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Perspectives of tearing modes control in RFX-mod

Perspectives of tearing modes control in RFX-mod. Paolo Zanca Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova, Italy. RFX-mod contributions to TMs control (I). Demonstrated the possibility of the feedback control onto TMs

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Perspectives of tearing modes control in RFX-mod

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  1. Perspectives of tearing modes control in RFX-mod Paolo Zanca Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova, Italy

  2. RFX-mod contributions to TMs control (I) • Demonstrated the possibility of the feedback control onto TMs • Clean-Mode-Control (CMC) based on the de-aliasing of the measurements from the coils produced sidebands

  3. RFX-mod contributions to TMs control (I) • Demonstrated the possibility of the feedback control onto TMs • Clean-Mode-Control (CMC) based on the de-aliasing of the measurements from the coils produced sidebands • Not obvious results: phase-flip instability?

  4. RFX-mod contributions to TMs control (I) • Demonstrated the possibility of the feedback control onto TMs • Clean-Mode-Control (CMC) based on the de-aliasing of the measurements from the coils produced sidebands • Not obvious results: phase-flip instability? • No-sign of phase-flip instability; equilibrium condition can be established where CMC induces quasi-uniform rotations of TMs

  5. RFX-mod contributions to TMs control (II) • Wall-unlocking of TMs with CMC • In general, the feedback cannot suppress the non-linear tearing modes requested by the dynamo. • The feedback keeps at low amplitude the TMs edge radial field • Improvement of the magnetic structure: sawtooth of the m=1 n=-7 which produces transient QSH configurations

  6. CMC optimizations • Increase the QSH duration → recipes under investigation • Which are the possibilities to reduce further the TMs edge radial field? → Model required

  7. 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 • It describes fairly well the RFX-mod phenomenology →L.Piron talk

  8. General analysis of the TM control

  9. Single-shell external coils Sensors Vessel Coils plasma

  10. Normalized edge radial field • The feedaback action keeps low the normalized edge radial field • At best b^senscan be made close but not smaller than the ideal-shell limit

  11. Feedback limit Sensors Vessel Coils plasma

  12. Feedback limit Sensors Vessel Coils plasma

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

  14. Role of the Vessel • The stabilizing effect of the vessel is crucial for having low b^sensand moderate power request to the coils • The shorterτwthe faster must be the control system (fc=1/Δt) to avoid feedback (high-gain) induced instabilities • Optimum range:τw>10ms better τw 100ms

  15. Single-shell Internal coils Coils Sensors Vessel plasma

  16. Single-shell Internal coils Coils Sensors Vessel plasma

  17. Single-shell Internal coils • Continuous-time feedback → solution ωω0 with br(rsens) 0 for large gains • Discrete-time feedback : including the latency Δt the high-gain instability may occur • The good control region is not accessible for realistic TM amplitudes. • For stable gains b^sensis determined by the ideal-shell limit, which is large due to the loose-fitting vessel required by the coils dimension

  18. RFP design for good TM control (a personal view)

  19. Premise • The passive stabilization provided by a thick shell does not solve the wall-locking problem • In the thick-shell regime wall-locking threshold ~σ1/4 • Feedback is mandatory to keep TMs rotating

  20. Design in outline • In-vessel coils not interesting • Single structure (vessel=stabilizing shell) with the coils outside • Close-fitting vessel to reduce the ideal-shell limit • τw10ms-100ms withΔt10μs-100μs

  21. RFX-mod perspectives (a personal view)

  22. RFX-mod layout • 3ms vacuum-vessel, 100ms copper shell, ~25ms mechanical structures supporting the coils • The control limit is mainly provided by the 100ms copper shell

  23. RFX-mod status Gain optimization guided by RFXlocking simulations for the RFX-mod case m=1 TMs

  24. Optimizations • Get closer to the ideal-shell limit (minor optimization) • Reduce the ideal-shell limit by hardware modifications (major optimization)

  25. Minor optimizations • Increase the coils amplifiers bandwidth: maximum current and rensponse time • Acquisition of the derivative signal dbr /dt in order to have a better implementation of the derivative control (to compensate the delay of the coils amplifiers) • Compensation of the toroidal effects by static decoupler between coils and sensors only partially exploited • Compensation of the shell non-homogeneities requires dynamic decoupler (work in progress)

  26. Majoroptimization • Approach the shell to the plasma edge possibly simplifying the boundary (removing the present vacuum vessel which is 3cm thick) • Moving the τw=100msshell from b=0.5125m to b=0.475m (a=0.459) a factor 3 reduction of the edge radial field is predicted by RFXlocking

  27. Conclusions • CMC keeps TMs into rotation • Edge radial field: ideal-shell limit found both with the in-vessel and out-vessel coils → br(a)=0 cannot be realized • The vessel=shell must be placed close the plasma → coils outside the vessel. Is a close-fitting vessel implementable in a reactor? • The feedback helps the vessel to behave close to an ideal shell→ τw cannot be too short

  28. spare

  29. Edge radial field control by feedback

  30. RFXlocking .vs. experiment

  31. Normalized edge radial field: weak brs dependence

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

  33. Locking threshold The present analysis valid for dw<<rw cannot be extrapolated to very long tw

  34. Edge radial field .vs. current time constant

  35. Single mode simulations: external coils a = 0.459m rw i = 0.475m c = 0.5815m

  36. Single-mode analysis: feedback performances dependence on tw

  37. Single-mode analysis: feedback performances dependence on tw

  38. Multi-mode analysis: power dependence on tw

  39. Edge radial field: tw dependence Data averaged on 0.1s simulation m=1

  40. Normalized edge radial field: rwi dependence m=1

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

  42. Out-vessel coils: signals 4x48 both for coils (c = 0.5815m) and sensors (rwi = 0.475m )

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

  44. External coils: discrete feedback τw=100ms

  45. External coils: discrete feedback τw=10ms

  46. External coils: discrete feedback τw=1ms

  47. The in-vessel coils

  48. Single mode simulations: frequency τw= 1ms100ms

  49. Single mode simulations: Ic, Vc

  50. Single mode simulations: edge br

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