Magnetic Steering and ECCD Mitigation of locked NTMs

1. Magnetic Steering and ECCD Mitigation of locked NTMs Presented byFrancesco Volpe In collaboration with: R.J. Buttery, R.J. La Haye, G. Jackson, M. Okabayashi, F.W. Perkins, R. Prater, H. Takahashi Acknowledgements to: J. Ferron, A. Hyatt, B. Johnson, B. Penaflor, C. Petty, H. Reimerdes, E.J. Strait & ECH Team Presented at the Friday Science Meeting, GA September 22, 2006

2. Motivation • ITER expected to have slow (few kHz) NTMs  high probability of locking. • Locked 2/1 modes are a recurrent cause of disruptions. • ECCD proved successful in suppressing rotating NTMs. • Success of ECCD for locked modes not guaranteed, due to island locking in a position not accessible by gyrotrons. • Resonant Magnetic Perturbations (RMPs) from I-coils used at DIII-D for RWM and ELM control. • Although more challenging (modes lie deeper in the plasma and are faster), here RMPs are used to control NTM rotation and assist their ECCD Stabilization

3. Overview of EFC+ECCD control proposals B1{ B2 B3 B4

4. How to predict a Locked Mode? 3 types of “dud detectors” were used: • dB/dt >20T/s for 200ms • Mode frequency drops below 1kHz • Born-locked mode >5G for 20ms B(G) f(kHz) dB/dt (T/s)

5. Exp B1: Controlled locking (accessible by gyrotrons). Then apply cw ECCD. Some beams inherited from mode onset phase (changing from shot to shot). Modify beams (via J.Ferron’s algorithm) to get low plasma and mode rotation, unless mode slows down “for free” NBI ctr co Dud detector detects oncoming locking f21 Due to overcorrection, mode locks even faster, but to toroidal phase optimal for CW ECCD 1kHz Generate a static error field with I-coils Ratio between IU30, IU90 and IU150 experimentally found (via phase scan) to lock O-point in front of gyrotron. If time, optimize strength of magn.perturbation by changing all currents in proportion DC currents in I-coils Max ECCD Other shots: inject in X-point and intermediate phase angles for comparison

6. Toroidal Alignment: at locking, 0.66Hz I-coil travelling wave and CW ECH were applied & change in mode amplitude observed.

7. CW ECCD +EFC +plasma:Mode amplitude is larger and varies (4-7.5G) with I-coil frequency

8. A Vacuum shot was taken to subtract I-coils and C-coils effect on magnetics. Saddle loops measure 1G, constant.

9. Even after subtraction of Vacuum Shot, locked mode changes amplitude when toroidally steered and illuminated by ECH Non-uniform rotation consistent with mode amplitude and stronger or weaker wall braking

10. Vacuum shot

11. No-ECH Ref. Shot has potential for isolating interaction between mode & intrinsic error, but it also affects ne, b, etc. and makes island more “sticky”

12. ECH deliberately misaligned by 3% lower Ip and BT: same heating, same mode-error field interaction but no ECCD stabilization. Subtraction made tricky by these instabilities Exp. can be improved by 2+2 gyrotrons or real time ECH steering

13. Clear Difference in Phase Consistent with the fact that two different phases are relevant: Between island and ECCD in ECH case (top) Between I-coil and Saddle-Loops in no-ECH case (bottom)

14. ECH aligned to island via radial jog of plasma. Improved stabilization for DR=-2cm. When ECH off, mode amplitude rises again.

15. Exp. B2: Prevent locking by sustaining mode rotation. Then apply ECCD (modulated or cw). Like B1 NBI ctr co Dud detector detects oncoming locking f21 Rotation at 1kHz sustained by dynamic EFC 1kHz Generate a dynamic error field by AC currents of f=1kHz. Repeat for various amplitudes at fixed ratios between pairs of coils. Amplitude of AC currents in I-coils ECCD modulated at 1kHz • Shotplan: • No ECCD (for ref.) • MECCD at 1.001kHz (equivalent to phase scan, if I-coils at 1kHz) • MECCD at 1kHz in O-point (and in another shot in X-point, for comparison) • CW ECCD for reference

16. Exp. B3: Spin mode up. No ECCD necessary.Exp. B4 is like B3 but wait for locking first, then unlock & spin up. Find co/ctr mix such that mode rotates at ~1kHz more or less constantly. No slowing down here. Alternatively, sustain rotation at 1kHz with I-coils (see B2), then apply steps in I-coil w/forms. NBI ctr co Apply EFC rotating at f≈f21 to control mode rotation 3 f I-coils f21 2 Apply EFC accelerating from f<f21 to f »f21 to make mode spin faster and faster. Invert ramp to study w12-b histeresis. 1 1 2 3 w21 Modulation of I-coil not necessarily commenced here: might have pre-existed, if rotation was magnetically sustained (see fig.2) 3 2 1 b Renounced to f/back on I-coil, not ready.

17. Gradual Entraining of I-coil travelling wave, 1-100Hz

18. Gradual Entraining of I-coil travelling wave, 1-100Hz

19. Gradual Entraining of I-coil travelling wave, 1-100Hz

20. Travelling Wave of up to 60Hz coupled to 2/1 mode

21. I-coil Travelling Wave less effective at high frequencies • Current II-coil delivered by SPAs falls off with f • Besides, for the same II-coil, the magnetic perturbation exerted on the plasma decreases due to: • Partial compensation from image currents in the wall (skin effect? Distance between I-coil & image comparable with wavelength?): -3dB @100Hz, -10dB @1kHz. • More Shielding associated with (faster) rotation • Furthermore, the same BI-coil couples less effectively with a faster, rotationally mitigated, weaker mode (= compass of reduced sensitivity immersed in the same field…) • Phase delays in SPAs • SPAs=Switching Power Amplifiers  discrete steps

22. Summary of Experiments carried out • “Preferential Locking” (CW ECCD + DC I-coils) • CW ECCD + 0.66Hz I-coil Travelling Wave • “Toroidal alignment” of island relative to gyrotrons • Radial Jog of Plasma • Radial alignment • CW ECCD + DC I-coils • Injection in O-point and, for comparison, in X-point. • “Sustained Rotation” (MECCD + AC I-coils) • Ramps of I-coil Frequency