field Structures(a) Heliotron J

1.3

1.1

ECHinjection port
1.00
B/BmaxB/B(ϕ=45deg)

1.2

1.00.90.95

ECH injection port
(for fixed N||)
(for N|| scan)straight section

0.8
0.7(b) TJ-II

1.05

Configuration
5:1
6:1
5:2
5:3
5:4 a
5:4 b01530(c) CHS

1.05

4560

75

L10.90900

Toroidal Angle (deg)15(d) LHD

1.05

L230

45607590

Toroidal Angle (deg)
R=3.53m
R=3.60m
R=3.75m

1.00

B/B0B/Bmax

1.00

0.95

Raxis

ECH injection port
0.90

01530

Toroidal Angle (deg)Horizontally elongatedflux surface

0.90

92.1 cm
94.9 cm
97.4 cm

Vertically elongatedcrossection0.95

450.85

ECH injection port
1.5L
051015

ECH injection port
2-O
2025

Toroidal Angle (deg)3035ECH/ECCD Systems
Heliotron J

TJ-IICHS

LHD

Frequency

70GHz

53.2GHz

53.2GHz

84GHz

Maximum
injection power

0.4MW

0.3MWx2

0.3MW

1.3MW

Maximum pulse
length0.2sec0.5sec0.1sec

3sec

Injection mode

Focused/nonfocused
Gaussian

FocusedGaussian

FocusedGaussian

FocusedGaussian

Injection angle

Controllable/
fixedControllable

Controllable

Controllable

Polarization

Controllable

Controllable

Controllable

Controllable

Injection mode

2nd X

2nd X

2nd X1st O/2nd X

ECH/ECCD Systems
Heliotron J
LCFS

TJ-II
Steerable flat mirror
Focusing mirror
BN window

HE11 corrugated waveguide

CHSLHD

Time Evolution of Plasma Current
Heliotron J#247611.519-3

ne (10 m )

1.0
0.5
0.0
10-1-2-3-4-5

Ip (kA)

1.070GHz ECH

TJ-II#122216

0.5
0.4
0.3
0.2
0.1
0.0
10-1-2-3-4
1.5

19 -3

ne (10 m )Ip (kA)

ECH

1.0

0.5
0.0

CHS

0.5200250Time (msec)

300

350

0.00LHD

20406080
Time (msec)

100120Dependence on Injection Angle• The EC current depends on the toroidal injection angle• The EC current is the order of a few kA• The current direction is determined by the Fisch-Boozer effect atnearly the ripple top power deposition
Heliotron J(a) Heliotron J

319#14945-14987-3

ne=0.5x10 m

1.0

PEC=300kW

8=0ECH QTL2 N||Ip (kA)

-0.5

0-1-1.0-2-1.5-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-3N||

6

R=94.9cm
19 -3
ne=0.5x10 m4Pinj=250kW

QTL1=010.0#113284-113340(c) CHSQTL2

ECH QTL1 N||20.5

Ip (kA)ECCD Exp. in TJ-II

Ip (kA)

1.5CHS

TJ-II

20-2-4-0.4

-0.2

0.0
N||

0.2

0.4

-6-0.2

-0.1

0.0

N||

0.1

0.2

Separation of BS and EC currents
2.0• The EC and BS currents can beseparated by reversing the magnetic
field direction in Heliotron J#20824-20857#21441-0ω /ω=-0.488

1.0
Ip [kA]• Theory predicts that the BS current
change its flow direction when themagnetic field direction is reversed,
while the EC current does notω /ω=0.488
00.0-1.0-2.00.0

0.5 1.0 1.5
19 -3n [10 m ]2.0e

2.0|ω /ω|=0.488

1.0

N =0.440||

Bootstrap current
ECCD
STD Configuration
P =320kWEC

I (kA)1 CW⎧CCW
I=I−I()P⎪⎪ BS 2 P⎨
⎪ I = 1 ( I CW + I CCW )P⎪⎩ EC 2 P

0.0-1.0-2.0
0.0

0.5

1.0191.5-3n (10 m )2.0

Reversal of EC Current Direction• ECCD is driven much at low ne/high Te, ripple top heating• The EC current is reversed at ripple bottom heating
Heliotron J
Magnetic field along axis
5:1
5:2
5:3

1.5
1.4
B (T)

Ripple bottom

IEC [kA]

1.6

2.0

Ripple bottom

1.3

ω=2ωce0.0-2.0
εEC
=0.15
b

Ripple top

1.2
1.1
1.0

ω0/ω=0.49-4.0

Ripple topECH injection port
01530

4560

Toroidal Angle (deg)75=0.06
εECb900.0=0.01
εECCDb0.5

1.019

1.5-3n [10 m ]e2.0

Density Dependence• The EC current decreases with electron density at ne<1x1019 m-3 inCHS• The total current is almost constant in TJ-II probably due to thecontribution of bootstrap current
0CHS
3#113830-113841
Rax=0.949m#14998-15020
2~1/ne-3

