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Study on Fluctuations during the RF Current Ramp-up Phase in the CPD Spherical Tokamak

4 th IAEA TCM on Spherical tori and 14 th International workshop on Spherical Torus ENEA Frascati, October 7-10, 2008. Study on Fluctuations during the RF Current Ramp-up Phase in the CPD Spherical Tokamak.

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Study on Fluctuations during the RF Current Ramp-up Phase in the CPD Spherical Tokamak

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  1. 4th IAEA TCM on Spherical tori and 14th International workshop on Spherical Torus ENEA Frascati, October 7-10, 2008 Study on Fluctuations during the RF Current Ramp-up Phase in the CPD Spherical Tokamak H. Zushi1), T. Ryoukai2), K. Kikukawa2), T. Morisaki3), R. Bhattacharyay2), T. Yoshinaga1,3), K. Hanada1), T.Sakimura2), H. Idei1), K. Dono 2), N. Nishino4), H. Honma2), S. Tashima2), T. Mutoh3), S. Kubo3), K. Nagasaki5), M. Sakamoto1), Y. Nakashima6), Y. Higashizono1), K. N. Sato1), K. Nakamura1), M. Hasegawa1), S. Kawasaki1) H. Nakashima1), A. Higashijima1) RIAM, Kyushu University, Kasuga, Fukuoka, Japan, 816-8580, 2) IGSES, Kyushu University, Kasuga, Fukuoka, Japan, 816-8580, 3) National Institute for Fusion Science, 4) Hiroshima University, 5) Kyoto University, 6) University of Tsukuba, zushi@triam.kyushu-u.ac.jp

  2. Motivation Why steady Bz is required for The initial condition ? Forrest PRL 1992 • Confine the trapped electrons • => toroidal precession current • Pressure driven/ Uni-directional • PS current • => seed toroidal current

  3. Motivation(Role of Bz) Slab-Annulus plasma is unstable because of bad curvature QUEST CPD Slab plasma in simple torus Fast camera image shows vertically moving modes

  4. Fluctuations & Conversion efficiency TOX v.s. Ln at f=8.2 GHz. Fluctuation level= 1% (solid-circle), 10% (dot-dashed), and 20% (dotted), and 40 % (solid-square). lpol =20 mm, and parallel refractive index N||=0.7. According to Laqua PRL 1997

  5. Open to Closed flux surfaces during the current ramp Closed Flux Surface on the Flattop #507034 External Bv Field Vertical shift is suppressed under Bv field with higher decay index. Center Post Ip increases slowly with the slow increase of Pinj (~60kW for Ip ~ 1.7kA).Less effective than current jump(Pinj ~ 30kW for Ip ~ 2kA). t = 0.146 s Yoshinaga ICPP2008

  6. OUTLINE • Diagnostics of fluctuations • Results during the current ramp-up • 2-1) fluctuations in slab-annulus plasma • role of Bz • 2-2) fluctuations during the current jump • 3. Summary

  7. I. RF antenna IR spectroscopy HX(CdTe-PHA) Li-BES (50 PMTs) 45 Flux loop coils (NIFS) (NIFS) (Hirosima Univ.) VUV spectrometer Fast camera (10ms) Stray rf power Visible spectroscopy Li-CCD CT injector (NIFS) CS Rotating Pump limiter (Hyougo Univ) Rogowskii coil (NIFS) Probe SX array AM-reflectometer Ha filter Visible monitor Stray rf power IR-TV camera (Tsukuba Univ.) Pump(TMP,Cryo) Medium speed camera(1ms)

  8. TF CS 50x50mm 10x10 fiber+ 50PMTs CCD Z -600mm Li injector R I. CPD Li-imaging (CCD & LBFS) Ex.１ Ex.２ 4 TFcoils Open Closed+Open fce fce 2fce Bt=0.29T, Bv=40G, Ip~3kA Rf 8.2GHz, 60kW

  9. Typical discharge

  10. Fluctuations in Annulus Plasma w/o Bz Rres=164 mm 1 kW

  11. LF mode with Long correlation length lR~ 5 cm lz> 2.5 cm

  12. Bz suppresses density fluctuations Bt=0.29T Bz< 50 G Density profile is not significantly affected by Bz (< 50G) However, the fluctuation level is much reduced.

