Overview of TJ-II experiments

Overview of TJ-II experiments

Presented by

Joaquín Sánchez

Laboratorio 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

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

ORNL, US

PPPL, US

Carlos III University, Spain

Univ Politecnica de Cayalunya, Spain

221st IAEA Fusion Energy Conference, Chengdu 2006

TJ-II Flexible Heliac

VF coils

TF coils

Vacuum vessel

Central conductor

Plasma

4.6 m


Neutral Beam Line

Neutral Beam Line

Heavy Ion Beam

High Resolution Scattering ThomsonTJ-II

B (0) ≤ 1.2 T, R (0) = 1.5 m, <a> ≤ 0.22 m0.9 ≤ ≤ 2.2 ECR and NBI heating


TJ-II goals

• Stellarator physicsprogress in the development of a disruption free, high density, steady state reactor based on the stellarator concept.

• Basic fusion physics, relevant also to tokamaks & ITER.


• Confinement, electric fields and transport

• Momentum transport

• Plasma – wall

• Conclusions

Overview




• Plasma – wall

• Conclusions


Global confinement and heat diffusion

Nr. shots a P n

TJ-II (ECH) 762 1.990.07 -0.620.02 1.060.02 0.350.04ISS04 1721 2.280.02 -0.610.01 0.540.01 0.410.01

Diffusivities range from 1 to 10 m2/s. e decreases with line density and iota in agreement with global confinement studies. (V. I. Vargas et al., EPS-2006)

E. Ascasíbar et al., Nucl. Fusion 45, 276 (2005)

0

1

2

3

4

5

-1 -0.5 0 0.5 1

n e (10

19m

-3)

0

0.2

0.4

0.6

0.8

1

1.2

-1 -0.5 0 0.5 1T

e (ke

V)

0

20

40

60

80

100

120

140

-1 -0.5 0 0.5 1

Ti (

eV)

Ne profiles show a gradual evolution from the hollow shape typical of ECH plasmas (on-axis) to bell-shaped profiles at the NBI phase (400 kW).

E aa nn PP

ne Te Ti

International Stellarator Confinement Data BaseA. Dinklage et al., IAEA EX/P7-1 (Friday)


Plasma potential profiles

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-600

-400

-200

0

200

400

600

800

1000

, V

ne1013cm-3

0.3 0.6 0.7 0.8 0.95 1.15 1.5 1.8 2.3 3.0

#15585

• Measurements of plasma potential show the evolution of the electric field from positive at ECH plasmas to negative at the NBI regime.

The smooth change from positive to negative electric field observed in the core region as density is raised is correlated with global and local transport results, showing a confinement time improvement and a reduction of electron transport.

A. Melnikov et al., EX/P7-3 Friday

0

1

2

3

4

-1 -0.5 0 0.5 1

#15569#15577#15582#15584#15585

n e (10

19m

-3)

0

0.2

0.4

0.6

0.8

1

1.2

-1 -0.5 0 0.5 1

Te (

keV

)

ne Te

p

Off-axis ECH + NBI


Plasma potential profiles

-150

-100

-50

0

50

100

0.6 0.65 0.7 0.75 0.8 0.85 0.9

<f>

(kH

z)

0.4

0.5

0.6

0.9

<ne>

In addition to edge shear layer (measured by probes) Doppler Reflectometry sees a second, deeper, shear layer which moves inwards as central density rises

T. Estrada et al. Nuclear Fusion 46 (2006) S792


Probabilistic Transport Models

• Based on Continuous Time Random Walk / Master Equation• Avoids assumption of locality• Mathematically sound approach to modelling of critical gradient• Earlier work showed:

• Power degradation• Stiffness / anomalous scaling with system size• Fast transport• etc.

• Now, studied effect of perturbations• Pulse propagation:

• Sign reversal (due to flux accumulation)• “Ballistic” and “instantaneous” propagation

Ballistic Instantaneous

B. van Milligen et al., TH/P2-17 (today)

tim

e

tim

e


Confinement and magnetic topology

Due to a rarefaction of resonant surfaces in the proximity of low order rationals which is expected to decrease turbulent transport

Due to the triggering of ExB sheared flows in the proximity of rationals

TJ-IIrole of different low order rationals

(3/2 vs 4/2, 5/3…)

Why do rationals trigger ITBs?: an open issue


Core Transitions : role of low order rationals

0

0.5

1

1.5

2

Te (k

eV)

0

0.2

0.4

0.6

0.8

1

n e (10

19m

-3)

-8

-6

-4

-2

0

1040 1080 1120 1160 1200 1240

I p (kA

)

time (ms)

1.2

1.3

1.4

1.5

1.6

1.7

1.8

0 0.2 0.4 0.6 0.8

100_60_69 + OH

vacuum1080 ms1140 ms1170 ms

5/3

3/2

4/3

Te

ne

Ip

Te

ne

Ip

•Positioning a low order rational (e.g. 3/2, 4/2, 4/3) near the core triggers controllably CERC (use, e. g., induced OH current or ECCD). But so far no CERC triggered by 5/3.

