Plasma potential evolution study by HIBP diagnostic during NBI - - PowerPoint PPT Presentation

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Plasma potential evolution study by HIBP diagnostic during NBI experiments in TJ-II stellarator

A.V. Melnikov, A. Alonso (1), E. Ascasibar (1) , R. Balbin (1), A.A. Chmyga(2), Yu.N.Dnestrovskij, L. G. Eliseev, T. Estrada (1) , J.M. Fontdecaba (1), C.Fuentes, J.Guasp, C.Hidalgo(1), A.D.Komarov(2), A.S. Kozachok(2), L.I.Krupnik(2), M.Liniers(1) , S.E. Lysenko, K. McCarthy(1), M.A. Ochando(1),

• J. L. de Pablos (1) , M. A. Pedrosa(1), S.V.Perfilov,

S.Ya.Petrov(3),V.I.Tereshin(2), TJ-II team (1)

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

(1) Laboratorio Nacional de Fusión por Confinamiento Magnético

Asociación EURATOM-CIEMAT, 28040-Madrid, Spain

(2)Institute of Plasma Physics, NSC KIPT, 310108 Kharkov, Ukraine (3)Ioffe Institute, St-Petersburg, Russia

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MOTIVATION

• One of the important achievements of the fusion community has been

the development of techniques to control plasma fluctuations based on the stabilizing effect of electric fields.

• In stellarator devices radial electric fields can affect both anomalous (via

sheared flows) and neoclassical transport. Both edge and core transport barriers are related to a large increase in the ExB sheared flows in fusion devices – both tokamaks and stellarators.

• This work reports the experimental investigation of plasma potential in

TJ-II heliac in ECRH and NBI heated plasmas.

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Outline

• TJ-II heliac: NBI heating and diagnostics
• Evolution of the plasma profiles
• Density-potential link
• Peripheral potential: HIBP versus Langmuir

probes.

• Summary

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Experimental Set-up in TJ-II

TJ-II is a four-field-period low-magnetic shear stellarator. TJ-II <R> = 1.5 m <a> = 0.22 m B0 = 1.0 T <ne> = 0.3–1.1x1019 m-3 PECRH = 200 - 400 kW PNBI = 200 - 400 kW

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Heating and Diagnostics

PNBI = 200 - 400 kW Ebeam = 28-30 keV Heavy Ion Beam Probe (Kharkov/Kurchatov) NPA – Ti (Ioffe Institute) Thomson Scattering Te, ne (CIEMAT-FOM Institute) Langmuir Probes (CIEMAT) ECE

HIBP PROBE INNER LIMITER OUTER LIMITER

NBI 1 NBI 2

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STATUS OF THE NEUTRAL BEAM INJECTORS

INJECTOR #1 “Co” Injector is operative

• Max. achieved parameters:

30 kV, 56 A @ ion source, H0 Injected power: 200-400 kW Ion Source is still not conditioned up to nominal values (40 kV, 100 A) INJECTOR #2 “Counter” Injector Presently undergoing commissioning Start beams: spring 2006

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TJ-II Heavy Ion Beam Probe

125 keV Cs+ HIBP allows us to

• btain plasma profiles from

the edge to the core each 10 ms Those are: ϕ ϕ ϕ ϕ - plasma electric potential from extra energy of the probing particles ne - plasma electron density from the total beam current ICs++ ϕ ϕ ϕ ϕ and ne fluctuations are also analyzed up to 50 kHz so far L.I.Krupnik et al. P3-23

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*Low density ECRH plasmas <n> = 0.3 - 1.1 × 1019 m–3 Te = 1000 – 800 eV ( Thomson sc., ECE) Ti = 80 eV (NPA)

τE ≤ 4 ms

Core positive plasma potential of order of + 1000 V to + 400 V. The minor area of the negative electric potential may appear at the very edge depending on the plasma density (Pedrosa et al. 2004). * NBI plasmas are characterized by significant density rise up to <n> = 2 – 5 × 1019 m–3 . Te = 200 eV Ti = 120 eV τE ≤ 8 ms (M. Liniers at al., P3-14 this workshop) Negative electric potential in the full plasma column from the center to the

• edge. The absolute value of the central potential is of order

– 300 V to – 600 V.

ECRH versus NBI plasmas

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NBI ECH 1 2 3 ne Prad Hα ne (1019m-3); Prad & Hα (a.u.) 0.5 1 1.5 2 2.5 3 1040 1080 1120 1160 1200 W

dia

T

i

Wdia (kJ), Ti (100 eV) time (ms)

Plasma parameters evolution during NBI

Quasi steady-state NBI plasma is occasionally

• btained with off- axis

ECRH

Density control in NBI (400 kW) discharges with a plasma target created by on-axis ECH has proven to be difficult. Target plasmas created by off-axis ECH, maintained during the NBI phase, are investigated. In this way NBI plasma discharges with density control (up to 130 ms) have been

• btained.

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1 2 3 4 5 6 NBI + ECH off-axis

ne (1019m-3)

0.2 0.4 0.6 0.8 1

• 1
• 0.5

0.5 1

Te (keV) ρ

1 2 3 4 5 ne Prad Hα ne (1019m-3); Prad & Hα (a.u.) 0.5 1 1.5 2 2.5 3 3.5 1100 1150 1200 Soft-X Wdia Ti W

dia (kJ), T i (100 eV)

time (ms) NBI ECH

ECH cut-off density

Plasma parameters evolution during NBI

Not steady-state NBI plasma

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Ti evolution

• 1,0
• 0,5

0,0 0,5 1,0 20 40 60 80 100 120 140

Ti, eV ρ ρ ρ ρ

c e f 100-44-64

ECRH Te (0) = 800 eV Ti (0) = 80 eV ne = 0.5x10 19 m -3 Decoupling NBI Te (0) =200 eV Ti (0) =120 eV ne = 3x10 19 m -3 Better coupling

• R. Balbin, J.M. Fontdecaba, S. Petrov P3-02.This workshop.

