 II  !
"#$%&'(Current Profile Control and Formation ofCurrent Hole in Tokamaks

T. Fujita
Japan Atomic Energy Research Institute,Naka Fusion Research Establishment

19th Annual Meeting of The Japan Society ofPlasma Science and Nuclear Fusion Research
Inuyama, Japan, Nov. 26-29, 2002

Contents
1. Introduction~ properties of current in tokamaks, measurement ofcurrent profile ~2. Sustainment of large bootstrap current fraction andcurrent profile control
3. Current hole and current drive in a current holeplasma
4. Summary and discussion

1. Introduction
j(r) or q(r) is closely related to transport and stability in tokamaks.
- Formation ITB with negative/weak magnetic shear.
- Stability improvement with high li.

Toroidal (parallel to magnetic field) current in tokamaks:(1) inductive (OH) (2) bootstrap current (3) externally driven
jBS∝∇ p(r)/Bpcurrent(by NB/RF)
jOH∝σNCEφnon-inductive current--- heat transport
jEX --- controllable.
---diffusion of Eφ. and inner current.
3

jOH

jBS

2
1
jBS or jEX

Eφp

Current profile measurements (1)
NB#9B
(co)
plasmaEspecially, MSE diagnostics ischaracterized by high spatialresolution (by local measurement) andhigh accuracy.

Direction of vxB"vπ

D (H)
atomB

λq(r), j(r)••• ••beamline

Direction of B
Bp/Btoptics
detectors

optical fibers

EL= v Bσ

NB#7B(counter)interference
filter
λ1.5

E36899Ip

10.50PNB
2345674
PEC 32108 9 10time [s]65432
10-1-2
3.2

3.4

3.6

3.8

R [m]

44.2

[MW]

Polarization angle

JT-60U

Ip

polarization angle [deg]q(r) or Bp(r) measurement--- Faraday rotation, motional Starkeffect (MSE) ,....

[MA]Current profile measurements (2)Internal toroidal electric field Eφ(r) can be evaluated in addition to q(r)
Ψp(r)/dt
from time evolution of equilibrium; Eφ (r)=-dΨjOH = σNCEφ, jNI = jtot-jOH, jEX = jNI-jBS
E3689934.15 s safetyfactor

j [MA/m2]

ρ

0.6

0.8

7.0 s

5.0 s

0.2

0.4

ρ

0.4

0.6

0.6

0.6

0.8

10.4

7-8 s
6-7 s

0.2

5-6 s

00.8

ρ

1.5

0.8

4.15 s
000.2

1

6.0 s
0.5-301

currentdensity

8.0 s

4.15 s

1-0.2
0EC

0.4

Vloop [V]

0.2

ECCD

5.0 s-2

8.0 s

1.5
1-1

226.0 s0[MA/m ]

7.0 s 6.0 s

7.0 s

1

4.15-5 s
0.2

0.4

ρ0.6

1

EC(exp.)jEC(calc.)

0.5

00.8

jj4

Ψp

8.0 s

2

Ψp [Wb]

5.0 s

q

600

Eφ

Ip ramp-up

108

flux surface
in a plasma

100.2 0.4 0.6 0.8
ρ1

2. Sustainment of large bootstrap
current fraction andcurrent profile control

Linkage of p(r) and j(r) under large fBSJ(r)Pressure
gradient

Bootstrap
current

P

In steady operation of tokamak, jOH is zero and most of theplasma current (>70%) should be carried by jBS while the rest byjEX. (Large jOH can be locally and transiently generated even in full
CD plasma.)
Study on high fBS plasma is important.

Heating
power0

Er shear0

E r (kV/m )6

Momentum-40

0.5
r/a

1

r/a

1

Transport
Reduction10χeχi-20

1000.5

χ (m2/s)Current
drive

MagneticShearq00.5
r/a1Turbulence
Suppression

0.100.5r/ap(r) and j(r) are closely linked to each other1

Linkage of p(r) and j(r) under large fBS(cont.) Two issues:(1) Is there a stationary point
where p(r) and j(r) stay insteady state ?
(2) Can j(r) be controlled bysmall jEX ?

q"well-aligned""not well-aligned"n or Tjbootstrap0ρ

1

Sustainment of large fBS
βN>~2), high confinement (HH>~2) RS with
 In JT-60U, high beta (β2.02015 PNB
10 [MW]
50βpqβN3
H892 HH98y2

HH98y2468time [s]1010.70.6 ρqmin
0.5
12JT-60U

j [MA/m2]

1.2

Ip 0.8[MA] 0.403
β N, βp 2
104
H89 324.5qmin 4
3.5
3

2.7 s (~6 τE)

E35037ne [1019m-3] Ti, Te [keV]fBS~80% and fCD>~100% was maintained for 2.7 s.
E35037
 Current profile was mainly
10Ti
8determined by the bootstrap current.
6 During 2.7s, ρqmin and qmin were
42 8.5 s Tekept stationarily.
04327.5s8.5s1
9.5s
0876
7.5s
58.5s
9.5s
43
1 j +j0.8 BS BD
0.6 8.5 s
0.4
0.2 jBD0 0 0.2 0.4 0.6
r/aITB

ITB

jtotmeajBS
0.81

Is there a stationary point ?
 In the experiment, raising βp (fBS) suppressed the shrinkage ofρqmin, which is a promising result for steady sustainment of ρqmin
by the bootstrap current.
 Oscillatory behavior may happen in a longer time scale and hence
the experiment with a longer duration is needed.
43βp 2100.80.7ρqmin 0.6
0.5
0.4
0.3