T. Estrada et al., PPCF 2005 /

FST 2006

5/3 3/2 4/3

-5

0

5

10

15

20

25

1.25 1.26 1.27 1.28 1.29 1.3 1.31 1.32

CERCno CERC

Er (

kV/m

)

R (m)0.3 -0.3

International Stellarator Profile Data Base

Yokoyama et al , EX5-3 Thursday


Core transitions triggered by 4/2 rational

0 1Effective radius

SXR profiles

0

0.2

0.4

0.6

0.8

1

<n e>

(10

19m

-3) a)

H (a.u.)

0

0.4

0.8

1.2

1.6

0

20

40

60

80

100

Te (k

eV) T

i (eV)

-4

-3

-2

-1

0

1000 1050 1100 1150 1200

I p (kA

)

time (ms)

1.9

2

2.1

2.2

0 0.2 0.4 0.6 0.8 1

vacuum1050 ms1100 ms1150 ms

/2

Difference between the SXR reconstructions before and during CERCTe

Ti

ne

Ip

• CERC triggered by the n=4/m=2 rational has been studied in TJ-II ECH plasmas.• Changes in both Te and Ti.

• The SXR tomography diagnostic shows a flattening of the profiles localized around ≈ 0.4 with a m=2 poloidal structure. The rational must be inside the plasma to trigger the transition.

T. Estrada et al.EX/P7-6 Friday




• Plasma – wall

• Conclusions


Momentum transport: plasma core

-1.0 -0.5 0.0 0.5 1.0

-800

-400

0

400

800

(V

)

ne (1019m-3) 0.41 ECRH 0.75 ECRH 1.90 NBI

#13542

•change of sign of the poloidal rotation direction I• depends abruptly on plasma density.

• In low-density plasmas the poloidal direction

corresponds to a positive radial electric field, at

higher densities negative radial electric fields are

deduced from the measured poloidal rotation.

•Results consistent with HIBP measurements• qualitative agreement with neoclassical theory

calculations that predict that the change of sign of

the radial electric field is mainly due to a change in

the ratio of the electron to ion temperature

B. Zurro et al., FST-2006

-6

-4

-2

0

2

4

6

0.2 0.4 0.6 0.8 1 1.2 1.4

Vpol (

km/s

)

ne [10

19 m

-3]

Er > 0

Er < 0


Momentum transport: plasma edge

Field lines

Poloidal limiter

View plane

View coneM.A. Pedrosa et al., EX/P4-40 ThursdayC. Hidalgo et al., EX/P7-2 Friday

•The development of the naturally occurring velocity shear layer requires a minimum plasma density.There is a coupling between the onset of sheared flow development and the level of turbulence (M.A. Pedrosa et al., PPCF-2005).

•Sheared flows can be developed in a time scale of tens of microseconds (A. Alonso et al., PPCF-2006)

3500

4500

5500

6500

Erm

s/B

(m

/s)

-2000

0

2000

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

v per

p (m

/s)

Line Average Density (x1019 m-3)

-80

-40

0

40

0.85 0.9 0.95 1 1.05

Floa

ting

Pot

enti

al (

V)

r/a

Increasing Density

M1

M3

M2


Phase transition model and edge transitions

˜ n k n0

2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.8 0.9 1 1.1 1.2

DataModel

Is /I

s|c

E/E

c

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.8 0.9 1 1.1 1.2

Data

Model

V'

Is /I

s|c

B.A. Carreras et al., Phys of Plasmas (2006)

• The emergence in TJ-II of the plasma edge shear flow layer as density increases can be described by a simple transition model.

• The mechanism used in the model is the resistive pressure driven turbulence.

• This model gives power dependence on density gradients before and after the transition consistent with experiment.

Instability drive

Nonlinear damping

Shear flow stabilization

Viscous dampingReynolds stress

E

N2/ 3E N 1/ 2 E2 N 1/3U2 E

N

dEN

U

aN1/ 3UE bU

Coupled nonlinear envelope equations for the fluctuation level and shear flow

E ( ˜ n / n)2 1/ 2

N (dp / dr) / ( p / a)

U V / r

TJ-II data


Energy transfer between global (parallel) flows and turbulence

ENERGY(DC flows)

ENERGYturbulence

EnergyTransfer

B. Gonçalves et al., Phys. Rev. Lett. 96 (2006) 145001.

First measurements of the production term (P) in the TJ-II stellarator show the importance of 3-D physics in the development of perpendicular sheared flows and the development of significant parallel turbulent forces.

C. Hidalgo et al., EX/P7-2 Friday

r

VP r

II

II


Biasing experiments: electric field damping and transport

•The ratio ne/ H (which is roughly proportional to the particle

confinement time p) increases substantially (~100%) during

biasing.

