Integrated Performance and Critical Issues

Towards Steady-State Operation

in JT-60U

Y. Sakamoto and the JT-60U team

Japan Atomic Energy Agency

I - 20

18th International Toki conference (ITC18)

on Development of Physics and Technology of Stellarator/Heliotrons en root to DEMO

9-12 December 2008, Toki Japan

Towards steady-state operation :

High integrated performance is required. High values of N, HHy2 (H89), fBS, fCD, ne/nGW, fuel purity, Prad/Pheat.

should be sustained for long time. Other reactor conditions:

q95~5, Te~Ti, low momentum input, etc.

JT-60U

Introduction

JT-60U: One of the largest Tokamak.

High fusion performance was achieved so far.

• Record value of fusion triple product : n T = 1.53x1021 m-3skeV

• Record value of DT equivalent fusion gain: QDTeq = 1.25

In JT-60U,

Weak Shear (WS) and Reversed Shear (RS) plasmas have been optimized towards high

integrated performance.

ITER SS(RS)

ne/nGW

2.93

1.0 0.82

HHy2

0.78

1.61

N

fBS

fCD

fuel purity

Prad

Pheat 0.46

0.53

JT-60U

Introduction (cont.)

Critical issues for the integrated performance:

–High beta and high confinement

• Enhancement of fusion performance

directly.

–Compatibility of high density with

high confinement

• Large amount of particle fueling by gas-

puffing degrades temperature in the core

region.

–Long sustainment

• Demonstration of steady-state plasma

with high non-inductive current drive

fraction over characteristic time scales

( E, R, ( W)) is required.

1 0 r/a

Weak Shear

q

Reversed Shear

j

p

1 0 r/a

Parabolic

ITB

Box

ITB

Confinement

Stability

BS current

Sustainment

High

Low

High

Difficult

Low

High

Low

Easy

JT-60U

Contents

Introduction

1. High beta operation above no wall beta limit.

2. High density operation with high confinement.

3. Long sustainment with high non-inductive current

drive fraction.

4. Integrated performance towards DEMO

Summary

1. High beta operation above

no wall beta limit

• Key operations:

– Suppression of Neoclassical Tearing Mode

(presented by A. Isayama on Tuesday)

– Stabilization of Resistive Wall Mode (RWM) by

toroidal rotation

JT-60U

High N ~3 above no wall beta limit was

sustained for 5s on WS plasma

• For the RWM stabilization, plasma rotation should be kept above critical rotation.

• In some discharges, duration is limited by

– Energetic particle driven wall mode (EWM), which is the coupling between

energetic particle and RWM.

– RWM precursor, which reduces toroidal rotation and/or its shear.

WS plasmas above no wall beta limit

2.0

2.5

3.0

3.5

0 1 2 3 4 5 6 7 8

N

Duration (sec)

JT-60U

High confinement RS plasmas with high

N above no wall beta limit

• N~2.7, HHy2~1.7, fBS~90% were

simultaneously achieved.

• qmin decreases towards stationary.

– Inner q=3 surface moves inward,

– Outer q=3 surface moves outward.

• VT at q=3 is reduced.

• Destabilization of RWM seems to be

attributed to decrease in toroidal rotation at

q=3 surfaces.

0.4

0.5

0.6

0.7

0.8

qmin

q=3_out

q=3_in

2

3

4

5

qm

in

-0.5

0

0.5

4 4.5 5 5.5 6

VT (

10

5m

/s)

time (sec)

at qmin

q=3_out

q=3_in

0

0.5

1

0

10

20Ip

(MA)

Ip

PNB

E048246

(MW

)

0

1

2

3

N N

N

no-wall ~ 3xl

i

3 4 5 6time (sec)

2

3

4

q

-0.4

0

0.4

0.4 0.6 0.8

5.4s6.0s

VT (

10

5 m

/s)

r/a

JT-60U

Simultaneous achievement of high beta

and high confinement

• Beta limit was improved by RWM stabilization, especially in RS plasma with

keeping high confinement.

• High N expected in ITER was achieved with high confinement.

• In WS plasma, high N with high confinement was obtained without wall stabilization.

• Lower confinement in WS plasma with wall stabilization is attributed to the lack of

strong central heating.

0.5

1

1.5

2

2.5

1 1.5 2 2.5 3 3.5

RS with wall stab.

RS w/o wall stab.

WS with wall stab.

WS w/o wall stab.

ITER SS

HH

y2

N

2. High density operation with

high confinement

• Key operations:

– Pellet injection in WS plasma

– Wide ITB in RS plasma

JT-60U

High confinement was obtained at high

density by pellet injection in WS plasma

pellet injection

Weak shear plasma (3.6T, 1.0MA, q95~6.5, ~0.45)

• Pellet injection (HFS(top)).

• HHy2=1.05, N=2.2 and fBS~60% at ne/nGW=0.7.

• Confinement was degraded with gas puffing at similar density.

• High confinement by pellet injection is attributed to keeping high

pedestal temperature at similar pedestal density.

High density above nGW in RS plasma

• Reversed shear plasma (2.5T, 1.0MA, q95~6.1, ~0.45)

• Large volume configuration for wide radius of density ITB.

• NB fueling only.

• HHy2=1.3, N=2 and fBS~70% at ne/nGW=1.1.

JT-60U

0

5

10

15

20

0

1

2

3

4

5

0

1

2

3

0

2

4

6

8

4 4.5 5 5.5 6 6.5 7 7.5 8

Time (s)

ne,

nG

W

(10

19 m

-3)

PN

B,

PL

H

(MW

)

W (

MJ)

Sn

(10

15 /

s)

I D (

a.u

.)

