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

NSC KIPT 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

MOTIVATION NSC KIPT • 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.

Outline NSC KIPT • TJ-II heliac: NBI heating and diagnostics • Evolution of the plasma profiles • Density-potential link • Peripheral potential: HIBP versus Langmuir probes. • Summary

Experimental Set-up in TJ-II NSC KIPT � TJ-II is a four-field-period low-magnetic shear stellarator. TJ-II <R> = 1.5 m <a> = 0.22 m B 0 = 1.0 T <n e > = 0.3–1.1x10 19 m -3 P ECRH = 200 - 400 kW P NBI = 200 - 400 kW

Heating and Diagnostics NSC KIPT � P NBI = 200 - 400 kW PROBE � E beam = 28-30 keV OUTER LIMITER NBI 1 NBI 2 � Heavy Ion Beam Probe (Kharkov/Kurchatov) � NPA – Ti (Ioffe Institute) � Thomson Scattering Te, ne (CIEMAT-FOM Institute) � Langmuir Probes (CIEMAT) � ECE INNER LIMITER HIBP

STATUS OF THE NEUTRAL BEAM INJECTORS NSC KIPT INJECTOR #1 “Co” Injector is operative Max. achieved parameters: 30 kV, 56 A @ ion source, H 0 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

TJ-II Heavy Ion Beam Probe NSC KIPT 125 keV Cs + HIBP allows us to obtain plasma profiles from the edge to the core each 10 ms Those are: ϕ - plasma electric potential ϕ ϕ ϕ from extra energy of the probing particles n e - plasma electron density from the total beam current I Cs++ ϕ and ne fluctuations are also ϕ ϕ ϕ analyzed up to 50 kHz so far L.I.Krupnik et al. P3-23

ECRH versus NBI plasmas NSC KIPT * Low density ECRH plasmas <n> = 0.3 - 1.1 × 10 19 m –3 Te = 1000 – 800 eV ( Thomson sc., ECE) τ E ≤ 4 ms Ti = 80 eV (NPA) 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 × 10 19 m –3 . Te = 200 eV τ E ≤ 8 ms Ti = 120 eV (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 – 6 00 V .

Plasma parameters evolution during NBI NSC KIPT Quasi steady-state NBI NBI ECH plasma is occasionally obtained with off- axis n e (10 19 m -3 ); P rad & H α (a.u.) 3 n e ECRH P rad 2 H α Density control in NBI (400 kW) discharges with a plasma target 1 created by on-axis ECH has proven to 0 be difficult. 3 W dia (kJ), T i (100 eV) W Target plasmas created by off-axis 2.5 dia T ECH, maintained during the NBI 2 i phase, are investigated. In this way 1.5 NBI plasma discharges with density 1 control (up to 130 ms) have been 0.5 obtained. 0 1040 1080 1120 1160 1200 time (ms)

Plasma parameters evolution during NBI NSC KIPT Not steady-state NBI plasma NBI ECH NBI + ECH off-axis 6 n e (10 19 m -3 ); P rad & H α (a.u.) 5 5 n e 4 n e (10 19 m -3 ) 4 ECH cut-off P rad 3 density 3 H α 2 2 1 1 0 0 1 3.5 i (100 eV) 3 Soft-X 0.8 W dia 2.5 T e (keV) 0.6 2 T i dia (kJ), T 1.5 0.4 1 0.2 0.5 W 0 0 1100 1150 1200 -1 -0.5 0 0.5 1 time (ms) ρ

Ti evolution NSC KIPT 100-44-64 ECRH 140 Te (0) = 800 eV 120 Ti (0) = 80 eV 100 Ti, eV n e = 0.5x10 19 m -3 80 Decoupling 60 40 c NBI e 20 f Te (0) =200 eV 0 Ti (0) =120 eV -1,0 -0,5 0,0 0,5 1,0 ρ ρ ρ ρ n e = 3x10 19 m -3 Better coupling R. Balbin, J.M. Fontdecaba, S. Petrov P3-02.This workshop.

