overview of tj-ii experiments - iaea · overview of tj-ii experiments presented by carlos hidalgo...

1

Overview ofTJ-II experiments

Presented byCarlos Hidalgo

On behalf of the TJ-II teamLaboratorio Nacional de Fusión, EURATOM-CIEMAT, Spain

Institute of Plasma Physics, NSC KIPT, Kharkov, Ukraine

Institute of Nuclear Fusion, RRC Kurchatov Institute, Moscow, Russia

Associação EURATOM/IST, Centro de Fusão Nuclear, Lisboa, Portugal

General Physics Institute, Russian Academy of Sciences, Moscow, Russia

Universitat Politècnica de Catalunya, Barcelona, Spain

A.F. Ioffe Physical-Technical Institute, St.Petersburg, Russia

Neutral Beam Line

Neutral Beam Line

Heavy Ion Beam

High Resolution Scattering ThomsonTJ-II

TJ-II heliacB (0) ≤≤≤≤ 1.2 T, R (0) = 1.5 m, <a> ≤≤≤≤ 0.22 m0.9 ≤≤≤≤ ιιιι (0)/2ππππ ≤≤≤≤ 2.2 ECR and NBI heating

3

Global confinement scaling and Enhanced confinement modes:

Spontaneous transitions and development of ExB sheared flows

Biasing induced improved confinement regimes

Effect of transformer induced toroidal current in confinement.

Electron internal transport barriers and role of rationals

Transport studies

Impurity and particle transport studies

Radial electric fields and transport

Plasma – wall studies

divertor like configurations

NBI plasmas

profiles and confinement

Conclusions

OUTLINE

4






Transport studies





NBI plasmas


Conclusions

5

TJ-II vs ISS95 scaling

•Previous experiments in all-metal wall conditionsare consistent with ISS95 predictions.

•Plasma discharges , produced in boronised wallconditions, yield different dependences onrotational transform and on plasma density.

•The possible physical origins: impact of improvedconfinement regimes as well as plasma-wallinteraction on TJ-II global confinement.

- 3 . 1

-3 .0

-2 .9

-2 .8

-2 .7

-2 .6

-2 .5

-2 .4

-2 .3

-2 .2

log

_ta

u A

ctu

al

- 3 . 1 -3 .0-2 .9-2 .8-2 .7-2 .6-2 .5-2 .4-2 .3-2 .2

log_tau Predicted P0.0000 RSq=0.86 RMSE=0.057

Nr.shots

αααα_P αααα_n αααα_iota

All-metalset

368 -0.75±0.03 0.63±±±±0.04 0.5±±±±0.1

Boron set 762 -0.62±0.02 1.06±±±±0.02 0.35±±±±0.04

ISS95 859 -0.59±0.02 0.51±0.01 0.40±0.04

E. Ascasibar et al., (2003)

6

Triggering of e-ITB in stellarator: role of rationals

before & during e-ITBbefore & during e-ITB0.4

0.8

1.2

1.6

-1 -0.5 0 0.5 1

ϕϕϕϕ (

kV)

ρρρρ0

10

20

30

40

50

60

-1 -0.5 0 0.5 1

I tota

l (nA

)

ρρρρ0

0.4

0.8

1.2

1.6

-1 -0.5 0 0.5 1T

e (ke

V)

ρρρρ

T. Estrada et al., Plasma Physic and Controlled Fusion (2004)

F.Castejón et al., IAEA-2004 (EX/8-1)

Great flexibility ofstellarator devices inmagnetic configuration(3/2, 4/2, 8/5,…)

•The rational 3/2 has to be positioned inside theplasma for the e-ITB to appear.

•Is the presence of rational surfaces in the plasmaessential to e-ITB formation or does it simplymodify the power threshold?

before & during e-ITBbefore & during e-ITB

7

Why do rationals trigger ITBs?

Due to a rarefaction of resonantsurfaces in the proximity of low orderrationals which is expected to decreaseturbulent transport.Romanelli PoP- 1993 / L. Cardozo 1997 / Brakel 2002 /Garbet 2001

Due to the triggering of ExBsheared flows in theproximity of rationalsHidalgo PPCF-2000 / Pedrosa 2000/Carreras 2001/ Ochando 2001 / Ida 2002/Shaing 2003/ Estrada 2004 / Castejón 2004:

•kinetic effect.

