10 th International ST workshop 29th Sep. -1 st Oct. 2004, Kyoto, Japan Recent Results from Helicity Injection Experiments on HIST M. Nagata, H. Hasegwa, K. Kawami, T. Takamiya, Y. Kagei N. Fukumoto and T. Uyama Graduate school of Engineering, University of Hyogo Contents • Background and Objectives • Hilights from helicity injection studies on HIST (Comparison between Spk and ST, Formation and sustainment of flipped ST) • Comparison with 3D MHD simulation results • Summary # The university name was changed from Himeji Inst. of Tech..
Helicity Injection Experiments and the Underlying Physics Coaxial helicity injection (CHI) technique was introduced to classical spheromaks and spherical tokamaks to start-up and sustain a plasma current . The ability of CHI to drive a current has been already examined and the related MHD relaxation have been observed in many spheromak/ST devices. Central Conductor Flux Conserver Closed flux SSPX NSTX Central open I g flux column Open Flux Ψ bias Bias Flux Coil Magnetized K inj = 2 Ψ bias V g Coaxial Gun HIT-II Helicity Injection Spherical Torus System HIST
Common Relaxation Phenomena Observed between Laboratory and Astrophysical Plasmas Kink behavior created by the guns Solar flares Yohkoh HIST Hsu and Bellan 、 PRL Plasmoid ejection, Helical twist, Magnetic reconnection, Rotation Helicity injection experiments are also useful to study similar MHD activities observed in space plasmas.
How to Approach to Understand Generic Properties of MHD Relaxation in Helicity-driven Toroidal System 2 Various utilizations of TF coil current in a single machine Spherical Tokamak A. Present works V g m=1/n=1 I inj q 1) Comparison study of MHD rational barrier 1 tim e Spheromak activites between SPK and T F ST during CHI current drive. 0.5 Spherical RFP 2) To see what happens to ST tim e by a rapid reversal of TF. 0 FRC Whether do ST plasmas collapse or survive after -0.5 they pass through the rational barrier ? Diamagnetic Low q ST -1 B. Future works New Prediction: (see Dr. Kanki’s presentation) R0 /r Diamagnetic high beta low-q STs with two fluid effect may be generated by driving fast flow by CT injection. Dynamics of driven spherical system Increase in MHD activity, self-organized Increase in classical diffusion, properties and large scale fluctuations stability and small scale fluctuations 0 I tf Two fluid Flipped ST Spheromak Low-q ST High-q ST Low-q ST TF coil current RFP
Helicity-driven Relaxation Theory Predicts the Existence of Flipped ST States in the Regime of TF < 0 B t. Flux conserver Magnetized plasma Gun ∇× B = λ B I tf λ0 =8.53 ( Eigenvalue ) Ψ t.e Ψ bias >0 λ ≅ 0 Ψ Flipped I tf > 0 ST t.e > 0 λ<λ c Ψ t.e > 0 RFP (a) near a Vacuum Field (b) ST λ 0 I tf = 0 Sph Flipped Sph Flipped RFP I tf < 0 ST λ>λ c Ψ t.e < 0 λ<λ c Ψ t.e < 0 (d) Spherical RFP (c) Flipped ST Sequence of poloidal flux topologies of driven plasmas as λ is increased from zero to above the eigenvalue λ c
HIST and Diagnostics Sustainment Bank Injection Current 20 kA, Formation Bank T oroidal Field Coil Injection Voltage < 600 V Bias Flux < 5 mWb V ertical TF coil current < 0.25 MA Field Coil Flux Outer Bias Coil R = 0. 30 m Conserver Inner Bias Coil a = 0.24 m A =1.25 I inj I tf Insulator Inner Gas Puff Surface Magnetic Probe(B t ,B p ) Rogow ski andFluxProbe Electrode Central Valves (4) z=-74m m z=0m m Spherical Flux Outer Electrode Conductor MagneticProbe 17 Conserver (B r, B φ ,B z ) 19 15 22 Central Vacuum Vessel Conductor 26 1 5 8 10 12 ST operation # 12 672 100 6 80 (10 1 9 m -3 ) T oroidal 4 I t < 150 kA (kA) FieldCoil 60 ∆ t = 4 - 8 ms 40 I t n e = 2 - 8 x 10 19 m -3 M agnetizedCoaxial 2 n e PlasmaGun T e ~ T i =20 -40 eV 20 CO 2 Laser Interferom eter Toroidal modeprobe (8 chs) 0 0 0 1 2 3 5 4 Time (ms) M. Nagata, et al., Phys. of Plasmas 10, 2932 (2003).
Comparison of Magnetic Fluctuations between Spk and ST Current generation on axis Spheromak Spherical Tokamak T i >T e T i ~T e I tf = 0 I tf >> 0 Toroidal current Current density on the magnetic axis n = 0 n = 1 Phase of n =1 n=1 kink mode and its rotation n=0 mode dominant Intermittent generation of the toroidal current at the magnetic axis was observed in both operations. Flux amplification/current generation in the spheromak case is associated with n=1 MHD activity. In the other hand, that in the ST is associated with repetitive merging of plasmoid injected from the gun which we proposed as a model of current drive so far.