NBI plasmas:confinement and fluctuations NSC KIPT 8 19 m -3 ) Density (x10 NBI Current (a.u.) 6 ECRH Power (a.u.) Combined ECRH and NBI # 11618 4 experiments reveal that, once ECRH heating power is switched-off, a 2 confinement regime characterized 0 by: Rad.(a.u.) H α (a.u.) 1.2 rms(Vf) (V) 20 • a strong reduction in ExB turbulent 0.8 transport 10 0.4 0 0 Frequency (kHz) • significant increase in the ratio 50 100 150 200 Time(ms) between density (n) and particle 200 transport (H α α ) is achieved. α α 150 100 5 0 E. Calder ó n et al. P3-04 0 1 5 5 0 8 5 120 155 190 225 Time (ms)

Plasma p _ tential profile evolution NSC KIPT #13542 #13542 1000 1000 0.41 0.41 800 800 0.75 0.75 600 600 1.9 1.9 400 400 ϕ , V ϕ , V 200 200 ϕ ϕ ϕ 0 0 -200 -200 -400 -400 -600 -600 -800 -800 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0 -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 Uscan, V ρ ρ ρ ρ Low density n e < 0.5 10 13 cm -3 –positive 3.0 potential-“Bell shape” 2.5 -3 m 2.0 Higher density n e > 0.5 10 13 cm -3 – negative 19 e *10 1.5 potential at the edge -“Mexican hat” n 1.0 High density n e > 1.5 10 13 cm -3 – negative 0.5 potential -“Cup” 0.0 1020 1040 1060 1080 1100 1120 1140 1160 1180 1200 t, ms (Patterns – see Fujisawa et al. PPCF 2000)

Plasma potential and magnetic configuration NSC KIPT Configuration (101_38_62) Configuration (100_44_64) Iota (a) ≈ 1.5 Iota (a) ≈ 1.6 #13496 #13543 0.65 1000 0.46 600 800 0.7 0.5 0.78 600 1.0 400 0.8 2.0 1.45 400 2.35 ϕ , V ϕ , V 200 200 0 0 -200 -400 -200 -600 -400 -800 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0 -6000 -4000 -2000 0 2000 4000 ρ Uscan, V The same tendency in both configurations

ϕ 0 versus <n e > - I NSC KIPT 1400 1200 The higher the #13536 1000 density – the 800 #13542 lower plasma 0 , V 600 #13501 potential 400 ϕ ϕ ϕ ϕ (TM-4 tokamak, #13542 200 NF 1983) 0 #13668 -200 #13543 -400 Records: -600 +1300 V 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 - 600 V <n e > 10 13 cm -3 Both configurations, various target density

ϕ 0 versus <n e > -II NSC KIPT 800 #13543 t1040 600 t1050 600 t1060 t1070 400 t1130 400 t1140 t1150 ϕ , V 200 200 Φ (0) NBI Φ Φ Φ 0 0 -200 ECRH -200 -400 -400 -1,0 -0,5 0,0 0,5 1,0 -600 0,5 1,0 1,5 2,0 2,5 3,0 ρ <ne> 1019 m-3 The clear potential -density link

Limiter Biasing - Density Rise NSC KIPT • 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 ϕ δ δ ϕ ~ - k ∆ ϕ ϕ ∆ n e . ∆ ∆ (A.V.Melnikov et al. FS&T 2004)

Edge potential versus density NSC KIPT Langmuir Probe HIBP #13278 0.12 1,0 Ion Saturation Current (A) (a) # 9748 n e ≈ 0.35x10 19 m -3 # 9749 n e ≈ 0.45x10 19 m -3 0,8 # 9751 n e ≈ 0.55x10 19 m -3 0.08 # 9752 n e ≈ 0.65x10 19 m -3 ne 0,6 0.04 0,4 0,2 0 1020 1040 1060 1080 1100 1120 1140 t, m s 1000 t=1038 n e = 0.5 10 19 m -3 40 (b) t=1084 n e = 0.63 10 19 m -3 750 Floating Potential (V) t=1135 n e = 0.67 10 19 m -3 500 t=1145 n e = 0.92 10 19 m -3 ϕ , V 0 250 0 -40 -250 -500 0.85 0.9 0.95 1 1.05 1.1 1.15 0.0 0.2 0.4 0.6 0.8 1.0 r/a ρ The edge negative Er is forming The edge negative Er is forming in ECRH plasma when n e > 0.5x10 13 cm -3 Er increases with further when n e > 0.5x10 13 cm -3 density rise (M.A. Pedrosa et al. PPCF 2004)

CONCLUSIONS NSC KIPT 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.