•Viscosity

•Neoclassical effects

•Turbulence driven flows

TJ-II

experiments

8

Biasing induced H-mode like transition in TJ-II

•Modification of edge radial electric fields by limiter biasing.•Improvement in particle confinement time and reduction of turbulence•No significant impurity influx.•Bursty behaviour in Hαααα

Fre

quen

cy (

kHz) 200

150

100

50

0

0

60

120

80 10 0 12 0 140 160 180

Bia

sing

(V

)

Time (ms)

C. Hidalgo et al., PPCF-2004

0.3

0.5

0.7

0.90

-40

-80

-120

-160

Lin

e A

vera

ge D

ensi

ty (

x1019

m-

3)

Biasing V

oltage (V)

0.8

1.2

1.6

2

100 140 180

Hα

(a.u

.)

Time(ms)

9

Sheared flow development and threshold plasma density

•The development of the naturally occurringvelocity shear layer requires a minimum plasmadensity.

•There is a coupling between the onset of shearedflow development and the level of turbulence.

•This mechanism can explain spontaneous improvedconfinement in TJ-II

2500

3500

4500

5500

6500

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

Eθrm

s/B

(m

/s)

Line Average Density (x1019 m-3)C. Hidalgo et al., Phys. Rev. E (2004);

M.A. Pedrosa et al., IAEA-2004 (EX/P6-21)

0

0.04

0.08

0.12# 9748 n

e≈0.35x10

19 m

-3

# 9749 ne≈0.45x1019 m-3

# 9751 ne≈0.55x1019 m-3

# 9752 ne≈0.65x1019 m-3

Ion

Satu

ratio

n C

urre

nt (

A)

-2000

-1000

0

1000

2000

0.85 0.9 0.95 1 1.05 1.1 1.15

Perp

. pha

se v

eloc

ity (

m s

-1)

r/a

10

Effects of Plasma Current: magnetic shear

•TJ-II plasmas confinement under transformerinduction conditions responds to plasma current.

•Plasma density: decreases/increases smoothly forvalues of positive/negative plasma current

•These observations show the link between magneticconfiguration (magnetic shear) and transport.

D. López Bruna et al., Nuclear Fusion (2004)

J. Romero et al., Nuclear Fusion 43 (2003) 386

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

0 0.2 0.4 0.6 0.8 1

Rot

atio

nal t

rans

form

ρ

+8.5 kA

-0.6 kA

-8.2 kA

5/3

3/2

4/3

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

#6922#6913#6907#6921

I p (

kA

)

a )

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

Loo

p V

olta

ge (

a.u.

)

b )

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1150 1200 1250 1300 1350

<n

e>

(10

19 m

-3 )

time(ms)

c )

11






Transport studies





NBI plasmas


Conclusions

12

Inpurity transport studies

Influence of density / collisionality:

Impurity confinement time (τ) slowly increases with density / collisionality up to a valuewhere it increases more rapidly. The threshold density / collisionality increases with iota.

Influence heating power:

Confinement time shows a strong dependence with ECRH heating power (τ ≈ P-3) whichturns out to be stronger than the one observed in the global energy confinement time (P-0.5).

Zurro B. et al., IAEA-2004 (EX/P6-32)

1

10

100

0,06 0,09 0,12 0,15

ι /2π = 1.6ι /2π = 2.2

ι /2π = 1.6ι /2π = 2.2

ττττ (m

s)

εεεε3/2 νννν*

0

10

20

30

40

50

60

150 200 250 300 350 400 450

bolometerphosphor

10116

10125

11186

ττττ (m

s)

power (kW)

13

Particle transport and perturbative experiments

0.20.4

0.60.8

1

1.21.4

1.61.8

-15

-10

-5

0

0.6 0.7 0.8 0.9 1 1.1

D (m

2 /s)

V r/a (m

/s)

ρ

---- 1145/1153 ms---- 1172/1190 ms

•Particle diffusivities are about 0.3 m2/s (r ≈ 0.6),increasing radially outwards. An inward pinch isnecessary to explain bulk particle transport.(S.Eguilor et al., 2004)

•However, evidence of non-diffusive transportmechanisms has been observed during thepropagation of edge cooling pulses experiments.(B. van Milligen et al., Nuclear Fusion 2002)

•A model has been developed, incorporating acritical gradient mechanism that separates a sub-critical diffusive and a super-critical anomaloustransport channel, depending on the local valueof the gradient.(B. Van Milligen et al., IAEA-2004 (TH/P6-10)

14

•The positive values of the radial electric field (50 V/cm) measured by the HIBP system at lowdensity plasmas are of the order of theneoclassical estimationsKrupnik et al., EPS-2004.

•Coupling between ECRH input power density,plasma potential and the energy of thesuprathermal electron tail.