  13. Power spectrum and coherency are much reduced Square coherency at 2kHz Low frequency components are reduced as Bz increases. Correlation length is drastically reduced. Better conversion

  14. Bz is desirable to reduce fluctuations Steady Bz is required for the initial condition ! Forrest PRL 1992 • Confine the trapped electrons • => toroidal precession current • Pressure driven/ Uni-directional • PS current • => seed toroidal current • Ryoukai, ICPP 2008 • Reduced Fluctuations • => more efficient conversion

  15. 2 Fluctuations during Current Jump

  16. Burst in LiI and F during Ip Jump 4 ms Blue: t=218ms Green:t=220 ms Red: t=221ms

  17. Contour of LiI(R,Z) during Jump Rres=194mm 159 <R<215 mm 27<Z< 52 mm Vertically aligned contour => horizontally aligned contour Open field => Closed field lines

  18. Profile flattening occurs 1ms before the burst

  19. Coherency of 1kHz during Jump Highly coherent mode at f~1 kHz dominates the viewing area

  20. Summary • Initial Bz suppresses the density fluctuations, suggesting that • the injected ECW can be converted efficiently to EBW • 2D structure of the density fluctuations shows that highly • coherent mode at low freq. with very long correlation length • grows just before the burst of LiI only during the current jump.

  21. 2 Current Jump in CPD Co-directional O-mode injection Normal X-mode injection Threshold P ~ 25 kW Threshold P ~ 20 kW Co-directional O-mode injection seems more efficient for achieving current jump than normal X-mode. The critical parameter for current jump phenomenon should be the value of Ip, since Ip just before and just after current jump are almost identical. The difference of the threshold power on the incident mode may be due to the heating efficiency in each mode. Ip itself may be determined from some equilibrium conditions. Yoshinaga IAEA (2008)

  22. Microwave (8.2GHz) Launching System in CPD • Incident wave must be converted into Electron Bernstein Wave (EBW) mode to heat the core plasma region in overdense plasmas. • Microwave launchers from 8 klystrons are separated into normal X-mode injectors and co-directional O-mode injectors to study injection mode dependencies. for O-mode injection (O-X-B scenario) (injection angle is adjustable) Reflecting mirror Midplane for X-mode injection (X-B scenario) (Normal to B-field) Reflector Shaft

  23. Current Jump discharge in CPD Closed Flux Configuration After Current Jump External Bv Field #506967 2nd ECR 2nd ECR Vertical Shift is observed both in magnetic flux and Ha image. ECR ECR t = 0.153 s t = 0.157 s Center Post Center Post 3ms Current Jump

  24. Non-Inductive Current Generation by ECH via Current Jump Current Jump occurs underrelatively high Bv (≥ 30 G). In this range of Bv, Ip saturates at ~ 1.8 kA without current jump. This suggests that the current jump is necessary to obtain higher Ip under higher Bv. Under Bv ≤ 30 G, Ip increases slowly and there is no clear current jump. Finally achieved Ip with and without current jumpincreases roughly proportional to Bv. Pinj < 60 kW • Summary of ECH current start-up experiments in CPD • Current jump discharges have been observed in CPD. • This suggests that the current jump occurs commonly in ST devices. • Current jump is necessary to achieve higher Ip under higher Bv.

  25. Z(cm) 60 40 20 0 -20 -40 -60 R(cm) 0 20 40 60 電流ジャンプにおけるポロイダル磁場構造の時間発展 Flat top 1.5 kA 外部垂直磁場 Ip = 0 kA 電流ジャンプ R=35.8cm 2nd ECR ECR 1.2 kA 1.4 kA 1.5 kA 1.0 kA 0.8 kA

  26. 磁場構造の変化にあわせて tip B の浮遊電位が低下 1.4 kA 1.2 kA Ip = 0.8 kA 1.0 kA tip A -30V tip B -100V -1000V

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