Evidence of Rotating Kink Behavior Driven by MCPG t=0.340 ms time Kinked central open flux t=0.365 ms Exponential growth of the kinked central column with the E × B toroidal rotation Kruskal Shafranov limit Fluctuations E + ˜ v × ˜ B = η j of v and B π 2 2 Kink mode 2 R I ~ MHD dynamo ~ > c t 1 〈 〉 〈 〉 v , B λ is unstable R I c 0 g Current drive v Z = 30 [km/s] , v R = 18 [km/s]
Dynamo Drive of Spk Demonstrated by 3D MHD Simulation Toroidal mode n=0, 1 Resisitve decay 1 10 0 0.8 W mag [a.u.] n=0 0.6 n=1 10 -1 I t 0.4 Sustainment 0.2 10 -2 0 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 t ( τ A ) Nonlinear evolution (Growth, nonlinear saturation and the following relaxation ) of the kinked flux column produces dynamo electric field. ~ ~ E dynamo = < v e × B > Closed flux surfaces are identified only as mean fields. # In collaboration with Y. Suzuki and Y. Kishimoto, JAERI
Multiple Pulse Operation for Improvement of Spheromak Confinement Gun voltage on Gun voltage off Multi-Pulse Helicity Drive is effective for suppressing the [ 10 -3 ] × 6 n = 1 fluctuation. 4 E inj 2 0 0 200 400 600 800 1000 1200 Improvement of confinement t 1 quality. 0.8 Driven phase 0.6 Chaotic scattering I t 0.4 Decaying phase of field lines 0.2 0 0 200 400 600 800 1000 1200 t 0 10 n=0 -1 10 n=1 n=2 10 -2 W mag n=3 n=4 10 -3 Poincare plot of magnetic field at -4 10 the time when the magnetic energy -5 in the n = 1 mode gets down to ~10 -4 10 0 200 400 600 800 1000 1200 t Closed flux surfaces are produced during the decay phase.
Plasmoid Ejection is Key Dynamics for Formation of ST Plasmoid ejection speed ～ 60km/s Stabilization of kink instability by TF In the ST case, the global relaxation like Taylor type does not seem to occur and it becomes more important Magnetic reconnection point can be clearly identified. to understand local features of reconnections around the X (null)- Reconnection layer ~Ion skin depth ~ 3.2 cm, point.(Ti >Te ? at X-point, Ti ~Te Electron skin depth ~ 0.07 cm, Ion-gyroradius ~ 0.4 cm in the core region) Two-fluid reconnection theory may become important.
Observation of Self-reversal of Magnetic Fields by Reversing TF ; Relaxation from the ST toward the Flipped ST State. ST RFP Shot #4586 70 > (kG) 0.4 Core toroidal field (a) 50 30 0.2 Toroidal current I t (kA) t.core 10 0 Self-reversal process -10 B t.e , <B Flipped ST -0.2 -30 -50 -0.4 Edge toroidal field -70 1 Large growth of the n=1 kink mode (c) 0.8 n mode (kG) n =0 0.6 Note that not only toroidal flux but also n =1 0.4 n =2 n =3 poloidal flux reverses the sign spontaneously 0.2 0 0.445 during the relaxation process . (d) 0.39 0.377 J t ( MA / m -2 ) R (m) 0.309 0.21 # M. Nagata et al. Phys. Rev. Lett. 90, 225001 (2003) 0.241 0.00 0.173 # In collaboration with S. Masamune, Kyoto Inst. of Tech. 0.105 -0.21 0.6 0.65 0.75 0.5 0.55 0.7 and M. Katsurai, U of Tokyo Time (ms) +J t -Jt
3D MHD Simulation of Self-organizing from ST to F-ST Relaxed States A B C D E F 12 Magnetic reconnection between the open and closed field lines. 10 t=80 Spontaneous reversal of not only toroidal but also poloidal flux. 8 t=20 The system relaxes to a lower energy state by rearranging 6 λ t=0 current distribution. The parallel current profile λ becomes 4 t=485 2 peaked. Kink of the central open flux is essential to the self- t=820 0 reversal process. 0.2 0.4 0.6 0.8 1.0 r # Y. Kagei et al. PPCF, 45, L17 (2003) # In collaboration with Y. Suzuki and Y. Kishimoto, JAERI and T. Hayashi, NIFS
Fast Camera Images Display Kink instability around the Center Conductor during the Current-reversal Process #10152 RF-ON 10 I g [kAturn] 0 TF -10 -20 -30 -40 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 60 40 I t ] I t [kA 20 0 -20 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Time[ms] (a) (b) (c) (d) (e) (f) (g) (h) 0 -5 ] I t [kA -10 -15 0.33 0.34 0.35 0.36 0.37 0.38 Tim e[ms] 0.340ms 0.345ms 0.350ms 0.355ms 0.360ms 0.365ms 0.370ms 0.375ms
Key Question; Can We Sustain the Flipped ST plasmas in Spite of No Central Open Flux ? The F-ST configuration is consisted of only closed flux surfaces so that it may have Non-flipped region Flipped region a better confinement quality ! ? But, the F-ST is isolated from the electrodes, so can we drive it by helicity injection? No Magnetic Reconnection How to drive current? A key point is to cause the kink deformation of the non-flipped field lines . Ejection condition: I inj > 2I tf Unique magnetic field lines geometry: B t : opposite direction, q ~ I tf /I t >1 I inj > 2I tf >2I t . B p : same direction Large injection current is required to sustain a large plasma current in the F-ST.
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