E35037E35029E35007

E32611

E35037

E3500756

E35029E32611

78
9time [s]10JT-60U experiment

11Simulation for Tore Supra

Current profile control demonstrated in RS
The radii of qmin and ITB-foot expandedby peripheral LHCD. HHy2=1.4, βN=2-2.2,
ne/nGW=0.8 due to large ITB radius.
 Reduction of the central q-value bycentral N-NBCD.
 Feedback control of q(r) with real-time
q(r) measurement, demonstrated in JET,
should be applied in future.
PLH

10[V]ρfoot

0.8
0.7
0.6

Expansion
66.5

ρqmin
time [s ]07

7.5

JT-60U

Te
7.24s
7.24s

42

6.15s

0730.3

Vloop

Ti

42

ρfoot

6.15s

5PP-NB

PN-NB

00.2
0-0.2

0

Vloop [V]

[MW]

0q

Wdia

3
[MJ ]

[MW]

6Ip = 0.9 MA, Bt = 2.5 T

E37964ne [1019m-3] Te, Ti [keV] Full non-inductive CD with BS(62%), LHCD and N-NBCD.

7.24s
6.85-7.05s

0-0.3
00.5
ρ1

3. Current hole and current
drive in a current hole plasmaCurrent hole as a limit of strong RS and large fBS

Increase of off-axis non-inductive(bootstrap) current-> Decrease of Etor(0) and j(0)(Formation of reversed shear)-> j(0) reach zero
(Formation of current hole)
No global instability with j(0)=0,and stable sustainment.
E39033

1.5
1
0.5
02

PNB[MW]
Ip[MA]

Wdia[MJ]

1015
10
5
03

3.5

4

4.5

5

time [s]

5.5

6

6.5

E390332520
1510501.2
1
0.8
0.6
0.4
0.2
01
0.8
0.6
0.4
0.2
0-0.2-0.40q(r)5.00 s5.20 s5.40 s5.60 s5.80 s6.00 s6.20 sj [MA/m2]

Vloop [V]5.0-5.2s
5.2-5.4s

5.4-5.8s
5.8-6.2s
0.2 0.4 0.6 0.8 1 JT-60U

ρStable existence of current hole

The current hole was observed in JT-60U and JET. It persists
stably (several seconds) without any global instabilities in JT60U.
 High temperature plasma confined by off-axis Bp and ITB.
 Extends operation region of j(r) and enables very high fBS.
IpPNBPECβN, βp 100.06
Bz 0.04
Bt 0.020-0.021

ρ1001βN

βp0PNB[MW][MW]

contourplot ofcurrent
density

current hole
4567time [s]8910E36639, 5.4s

8ITB

Ti

6Te

4
20

ρ~0.27
ρ~0.17
ρ~0.08ρ~0.37

ρ~0.47100.5 PEC

0.5
0320

[keV]

E36639

00.2

0.4

0.6

0.8

120

j15

1.2
1

[MA/m2]1.5
1Ip[MA] 0.502

q0.8
0.6

10currenthole

0.4

50.2
000.20.4

ρ0.6

0.8

10JT-60U

“Current clamp” in a current hole
 A large current hole will be a problem for confinement of α

particles in reactors. Control of current hole radius is required.
 No response to ECCD in the current hole both for co- andcounter-CD. (“current clamp”).
 Some mechanism to maintain the structure.
~~ ~--- anomalous resistivity? dynamo (vxB) due to instability ?
 CD in the center of current hole is difficult
E47735(co-ECCD)
CD outside the current holeE41777(ctr-ECCD)

E41735, t=5.8s

EC32

Wdia021neTe (0)544.50OH66.500.2

0.4 0.6r/actr-ECCD2

total

1-2
55.5
time [s]co-ECCDBS-101003EC02
1E41777, t=5.8sj [M A/ m 2 ]Te(0) [keV] ne [1019m-3] Wdia [MJ]

10

j [M A/ m 2 ]PNB [MW]

20BS

10OH-10.8

1total-2EC
00.2

0.4 0.6
r/a0.81

Control of current hole radius Reduction of the central q-value (outside the current hole) andcurrent hole radius has been demonstrated by central N-NBCD.
N-NBCD
LHCD

87qq

ρ = 0.4

62

6

6.5
7time (s)57.24s

0.6

4

6.15s

30JT-60U

0.5
ρ

7.5
3

1

E41467, 6.2s

unordinary even outside the current
hole. This should be investigated
for the j(r) control outside thecurrent hole.

j [MA/m2]

 The response of j(r) seems to bejOH = σNCEφ2?
10

jtot
00.2

0.4 0.6
r/a

0.8

1

4. Summary and discussions
 Development in j(r) measurement has progressed the j(r)control research.
 Control of j(r) with high fBS is important, where p(r) and j(r)are linked through heat transport and Eφ diffusion.
 Quasi-steady sustainment of 80% of fBS for several τE.

Next: longer sustainment for current diffusion time.
 j(r) control in RS with fBS~60% demonstrated.

Next: (i) evaluate controllability in larger fBS. and (ii)apply feedback control with real-time q(r) measurement.
 Current hole appears as a limit of strong RS and high fBS.

Stable existence and mechanism for clamping j(0)~0.
 The radius of current hole can be controlled, but theresponse should be carefully investigated.

Feedback control of current profile Real time q(r) from polarimetry(Faraday rotation).

3.0q( ρ=0.5)

to minimize q-qref.

2.0
Without LH control

1.0

plasma. Application to large fBS
plasma is expected.

JET experiment

Plhcd [MW]0.50

 At present, low beta, small fBSq ref.

2.5

1.5

 LHCD power was controlled

JET #55873

4681012
Time [s]1416