Study on Fluctuations during the RF Current Ramp-up Phase in the CPD Spherical Tokamak H. Zushi 1), T. Ryoukai 2), K. Kikukawa 2), T. Morisaki 3), R. Bhattacharyay 2), T. Yoshinaga 1,3), K. Hanada 1), T.Sakimura 2), H. Idei 1), K. Dono 2), N. Nishino 4), H. Honma 2), S. Tashima 2), T. Mutoh 3), S. Kubo 3), K. Nagasaki 5), M. Sakamoto 1), Y. Nakashima 6), Y. Higashizono 1), K. N. Sato 1), K. Nakamura 1), M. Hasegawa 1), S. Kawasaki 1) H. Nakashima 1), A. Higashijima 1) 1)RIAM, Kyushu University, Kasuga, Fukuoka, Japan, , 2) IGSES, Kyushu University, Kasuga, Fukuoka, Japan, , 3) National Institute for Fusion Science, 4) Hiroshima University, 5) Kyoto University, 6) University of Tsukuba, 4 th IAEA TCM on Spherical tori and 14 th International workshop on Spherical Torus ENEA Frascati, October 7-10,

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

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

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

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

OUTLINE 1.Diagnostics of fluctuations 2.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 (10  s) Rotating Pump limiter Pump(TMP,Cryo) Probe Ha filter Li-BES (50 PMTs) Rogowskii coil Medium speed camera(1ms) AM-reflectometer Stray rf power IR-TV camera IR spectroscopy Visible spectroscopy (Hyougo Univ) CT injector (NIFS) (Hirosima Univ.) (Tsukuba Univ.) CS I. 45 Flux loop coils

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

Typical discharge

Fluctuations in Annulus Plasma w/o Bz 1 kW R res =164 mm

LF mode with Long correlation length R ~ 5 cm z > 2.5 cm

Bz suppresses density fluctuations Density profile is not significantly affected by Bz (< 50G) However, the fluctuation level is much reduced. Bt=0.29TBz< 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 to reduce fluctuations Steady Bz is required for the initial condition ! Forrest PRL Confine the trapped electrons => toroidal precession current 2.Pressure driven/ Uni-directional PS current => seed toroidal current Ryoukai, ICPP Reduced Fluctuations => more efficient conversion

2 Fluctuations during Current Jump

Burst in LiI and  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 <R<215 mm 27<Z< 52 mm R res =194mm

Profile flattening occurs 1ms before the burst

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

Summary 1.Initial Bz suppresses the density fluctuations, suggesting that the injected ECW can be converted efficiently to EBW 2.2D 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.

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 normal X-mode. The critical parameter for current jump phenomenon should be the value of I p, since I p 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. I p itself may be determined 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 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.

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

Non-Inductive Current Generation by ECH via Current Jump Current Jump occurs under relatively high B v (≥ 30 G). In this range of B v, I p saturates at ~ 1.8 kA without current jump. This suggests that the current jump is necessary to obtain higher I p under higher B v. Under B v ≤ 30 G, I p increases slowly and there is no clear current jump. Finally achieved I p with and without current jump increases roughly proportional to B v. P inj < 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 I p under higher B v.

外部垂直磁場 Ip = 0 kA 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 電流ジャンプ R(cm ) ECR 2nd ECR 0.8 kA Z(cm ) 1.5 kA1.4 kA1.2 kA 1.0 kA R=35.8cm

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