•Flows decay after biasing in about 30 s: similar results have been found in other stellarator (HSX) and tokamaks (CASTOR)

M.A. Pedrosa et al., EX/P4-40 Thursday

10

20

30

40

50

0.4 0.5 0.6 0.7 0.8 0.9 1

(s)

ne (1019 m-3)

-150

-100

-50

0

50

100

-0.05 0 0.05 0.1 0.15 0.2

Floa

ting

Pot

enti

al (

V)

Time (ms)

# ne 0.5 x 10 19 m-3 13 s

# ne 0.7 x 10 19 m-3 40 s

# ne 0.9 x 10 19 m-3 20 s




• Plasma – wall

• Conclusions


Plasma Wall Interaction Studies in TJ-II

0 1m

HeTurbopumps

Observationoptics

He beam

pulsed valve & skimmer

=85,3°

Manometer

PLASMA

VACUUMVESSEL

LASER

Vacuum chamber 1

Vacuum chamber 2

Comparison with reflectrometry, Li beam and TS--> density profile

Comparison with ECE Langmuir probes and TS--> temperture profile- Self consistency: reproduction of full emission radial profile

emiss.(nm) 667 706 728

Relative line intensity (A.U.)

Validation of the C-R model for He in a supersonic He beam

0

200

400

600

800

-0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8

ThomsonECEHaz de He

Te(e

V)

0

200

400

600

800

1000

1200

1400

-1 -0.5 0 0.5 1

13574

Sonda de LangmuirThomsonHaz de He

T e(e

V)

0

2 1012

4 1012

6 1012

8 1012

1 1013

1,2 1013

-1 -0,5 0 0,5 1

#5743

ThomsonreflectómetroHaz de He

n e(cm

-3)

0

2 1012

4 1012

6 1012

8 1012

1 1013

-1 -0.5 0 0.5 1 1.5

#13574

Haz de Li

Haz de HeThomson

n e(cm

-3)

ne neTe

0

2 1012

4 1012

6 1012

8 1012

1 1013

1.2 1013

-1 -0.5 0 0.5 1

#6296

ThomsonreflectómetroHaz He

n e(cm

-3)

0

100

200

300

400

500

600

700

800

-1 -0.5 0 0.5 1

#6296

Haz de HeECEThomson

Te(e

V)

0

0.5

1

1.5

2

2.5

3

3.5

0.75 0.8 0.85 0.9 0.95 1

#13567

I667expI706expI728exp

I667simulatedI728simulatedI706simulated

I(a.

u.)

0

2 1012

4 1012

6 1012

8 1012

1 1013

1.2 1013

0

20

40

60

80

100

0.75 0.8 0.85 0.9 0.95 1

#13567

ne

Te

n e(c

m-3) T

e(eV

)


Plasma Wall Interaction Studies in TJ-II

Effects of limiter insertion in the plasmas depending on hydrocarbon deposition. Limiter C, contaminated. Limiter A, clean

0

1

2

3H C3/neH A3/ne Isat /ne (Lim.A)

Limiter C in (A Out)

Z< 2.5 cmLimiter A in (C out)Z< 2.5 cm

Erosion / transport of C

Source of C: bulk graphite or pre-deposited CxHy films ?

Experiment: limiter insertion (shot by shot)

Limiter C: graphite contaminated with ethylene (regenerated every shot)

Limiter A: clean graphite

• Recycling (H) similar

•C influx, mainly from contaminated limiter

Contaminated

Clean

0

30

60

CV/ne Te (eV)

135601356513570 135751358013585

0.4

0.8

shot number


Tritium Inventory Control Through Chemical Reactions

Cold plasma experiment + new diagnostic (Cryo-trapping assisted mass spectroscopy)

C deposition can be inhibited by N2 injectionDemonstrated Asdex Up + Laboratory experiments Mechanism?

Chemical sputtering of deposited film -> HCN (doubts for ITER, needs energetic ions , E>50 ev)

Film precursor recombination prevents deposition -> C2Hx (would work at room temp energies )

H2/N2 plasmas. Chem. Sputtering HCN/C2 Hc’s=3

H2/N2/CH4 plasmas. Scavengers HCN/C2 Hc’s=0.1

C2H2

HCN

HCNC2H2

Pre-deposited film+ N2 Plasma

Pre-deposited film+ N2 /Methane Plasma

Tabarés et al. EX/P4-26 Thursday


Near term plans for TJ-II

Lithium deposition

evaporation +Ne GD Plasma 4 ovens, 1g/each. Symmetric

Route to high ne high operation> 1.6 MW NBI by early 2007




• Plasma – wall

• Conclusions


Conclusions

The investigation of plasma potential profiles reveals a direct link between electric fields, density and plasma confinement. Statistical description of transport is emerging as a new way to describe the coupling between profiles, plasma flows and turbulence.

TJ-II experiments clearly show that the location of rational surfaces inside the plasma can provide a trigger for core transitions. These findings provide critical test for different models proposed to explain the appearance of CERCs linked to magnetic topology.

In the plasma core, perpendicular rotation is strongly coupled to plasma density, showing a reversal consistent with neoclassical expectations. Contrarily, spontaneous sheared flows appear to be strongly coupled to plasma turbulence in the plasma edge, consistent with theoretical models for turbulence-driven flows.

Carbon erosion & redeposition studies: - carbon influx from film dominant over bulk graphite release

- scavenger effect dominant over chemical sputtering in N2 puffing experiments .

Overview of TJ-II experiments

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