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1、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. Nag
2、asaki5), 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 Fus
3、ion Science, 4) Hiroshima University, 5) Kyoto University, 6) University of Tsukuba, zushitriam.kyushu-u.ac.jp,4th IAEA TCM on Spherical tori and 14th International workshop on Spherical Torus ENEA Frascati, October 7-10, 2008,1,Motivation,Why steady Bz is required for The initial condition ?,Forres
4、t PRL 1992,Confine the trapped electrons = toroidal precession current Pressure driven/ Uni-directional PS current = seed toroidal current,Motivation (Role of Bz),Slab-Annulus plasma is unstable because of bad curvature,QUEST,CPD,Fast camera image shows vertically moving modes,Slab plasma in simple
5、torus,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,Open to Closed flux surfaces during the current ramp,
6、#507034,External Bv Field,Closed Flux Surface on the Flattop,Ip increases slowly with the slow increase of Pinj (60kW for Ip 1.7kA).Less effective than current jump(Pinj 30kW for Ip 2kA).,Center Post,Vertical shift is suppressed under Bv field with higher decay index.,t = 0.146 s,Yoshinaga ICPP2008,
7、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,RF antenna,HX(CdTe-PHA),VUV spectrometer,SX array,Visible monitor,Li-CCD,Fast camera (10ms),Rotating Pump limiter,Pump(TMP
8、,Cryo),Probe,Ha filter,Li-BES (50 PMTs),Rogowskii coil,Medium speed camera(1ms),AM-reflectometer,Stray rf power,Stray rf power,IR-TV camera,IR spectroscopy,Visible spectroscopy,(Hyougo Univ),CT injector,(NIFS),(NIFS),(NIFS),(NIFS),(Hirosima Univ.),(Tsukuba Univ.),CS,I.,45 Flux loop coils,8,Ex.,Ex.,2
9、fce,fce,fce,Open,Closed+Open,Bt=0.29T, Bv=40G, Ip3kA,Rf 8.2GHz, 60kW,I. CPD Li-imaging (CCD & LBFS),4 TFcoils,Typical discharge,Fluctuations in Annulus Plasma w/o Bz,1 kW,Rres=164 mm,LF mode with Long correlation length,lR 5 cm lz 2.5 cm,Bz suppresses density fluctuations,Density profile is not sign
10、ificantly affected by Bz ( 50G) However, the fluctuation level is much reduced.,Bt=0.29T,Bz 50 G,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,Bz is desirable t
11、o 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,2 Fluctuations
12、during Current Jump,Burst in LiI and F during Ip Jump,4 ms,Blue: t=218ms Green:t=220 ms Red: t=221ms,Contour of LiI(R,Z) during Jump,Vertically aligned contour = horizontally aligned contour Open field = Closed field lines,159 R215 mm 27Z 52 mm,Rres=194mm,Profile flattening occurs 1ms before the bur
13、st,Coherency of 1kHz during Jump,Highly coherent mode at f1 kHz dominates the viewing area,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
14、. with very long correlation length grows just before the burst of LiI only during the current jump.,2 Current Jump in CPD,Normal X-mode injection,Co-directional O-mode injection,Threshold P 20 kW,Threshold P 25 kW,Co-directional O-mode injection seems more efficient for achieving current jump than
15、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 determine
16、d from some equilibrium conditions.,Yoshinaga IAEA (2008),for X-mode injection (X-B scenario) (Normal to B-field),for O-mode injection (O-X-B scenario) (injection angle is adjustable),Midplane,Reflector Shaft,Reflecting mirror,Microwave (8.2GHz) Launching System in CPD,Incident wave must be converte
17、d 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.,Current Jump discharge in CPD,ECR,2nd ECR,ECR,2nd E
18、CR,External Bv Field,Closed Flux Configuration After Current Jump,Center Post,Center Post,t = 0.153 s,t = 0.157 s,#506967,Vertical Shift is observed both in magnetic flux and Ha image.,Non-Inductive Current Generation by ECH via Current Jump,Current Jump occurs under relatively high Bv ( 30 G). In t
19、his 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 jump increases roughly proportional
20、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.,外部垂直磁場 Ip = 0 kA,電流磁場構造時間発展,Flat top 1.5 kA,電流,ECR,2nd ECR,0.8 kA,1.5 kA,1.4 kA,1.2 kA,1.0 kA,R=35.8cm,Ip = 0.8 kA,1.2 kA,1.0 kA,1.4 kA,磁場構造変化 tip B 浮遊電位低下,tip A,tip B,-100V,-1000V,-30V,