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) 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) ( )

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

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

Model-based optimization for m=1

Latency reduction m=1,n=8-16 b r (a) Empirical gains

Latency reduction m=1, n=8-16 b r (a) Empirical gains

Latency reduction m=1, n=8-16 b r (a) Empirical gains

Derivative control db/dt is currently obtained numerically from b Acquisition of the derivative signal db/dt is preferable This allows a better PD gains optimization

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) I coil m,n (A) B r m,n (T) decoupler ON m=1, n=-7 m=0, n=7 m=1, n=7 m=2, n=7

b φ systematic errors correction Unavoidable misalignment of the pick-up coils determines a spurious I p contribution to b φ Real-time subtraction of this term Similar for m=±1,2

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

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

Independent feedback on b r and b φ I ref = K r b r + K φ b φ Suggested by J.Finn and co-workers A more general control could allow finding a new optimum Preliminary RFXlocking simulations are planned

Feedback on the plasma response b plasma = b r – b coils (vacuum) b coils 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 b r is comparable to the standard feedback case upon PD gains optimization

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

Spare

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

Model-based optimization

Simulation of the derivative control

plasma SensorsCoilsVessel Feedback limit

plasma SensorsCoilsVessel Feedback limit

plasma Sensors b r =0 everywhere: impossible CoilsVessel Feedback limit

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

External coils: discrete feedback τ w =100ms

External coils: discrete feedback τ w =10ms

External coils: discrete feedback τ w =1ms

Edge radial field control by feedback

RFXlocking simulation of the plasma response

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

b r (r m,n ) vs b r (a) experimental

Edge radial field.vs. current time constant

Normalized edge radial field: no r f dependence m=1