F.Medina el et EPS-2004.

•An approach based on Langevin equations hasbeen recently introduced to estimate this ECHinduced flux.

F. Castejón et al., PPCF 2003 /

IAEA (2004) EX/8-1

Electric fields: neoclassical and kinetic effects

15

Ion energy confinement and electric fields

ντei e i e

i

EiiT T n

Tn −( ) − =0

•The ion power balance equation showsdifferent confinement regimescharacterized by different ion energyconfinement times.

•Results are compatible with changes inthe ambipolar electric field computedfrom neoclassical calculations.

R. Balbín et al., 2004

16

Radial profiles of plasma potential and flows showevidence of “structures” in configurations with loworder rationals.M.A. Pedrosa et al., PPCF-2004L. Krupnik et al., EPS-2003

Plasma potential, flows and rational surfaces

0

0.2

0.4

0.6

0.8

185 195 205 215

Mac

h N

umbe

r

Probe Position (mm)

With (n=4/m=2)at the plasma edge

Without low orderrational surfaces

17

Magnetic topology: momentum re-distribution viaturbulence

Experiments carried out in the plasma boundary of TJ-II stellarator and JET

tokamak have shown the existence of significant gradients in the cross-correlationbetween parallel and perpendicular flows near the LCFS.(B. Gonçalves et al., IAEA-2004)

Energy transfer:

18






Transport studies





NBI plasmas


Conclusions

OUTLINE

19

58_83_61 100_28_59

-0.36

-0.24

-0.12

0

0.12

-0.2 0 0.2

Z (

m)

R-R0 (m)-0.2 0 0.2

R-R0 (m)

• Enhanced screening ofinjected impurities in edgeisland configurations takesplace.

• These results indicate thatconfigurations with rationalnumbers that give rise to islandchains at the edge are goodcandidates for application asdivertor-type plasmas in TJ-II.

0

1

2

3

0 10 20 30

divertor-type conf.standart conf.

ZLCFS

-Z (mm)

∆ CV

/ne (

a.u.

)

Plasma wall studies: configurations with edge islands

I. García-Cortés et al., JNM-2004

20






Transport studies





NBI plasmas


Conclusions

21

NBI plasmas: profile evolution

•The electron and density profiles show agradual evolution from the hollow shapetypical of ECH plasmas (on-axis) to bell-shaped profiles at the NBI phase (400 kW).

•Measurements of plasma potential show theevolution of the electric field from positive atECH plasmas to near sign reversal at theNBI regime.

-0.6 -0.4 -0.2 0.0 0.2 0.4-400

-200

0

200

400

600

800

1000

1200

#10533

80 ms ECRH

160 ms ECRH + NBI

180 ms NBI

M. Liniers et al., EPS-2004L. Krupnik et al., EPS-2004

TeNe

Φp

0

1

2

3

-1 -0,5 0 0,5 1

n e[x 1

019 m

-3]

ρ

0

0,2

0,4

0,6

0,8

1

-1 -0,5 0 0,5 1

Te

[keV

]

ρ

22

0

2

4

6

8

# 11618

Density (x1019

m-3

)NBI Current (a.u.)ECRH Power (a.u.)

0

10

20

0

0.4

0.8

1.2

50 100 150 200

Hα (a.u.)rms(Vf) (V)

Rad.(a.u.)

Time(ms)

1 5 5 0 8 5 120 155 190 225

200

150100

5 00

Time (ms)

Fre

quen

cy (

kHz)

NBI plasmas:confinement and fluctuations

Combined ECRH and NBIexperiments reveal that, onceECRH heating power is switched-off, a confinement regimecharacterized by:

•a strong reduction in ExBturbulent transport

•significant increase in the ratiobetween density (n) and particletransport (Hαααα) is achieved.

23

Final Conclusions

• Global confinement studies have revealed a positive dependence of energyconfinement on the rotational transform and plasma density.

• Spontaneous and biasing-induced improved confinement transitions, withsome characteristics that resemble those of previously reported H-moderegimes in other stellarator devices, have been observed.

• Magnetic configuration scan experiments have highlighted the interplaybetween magnetic topology, transport and electric fields.

• Experiments configurations with a low order rational located in theproximity of the LCFS have shown the impurity screening propertiesrelated to the expected divertor effect.

• The evolution of transport and turbulence at the transition from ECRH toNBI plasmas plasmas points in the direction of an improved particleconfinement regime.

overview of tj-ii experiments - iaea · overview of tj-ii experiments presented by carlos hidalgo...

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