Japan-Korea Workshop on Heating and Current Drive KSTAR Conference, Daejeon Convention Center, Feb. 5-7, 015 Effect of Magnetic Shear on Propagation and Absorption of EC Waves Presented by K. Nagasaki
The Heliotron J Group F. Sano, T. Mizuuchi, K. Nagasaki, Y. Nakamura, Y. Kishimoto, H. Okada, T. Minami, S. Kado, J. Li, S. Kobayashi, S. Yamamoto, S. Ohshima, K. Imadera, G. Weir, S. Konoshima, C. Takahashi, L. Zang, K. Nishioka, N. Kenmochi, Y. Ohtani, T. Harada, M. Kirimoto, K. Murakami, Y. Nakayama, S. Tei, M. Yasueda, A. Suzuki, X. Lu, K. Nishikawa, S. Kitani, Z. Hong, M. Motoshima, Y. Jinno, H. Kishikawa, H. Matsuda, Y. Nakano, A. Nuttasart, D. Oda, R. Tsukasaki, K. Yaguchi, T. Senju, M. Shibano, K. Tohshi, K. Sakamoto, Y. Ijiri Kyoto University T. Mutoh, K. Y. Watanabe, K. Ida, K. Tanaka, M. Yokoyama, Y. Yoshimura, T. Akiyama, K. Nagaoka, Y. Suzuki, H. Igami, S. Nishimura, G. Motojima, H. Takahashi, K. Mukai, R. Yasuhara, K. Ogawa, K. Narushima, National Institute for Fusion Science N. Nishino Hiroshima University S. Kitajima Tohoku University T. Fukuda Osaka University Y. Nakashima University of Tsukuba F. Volpe Columbia University (USA) G. Weir, D. T. Anderson University of Wisconsin (USA) N. B. Marushchenko PP, Greifswald (Germany) L. Yan, Q. Yang SWIP (China) E. Ascasíbar, R.Jiménes-Goméz, T. Estrada, A. Cappa CIEMAT(Spain) B. Blackwell, D. Pretty ANU (Australia) I. M. Pankratov, V.V. Chechkin NSC (Uknaine)
Background ECH power should be absorbed in single path for localized electron heating, effective current drive and accurate transport study Suppression of EC stray field is effective for protecting invessel materials In stellarator/heliotron configurations as Heliotron E and LHD, the magnetic field structure forms a high magnetic shear, causing mode mixture between O-and X-mode. The single pass absorption is not fully optimized in LHD experiments What is the best launching condition for localized ECH/ECCD?
Outline High magnetic shear configuration Coupled wave Equations including magnetic shear terms Experimental results from the Heliotron E device Comparison between experiment and calculation Low magnetic shear configuration Experimental results from the Heliotron J device Summary
Magnetic Field Profiles in Heliotron E (High Shear) The poloidal field is comparable to the toroidal field particularly at edge region The ratio changes spatially, resulting in a strong magnetic shear
Wave Equations Including Magnetic Shear Effect 1-D second order coupled equations d Eˆ dρ d Eˆ dρ // ω + ( N c ω + ( N c O X φ ) Eˆ // φ ) Eˆ deˆ dφ = φ + Eˆ dρ dρ deˆ // dφ = φ Eˆ dρ dρ // θ φ = d dr E ˆ cosθ ˆ ˆ// = E ς + E z sinθ E ˆ = Eˆ sinθ + ˆ cosθ :magnetic shear ς E z N O = 1 p N = {( 1 p) q} / ( 1 p q) p X = ω p / ω q = ω / ω c This equation is valid under the condition that the radial magnetic field component is weak I. Fidone and G. Granata, Nucl. Fusion 11, 133 (1971)
Mode Power Rate along Propagation Path w/o absorption w/ absorption r (m) r (m) The launched waves are coupled to the plasma at the edge region. The mode conversion occurs along the propagation path. When E wave B res, only 63% of the total power should be absorbed.