1
Ip (kA)

IECCD,CCW (kA)

-1-2

ECCD Exp. in TJ-II, March 29, 2006

0-1-4
-5
0-2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
ne,average (1019/m3)-3
0.00.20.40.619-3

ne (10 m )0.81.0Dependence on Power Absorption Position• On-axis power deposition is preferable for high ECCDCHS

Role of Single Pass Absorption• The X-mode fraction was changed bypolarization control angle in order todetermine the role of single pass absorption

1.6

B (T)

1.4

Ripple bottom

1.3

ω =2 ω ce

1.2
1.1
1.0

Ripple top ECH injection port
0Bootstrap current

ECCDω /ω = 0.49

εb = 0.1502

εb = 0.06

1εb = 0.01

Ip [kA]

0Ip [kA]

5:1
5:2
5:3

1.5

• EC current decreases when the X-modefraction is decreased in all three
configurations, indicating that the ECCD isdriven by single pass absorption
2Magnetic field along axis

-1-2

15304560

Toroidal Angle (deg)ω /ω = 0.500(a) #19964-20004

ε = 0.06
b1-3-4-500

Heliotron J

204060exp

Pabs [%]

80

100

0204060

P[%]

exp

abs

80

1007590Effect on Rotational Transform Profile (I)• Rotational transform profile has been measured in LHD by MSE
diagnostic• Co-ECCD increases the central rotational transform, and vice versa.
• This tendency qualitatively agrees with the direction of poloidal
magnetic field generated by the measured EC current
LHD0.7

10.5

ECHCtr ECCD
Co ECCD

0.6iotaΔpol. ang. (deg.)

t=1.8(s)

0-0.5

-1
3.8

3.9

4R(m)

4.1

4.2

0.5

co-ECCD

0.4

ctr-ECCD

0.3
0.2

0.3

0.4

co 1.2s
co 1.8s
ctr 1.2s
ctr 1.8s
0.5
ρ

0.6

0.7

0.8

Effect on Rotational Transform Profile (II)• Rotational transform profile is calculated by HINT2 code

Ip=-5kAεb = 0.01

1.0rotational transform

Ip=+5kAj = j0(1-s)10

Ref. Y. Suzuki. et al, Nucl. Fusion 46 (2006) L19

0.8
0.5vacuum
0 kA
5 kA
-5 kA

0.3
0.0
0.0

0.1reff

0.2[m]• Localized toroidal current changes rotational transform• No significant degradation of plasma confinement due tothe change in rotational transform has been observed

Estimation of ECCD Efficiency• ECCD efficiency in linear theory(jEC
4 s ⋅∇ v / vth ( v / vth )=η=
2PEC 5 + Zeffs ⋅∇ ( v / vth )3)

3emv12=4π e4neΛ(5 + Zeff )
I nR
PEC

γ = EC e ∼Te
5 + ZeffFisch, PRL 41 (1980) 720

ECCD is a factor of 3/4 as efficient as LHCD

• Dimensionless figure of merit

I EC ne R
ζ=Te PEC

Prater, PoP 11 (2004) 2349

Current Drive Efficiency
Heliotron J

TJ-II

CHS

Mode

2nd X

2nd X

2nd X

EC Power

320 kW

200+200 kW

300 kW

Max IEC

4.6 kA

2 kA

6 kA

η=IEC/PEC

14 A/kW

10-15 A/kW

35 A/kW

γ=neIECR/PEC∼ 8x1016
A/Wm2∼ 9x1016
A/Wm2∼16x1016
A/Wm2

ζ=32.7n20IARm/
PWTkeV∼ 0.05

∼0.03

∼0.04

Control of Net Current in ECH Plasmas• Net current free plasma is realized by compensating the bootstrap
current by the EC current in Heliotron J• Co- and counter-ECCD compensate total EC current in TJ-II
Heliotron J#208542TJ-II

19 -3

ne (10 m )1No ECCD

02

Wp (kJ)
102

Ip (kA)0-2

Co + Ctr ECCD
70GHz ECH
200250Time (msec)

300

Conclusion• The ECCD has been studied in Heliotron J (Kyoto Univ), TJ-II(CIEMAT), CHS (NIFS) and LHD (NIFS)• The EC current is controlled by N||, the EC power deposition andmagnetic field configuration• The ECCD is determined by the ripple structure at the EC power
deposition due to the balance between the Fisch-Boozer and Ohkawaeffects• The current drive efficiency is similar in Heliotron J, TJ-II and CHS• The EC current is comparable to the BS current• Net free current state has been experimentally demonstrated bycompensating the bootstrap current with the EC current

Future Plan
• Application of ECCD to plasma confinement improvement
Modification of rotational transform profile
Demonstration of MHD suppression by ECCD
• Is local cancellation of BS current required?
• Enhancement of ECCD efficiency• Extension of ECCD databases
ECCD efficiency
Dependence on ripple structure• Nonlinear interaction between BS current and EC current• Comparison with tokamak experiment and theory