Pra

d (

MW

)

Compatibility of high density and high

confinement

• High density operation region was obtained at ne/nGW ~ 1 with high

confinement, especially in RS plasmas.

• In WS plasmas, high density operation region was extended by pellet

injection or impurity seeding with small degradation of confinement.

• In the cases of impurity seeding (Ar or Ne), high radiation loss fraction (frad >

0.9) was also obtained.

• Furthermore, in the wall saturated condition, HHy2~0.95 at ne/nGW~0.7.

JT-60U

0.5

1

1.5

2

0.2 0.4 0.6 0.8 1 1.2

RS with Ne

RS

WS with Ar+pellet

WS with pellet

WS

ELMy H-mode

Reactor

HH

y2

ne / nGW

ITER SS

SlimCS

WS + Ar

WS + pellet

WS

H-modeRS + Ne

RS

Wall saturated

3. Long sustainment with high non-

inductive current drive fraction

• Key operations:

– NTM suppression in WS plasma

– ITB control in RS plasma

fBS~45% sustained for ~5.8s (~2.8 R) under

nearly full CD in WS plasmaJT-60U

0

1

2

3

N

p

-1

0

1

0

1

2

3

5 7 9 11 13 15

(V)

(a.u.)

Time (sec)

(MW)

Vloop

D div

0

10

20P

NB

PNNB

E44104~5.8 s (~26

E, ~2.8

R)

Weak shear plasma (2.4T, 1.0MA, q95~4.5)

Removal of q=1.5 for 3/2 NTM suppression by p(r) & j(r) optimization using FB control of stored energy and injection timing of NBs.

N~2.4, fBS~45% and HHy2~1 for 5.8s

jBS+jBD = 90-100% is close to jtotmea Near y full CD

0

1

2

3

4

q

0

2

4

6

8

Ti

Te

(ke

V)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

j (M

A/m

2)

r/a

jBDcal jtot

mea

jBScal

jBDcal+jBS

cal

fBS~75% sustained for ~7.4s (~2.7 R) under

nearly full CD in RS plasmaJT-60U

012

p, NN p

-101Vloop

(V)

0123

4 6 8 10 12

D div

(a.u.)

Time (sec)

2

4H89

0

2

4Prad

(MW)

0

0.5

1

0

10

20Ip

(MA)

PNB

(MW)

E43046~7.4 s (~16

E, ~2.7

R)

0

2

4

6

8

10

6.3s10.8s12.4s

(ke

V)

Ti

0

0.4

0.8

1.2

0 0.2 0.4 0.6 0.8 1j (M

A/m

2)

r/a

jBD

cal

jBS

cal+j

BD

caljtot

mea

2

4

6

8

10

125.1s

6.3s

10.8s

12.4s

q

Reversed shear plasma (3.4T, 0.8MA, q95~8.3)

N~1.7 & fBS~75% were kept constant by feedback control of stored energy.

Although q95 is high, stationary condition of p(r) & j(r) was confirmed.

jBS+jBD = 95% is close to jtotmea Nearly full CD

Long sustainment of high fBS plasmas

under nearly full-CDJT-60U

SlimCS

ITER SS

0

20

40

60

80

100

0 2 4 6 8 10

WSRS

f BS (

%)

Duration (sec)

fCD

>90%

Double symbol: ~Full CDf

CD>90%

• By optimizing high confinement WS and RS plasmas, sustained duration of

high fBS is extended under the nearly full-CD condition.

• High fBS expected in ITER steady state scenario and DEMO reactor is

sustained for longer than current diffusion time scale ( R).

• Durations are limited by pulse length of NB or NNB.

4. Integrated performance towards

DEMO reactor

JT-60U

High integrated performance achieved in

WS and RS plasmas

ne/nGW

2.92

1.0

0.8

HHy2

0.82

1.56

N

fBS

fCD

fuel purity

Prad

Pheat 0.49

0.53

ITER SS(WS)

High HH & N

Full CD

1.4

2.51

0.51

1.08

0.59

0.64

0.41

High ne

pellet

ITER SS(RS)

High HH & N

q95~5.3

ne/nGW

2.93

1.0 0.82

HHy2

0.78

1.61

N

fBS

fCD

fuel purity

Prad

Pheat 0.46

0.53

1.66

2.7

0.920.94

0.54

0.46

0.87 Long RS

q95~8.3

WS plasmas RS plasmas

• Some parameters are not satisfied simultaneously

• Long sustainment : Remaining issues

JT-60U

JT-60SA will bridge large gaps between

DEMO and ITER

• SlimCS: Economical & compact DEMO reactor with high N and fBS.

• Large gaps in design parameters between SlimCS and ITER, especially

N, fBS, Prad/Pheat.

• JT-60SA will address key issues for DEMO, as satellite tokamak of ITER.

– Demonstration of high beta operation by RWM control coils.

– Heat and particle control with divertor pumping capability.

• ITER scenario can be improved by results of JT-60SA.

ITER SS(RS)ne/nGW

2.93

1.0

0.82

HHy2

0.78

1.61

N

fCD

fuel purity

Prad

Pheat 0.46

0.53

fBS

SlimCS

0.9

0.981.3

4.3

0.77

1.0

0.91

JT-60SA

JT-60SA

JT-60U

Summary

JT-60U tokamak optimized WS and RS plasmas towards steady-state

operation of tokamak and demonstrated

– High beta and high confinement simultaneously

– Compatibility of high density with high confinement

– Long sustainment

High integrated performances were achieved in both plasma regimes.

There are still remaining issues for high integrated performance and

long sustainment.

JT-60SA will address the remaining issues, as satellite tokamak of

ITER.