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2 4 6 8 # 11618

Density (x10

19 m

• 3)

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

10 20 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 5 120 155 190 225 200 150 100 5 0 Time (ms) Frequency (kHz)

NBI plasmas:confinement and fluctuations

Combined ECRH and NBI experiments reveal that, once ECRH heating power is switched-off, a confinement regime characterized by:

• a strong reduction in ExB turbulent

transport

• significant increase in the ratio

between density (n) and particle transport (Hα

α α α) is achieved.

• E. Calderón et al. P3-04

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Plasma p_tential profile evolution

• 5000
• 4000
• 3000
• 2000
• 1000

1000 2000 3000

• 800
• 600
• 400
• 200

200 400 600 800 1000

ϕ, V Uscan, V 0.41 0.75 1.9 #13542

Low density ne < 0.5 1013 cm -3 –positive potential-“Bell shape” Higher density ne > 0.5 1013 cm -3 – negative potential at the edge -“Mexican hat” High density ne > 1.5 1013 cm -3 – negative potential -“Cup”

• 1,0
• 0,8
• 0,6
• 0,4
• 0,2

0,0 0,2 0,4 0,6 0,8 1,0

• 800
• 600
• 400
• 200

200 400 600 800 1000

ϕ ϕ ϕ ϕ, V ρ ρ ρ ρ

0.41 0.75 1.9 #13542

1020 1040 1060 1080 1100 1120 1140 1160 1180 1200 0.0 0.5 1.0 1.5 2.0 2.5 3.0

n

e *10 19

m

• 3

t, ms

(Patterns – see Fujisawa et al. PPCF 2000)

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Plasma potential and magnetic configuration

• 1,0 -0,8 -0,6 -0,4 -0,2

0,0 0,2 0,4 0,6 0,8 1,0

• 400
• 200

200 400 600

ϕ, V ρ

0.5 1.0 2.0

#13543

Configuration (100_44_64) Iota (a) ≈ 1.6

• 6000
• 4000
• 2000

2000 4000

• 800
• 600
• 400
• 200

200 400 600 800 1000

ϕ, V Uscan, V

0.65 0.46 0.7 0.78 0.8 1.45 2.35

#13496

Configuration (101_38_62) Iota (a) ≈ 1.5

The same tendency in both configurations

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ϕ0 versus <ne> - I

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5

• 600
• 400
• 200

200 400 600 800 1000 1200 1400

#13668 #13501 #13542 #13542 #13543

ϕ ϕ ϕ ϕ

0 , V

<ne> 1013 cm

• 3

#13536

The higher the density – the lower plasma potential

(TM-4 tokamak, NF 1983)

Records: +1300 V

• 600 V

Both configurations, various target density

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ϕ0 versus <ne> -II

• 1,0
• 0,5

0,0 0,5 1,0

• 400
• 200

200 400 600

ϕ, V ρ

t1040 t1050 t1060 t1070 t1130 t1140 t1150 #13543

The clear potential -density link

0,5 1,0 1,5 2,0 2,5 3,0

• 600
• 400
• 200

200 400 600 800

Φ Φ Φ Φ (0)

<ne> 1019 m-3

NBI ECRH

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Limiter Biasing - Density Rise

• Slow plasma potential

modifications induced by biasing are linked to the plasma density evolution: the higher the density the lower the potential value.

• In observed density

range the dependence is linear δ

δ δ δϕ ϕ ϕ ϕ ~ - k∆ ∆ ∆ ∆ne.

(A.V.Melnikov et al. FS&T 2004)

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Edge potential versus density

0.0 0.2 0.4 0.6 0.8 1.0

• 500
• 250

250 500 750 1000

ϕ, V ρ

t=1038 ne = 0.5 1019m

• 3

t=1084 ne = 0.63 1019m

• 3

t=1135 ne = 0.67 1019m

• 3

t=1145 ne = 0.92 1019m

• 3

1020 1040 1060 1080 1100 1120 1140 0,2 0,4 0,6 0,8 1,0

ne t, m s

#13278

• 40

40 0.85 0.9 0.95 1 1.05 1.1 1.15 Floating Potential (V) (b) r/a 0.04 0.08 0.12

# 9748 ne≈0.35x10 19 m-3 # 9749 ne≈0.45x10 19 m-3 # 9751 ne≈0.55x10 19 m-3 # 9752 ne≈0.65x10 19 m-3

Ion Saturation Current (A) (a)

The edge negative Er is forming in ECRH plasma when ne > 0.5x1013 cm -3 The edge negative Er is forming when ne > 0.5x10 13 cm -3 Er increases with further density rise Langmuir Probe

(M.A. Pedrosa et al. PPCF 2004)

HIBP

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CONCLUSIONS

The recent study of NBI regimes in TJ-II stellarator shows:

• 1. The evidence of positive electric potential up to + 1300 V in

the low density target ECRH plasma.

• 2. The evidence of negative electric potential up to – 600 V in

the whole NBI heated plasma column for the first time in heliac configuration.

• 3. Proof of the potential patterns like “Dome”, “Mexican hat”

and “Cup”, found in CHS torsatron, for heliac configuration.

• 4. The density/potential link: the higher the density - the lower

the plasma potential at the core and at the edge.