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Tearing modes control in RFX-mod: status and perspectives

Tearing modes control in RFX-mod: status and perspectives. P.Zanca , R.Cavazzana, L.Piron, A.Soppelsa Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova, Italy. Milestones. Intelligent Shell: egde radial field control (2005)

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Tearing modes control in RFX-mod: status and perspectives

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  1. Tearing modes control in RFX-mod: status and perspectives P.Zanca, R.Cavazzana, L.Piron, A.Soppelsa Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova, Italy

  2. Milestones • Intelligent Shell: egde radial field control (2005) • Clean Mode Control (de-aliasing of the measurement): TM wall-unlocking (2007) • Coils amplifiers improvements: maximum current and rensponse time (2008, 2010) • MHD model of the feedback (RFXlocking) (2007-2010)

  3. Optimizations • Edge radial field reduction: get closer to the ideal-shell limit (determined by vessel/copper shell) • Increase the QSH duration: non-zero reference for the dominant mode • m=2, n=1 control in tokamak dicharges

  4. Optimizations • Edge radial field reduction: get closer to the ideal-shell limit (determined by vessel/copper shell) • Increase the QSH duration: non-zero reference for the dominant mode • Control in tokamak dicharges

  5. Model-based optimizationfor m=1

  6. Latency reduction m=1,n=8-16 br(a) Empirical gains

  7. Latency reduction m=1, n=8-16 br(a) Empirical gains

  8. Latency reduction m=1, n=8-16 br(a) Empirical gains

  9. Derivative control • db/dtis currentlyobtained numerically from b • Acquisition of the derivative signal db/dt is preferable • This allows a better PD gains optimization

  10. m=1, n=-7 m=0, n=7 m=1, n=7 m=2, n=7 Brm,n (T) Icoilm,n (A) decoupler ON Dynamic decoupler • The dynamic decoupler reduces the side-harmonic components of the magnetic field produced • A “modal” decoupler could be designed considering a limited number of harmonics (i.e. only the poloidal sidebands)

  11. bφ systematic errors correction • Unavoidable misalignment of the pick-up coils determines a spurious Ip contribution to bφ • Real-time subtraction of this term Similar for m=±1,2

  12. M=0 control • Little affected by the feedback • High gains test on m=0, n<6 has not shown any improvement on F shallow discharges • m=0 control at deeper F still to be investigated • m=0 n≥7 spurious contribution should be removed by the dynamic decoupler

  13. M=0, n<6 feedback with the toroidal circuit • Enhance the natural reaction of the 12 toroidal sectors to the m=0 low n TM • Present circuit too slow to follow the m=0 dynamic (2.5ms delay according to 2006 experiments) • Upgrade of the internal circuit control by reducing the latency

  14. Independent feedback on br and bφ Iref = Kr br + Kφbφ • Suggested by J.Finn and co-workers • A more general control could allow finding a new optimum • Preliminary RFXlocking simulations are planned

  15. Feedback on the plasma response bplasma = br – bcoils(vacuum) • bcoils from the cylindrical model used in the de-aliasing or from a state-space model which includes the shell frequency response • The hope is to reduce the TM amplitude at the resonant surface • According to RFXlocking edge br is comparable to the standard feedback case upon PD gains optimization

  16. Synopsis • Control system upgrade • Latency reduction • db/dt acquisition • Improved toroidal circuit control • New algorithms • Dynamic decoupler • bφ sistematic errors removal • M=0 low n control with the toroidal circuit (partially developed) • Plasma response • Independent br bφ feedback • Other schemes • M=0 control at deep F • Non-zero reference control to sustain QSH } Gains optimization

  17. Spare

  18. RFXlocking • Semi-analitical approach in cylindrical geometry • Newcomb’s equation for global TMs profiles • Resonant surface amplitudes imposed from experiments estimates • Viscous and electromagnetic torques for phase evolution • Radial field diffusion across the shell(s) • Feedback equations for the coils current

  19. Model-based optimization

  20. Simulation of the derivative control

  21. Feedback limit Sensors Vessel Coils plasma

  22. Feedback limit Sensors Vessel Coils plasma

  23. Feedback limit Sensors Vessel Coils plasma br=0 everywhere: impossible

  24. Single-shell: discrete feedback Δt = latency of the system

  25. External coils: discrete feedback τw=100ms

  26. External coils: discrete feedback τw=10ms

  27. External coils: discrete feedback τw=1ms

  28. Edge radial field control by feedback

  29. RFXlocking simulation of the plasma response

  30. Toroidal circuit dynamic response Control system + internal control latency: 2.5 ms Power supply time constant: 3 ms

  31. Toroidal circuit dynamic response Simulation of power supply behaviour with latency = 1.6 ms - Kp = 0.04 Simulation of power supply behaviour with latency = 0.1 ms - Kp = 0.7

  32. br(rm,n) vs br(a) experimental

  33. Edge radial field .vs. current time constant

  34. Normalized edge radial field: no rf dependence m=1

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