Recent Results from the STOR-M Tokamak

1. Recent Results from the STOR-M Tokamak A .Hirose, M. Dreval, S. Elgriw, O. Mitarai(1), A. Pant, M. Peng(2), D. Rohraff, A.K. Singh(3), D. Trembach, C. Xiao Plasma Physics Laboratory University of Saskatchewan, Canada (1) Institute of Industrial Science and Technical Research, Kyushu Tokai University, Kumamoto, Japan (2) Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA (3) Present address: Dept. of Physics, Utah State Univ., Logan, Utah, USA IAEA TM RUSFD, October 22-24, 2007, Lisbon, Portugal

2. STOR-M tokamak ( • H-mode studies (biasing, TH, CT), turbulence/fluctuations • AC operation • Compact torus injection

3. Outline Part I Effects of Compact Torus (CT) Injection on the MHD Fluctuations in STOR-M Discharges Motivations Experimental Setup Singular Value Decomposition (SVD) Algorithm Experimental Results Future Work Summary I

4. Outline (cont.) Part II Simulation of Spherical Tokamak Current Start-up by Outer Vertical Field Coils in STOR-M • Motivations • STOR-ST configurations • Numerical simulation of the plasma current start-up in STOR-M • ST simulation start-up experiments in STOR-M • Proposed experiment of saturable iron core operations in STOR-M • Summary II

5. Motivations (CT/MHD) • Compact Torus (CT) injection is a promising technique to fuel large tokamak fusion reactors • Magnetic interactions between CT and tokamak plasma are important parts for • CT penetration into tokamak • Fuel deposition • Localized fuelling may alter pressure profile Does CT injection excite MHD instabilities?

6. CT toroidal field/current

7. Motivations (CT/MHD) M=2 mode suppressed M=3 mode intact (not shown here) M=2 mode reappear Before H-L transition Tangential CT injection induced H-mode like phase

8. Experimental Setup: 12 discrete Mirnov coils evenly distributed in the poloidal direction. Measures

9. Singular Value Decomposition Algorithm: Principal Axis SJJ: mode energy t US modes t Principal component SVD algorithm decomposes matrix A=USVT

10. Experimental Results CT is injected at t=15.25 msec MHD activities are is suppressed after CT injection MHD activities return to higher level at t=16.35

12. Features during the MHD reemergence phase • Starts with non-propagating strong signals near the inner board • Followed by a propagating (rotating) m=2 mode

13. A dominant Principal Axis pair (PA1, PA2)  an m=2 structure (Sine and Cosine parts) Corresponding Principal components oscillating at 30 kHz  seen by a fixed Mirnov coil when the m=2 mode rotates at 15 cycles/msec

14. The third dominant mode  m=1 spatial structure Short-lived burst non-propagating  the gong mode The gong mode appears to be the precursor triggering the MHD activities and H-L back transition

15. Direct spatial FFT based on 12 Mirnov raw signal  evolution of the m=1 and m=2 components The gong mode burst occurs well before the development of m=2 mode

16. What triggers the gong mode? • Sawtooth crash in the core region near q=1 surface excites the gong mode. • Does gong mode travel in toroidal direction? • Is this also true in STOR-M? • Does CT injection cause sawtooth crash as the pressure build up?

17. Future Work: Install another poloidal array offset from the first by 180° in toroidal direction

18. Future Work (cont.): • Develop Soft X-ray (SXR) pin-hole cameras to investigate • Relation between Core and edge MHD oscillation • Relation between sawtooth crash and gong mode

19. Summary I: CT injection suppressed MHD during the induced H-mode like phase. Reemergence of the MHD may have terminated H-mode phase. The return of the relatively large propagating m=2 MHD oscillations is led by a non-propagating, gong mode like m=1 burst. Further investigation is necessary to better understand the effects of CT injection on MHD activities in tokamak plasma.

20. Part II Simulation of Spherical Tokamak Current Start-up by Outer Vertical Field Coils in STOR-M

21. Motivations (ST Current Start-up) • Small aspect ratio spherical tokamaks (STs) have limited space for central solenoid (CS) for current ramp-up • Current start-up without CS is necessary for Component Test Facility, CTF, (R=1.2 m, a=0.8 m, A=1.5, Bt=2.5 T, Ip=16 MA, k=3.2, Pf=300 MW) • Small iron core for the current start-up was proposed by Nishio for Vector

23. STOR-M Configurations

24. STOR-ST Configurations (cont.)

25. Numerical simulation of the plasma current start-up in STOR-M • magnetic field induced by the current through various poloidal field coils • imaging field due to iron core is included

26. Numerical simulation results

27. Experimental results

29. Proposed experiment of saturable iron core operations in STOR-M • STOR-ST current terminates at ~22 msec • The iron core became nearly saturated air core transformer • Third bank will be used to sustain the current

31. Proposed experiment of saturable iron core operations in STOR-M

32. Summary II • Iron core ST current start-up using vertical field windings has been successfully simulated using STOR-M • Experimental results agree well with numerical calculations • Experiments for saturable iron core current start-up and maintenance have been proposed

33. Thank you!