Dependence of Single Pass Absorption Rate on Polarization Conditions in Heliotron E 1.0 Left hand rotation left hand rotation 1.0 Right hand rotation right hand rotation β= 0(deg) β=15(deg) β=30(deg) β=45(deg) P abs 0.5 P abs 0.5 β= 0(deg) β=15(deg) β=30(deg) β=45(deg) 0.0-90 -60-30 0 30 60 90 α (deg) The best single pass absorption in Heliotron E Rotation α=-35deg Ellipticity β= +15deg (left hand rotation) 0.0-90 -60-30 0 30 60 90 α (deg) ρ z β α E y
Polarization Control Experiment in Heliotron E (High Shear) Currentless plasmas are produced by second harmonic ECH, then heated by NBI A grating polarizer is used for controlling the polarization of launched waves Second harmonic ECH f=106.4ghz P=300kW perpendicular outside launch focused on the magnetic axis arbitrary polarization rotation B(0)=1.96T B(0)=1.84T K. Nagasaki, Phys. Plasmas 6, 556 (1999)
Comparison between Experiment and Theory 3.0 ECH #71797-71817 100 1.5 ECH+NBI #71797-71817 100 n e =1.5*10 19 m -3 ne=.0*10 19 m -3 105GHz T e (0) (kev).0 1.0 EE 0 BB res 50 η cal (%) T e ECE (kev) 1.0 0.5 EE 0 BB res 50 η cal (%) 0.0-90 -60-30 0 30 60 90 0 α (deg) 0.0-90 -60-30 0 30 60 90 0 α (deg) The measured tilting angles agree with the theory α=-9deg for ECH α=-36deg for ECH+NBI The calculated best single pass absorption Rotation α=-35deg, Ellipticity β= 15deg (left hand rotation)
Best Launching Conditions for Single Pass Absorption The magnetic shear effect must be taken into account for the best single pass absorption in the helical devices with high magnetic shear frequency α (rotation) β (ellipticity) rotation direction Heliotron E 106.4GHz -35 15 left CHS 106.4GHz 5 15 right LHD 84GHz -30 5 left
Heliotron J Device Toroidal Coil A Inner Vertical Coil Plasma Outer Vertical Coil Vacuum Chamber Toroidal Coil B Helical Coil Coil System L=1/M=4 helical coil Toroidal coil A Toroidal coil B Main vertical coil Inner vertical coil 0.96MAT 0.6MAT 0.18MAT 0.84MAT 0.48MAT Major radius 1.m Minor radius of helical coil 0.8m Vacuum chamber.1m 3 Aspect ratio 7 Port 65 Magnetic Field 1.5T Pulse length 0.5sec Pitch modulation of helical coil
STD configuration Magnetic Configuration of Heliotron J (Low Shear) The Heliotron J device features low rotational transform, low magnetic shear and magnetic well ι/π 0.5 10 B (T) 0.0-0.5-1.0 B R B φ B z -0. 0.0 0. z (m) θ deg) 110 100 90-0. 0.0 0. z (m)
Measurement of EC Power Transmission in Heliotron J (Low Shear) Polarizer The transmitted waves are measured with directional couplers located at the bottom port facing the ECH launcher The EC absorption rate agrees with a ray tracing calculation without magnetic shear effect Positive B B=+1.5T, STD 100 #19964-#0004 80 n e = 0.5 x10 19 m -3 100 80 Negative B B=-1.5T, STD #19638-#19641 BL-1+ #19941-#19945 BL- P abs (%) 60 40 P abs (%) 60 40 n e =0.4 x10 19 m -3 0 Experiment Theory 0 Experiment Theory Detectors 0 0 30 60 90 10 150 180 Polarizer Rotation Angle (deg) 0 0 30 60 90 10 150 180 Polarizer Rotation Angle (deg)
Electron Temperature in Heliotron J (Low Shear) ECH Plasmas Internal transport barrier has been observed in ECH plasmas on the Heliotron J device ITB is formed when ECH beyond a critical power is injected on magnetic axis The central electron temperature is dependent on the total injection power and the polarization of launched waves
Dependence of T e on Polarization in Heliotron J (Low Shear) T e (0) is sensitive to single pass absorption rate of nd harmonic X-mode, while T e (r=0.4) is insensitive It appears that maximum T e (0) is obtained when the single pass absorption rate is maximal under vacuum condition, suggesting that the magnetic shear effect is weak T e (kev) 4 3 1 #54889-5498 Te(ρ=0) Te(ρ=0.4) 100 80 60 40 0 X-mode Power Ratio (%) More accurate measurement is required to determine the best launching condition 0 0 30 60 90 10 150 Polarizer Angle (deg)
Summary The polarization of launched waves should be adjusted by considering the magnetic shear effect in high magnetic shear devies The experimental results from a high magnetic shear device, Heliotron E, are in good agreement with the calculation results using 1-D second order coupled equations Experiments in the low shear device, Heliotron J, show that the magnetic shear effect is not so important for determining the polarization condition The mode coupling at edge region will be investigated in LHD for launch optimization Is the magnetic shear effect important in tokamaks? Maybe, yes for high-power (>1MW) long-pulse (>100sec) operation
Polarization Dependence of Plasma Breakdown in Heliotron J (Low Shear) It is known that the plasma breakdown using nd harmonic ECH depends on polarization of launched waves The dependence of plasma breakdown on polarizer angle is consistent with single pass absorption rate calculation nd harmonic X-mode ω 0 /ω=0.50 (B(0)=1.5T) Growith rate (1/s) 800 600 400 00 #54880-5490 STD (H+V=86%) ECH 331(kW) N =0.0 100 80 60 40 0 X-mode Ratio (%) Tangential View of 70GHz ECH Plasma measured with a CCD Camera 0 0 0 30 60 90 10 150 180 Polarizer Angle (deg)