23 rd SOFT September 20041/31 Control of non-axisymmetric magnetic fields for plasma enhanced performances: the RFX contribution P. Sonato, R.Piovan, A.Luchetta and the RFX team

23 rd SOFT September 20042/31 Outline Introduction to MHD instabilities in tokamaks & RFPs –Error field control in tokamaks –RWM stabilization in tokamaks –Error field reduction in RFX –Control of m=1 modes in RFX The new experiments on the modified RFX –The machine modification and the saddle coil system –Power supply –Magnetic measurements –Control system –Control strategies Conclusions

23 rd SOFT September 20043/31 Introduction: MHD plasma instabilities The MHD instabilities limit the operational space of the plasma in any magnetically confined plasma operating at the highest performances MHD instability sources: –current gradients –pressure gradients MHD instability types: –Ideal instabilities –Resistive instabilities

23 rd SOFT September 20044/31 Introduction: MHD description 2-D Fourier decomposition of the magnetic field: –Poloidal spectrum: m –Toroidal spectrum: n m=1, n=3 Resonant surfaces in toroidal geometry

23 rd SOFT September 20045/31 Introduction: MHD resonance Tokamak: –Internally resonant modes –Externally resonant modes Resonance for rational q

23 rd SOFT September 20046/31 MHD instabilities in the Tokamak – error fields The non-axisymmetric magnetic error fields are sources of instabilities: –coils misalignments –coil feed connections –inhomogeneity of conductive passive structures –ferromagnetic structures –ripple –…….. The error fields exert a braking torque against the plasma rotation Problems of present error field studies: –Many sources of error fields are not completely evidenced –Sidebands of correction coils

23 rd SOFT September 20047/31 Error field control in DIII-D Static error field compensation to attain low density regimes Recent multi error field compensation with both N=1 coil and C-coils  limit vs. B r (2,1) Reference discharge Multi-mode Error field compensation

23 rd SOFT September 20048/31 MHD instabilities in the Tokamak – RWMs Advanced scenarios require: –sufficiently high  : high boostrap current fraction flat or reversed shear Consequence: Resistive Wall Modes (RWMs) appearance –external kink modes: n=1,2 and various m –stabilized only by an infinitely conductive wall close to the plasma surface –severe limit in 

23 rd SOFT September 20049/31 RWMs stabilization and control in DIII-D C-coil feedback control of RWMs and pre-programmed similar correction obtain similar improvement RWMs avoidance strategies: –Stabilisation by rotation through tangential NBI –Careful error field control –Feedback stabilisation with additional coils No feedback Pre-programmed Feedback

23 rd SOFT September /31 Error fields & RWM extrapolation to ITER The RWMs can be stabilized by feedback control acting on the outer correction coils

23 rd SOFT September /31 MHD instabilities in the RFP experiments: mode classification

23 rd SOFT September /31 RWMs in the RFP experiments EXTRAP-T2R HBTX-1C internally nonresonant on-axis RWM, m=1, n=-10 Internally resonant tearing mode m=1, n=-12

23 rd SOFT September /31 Error fields in RFX - ’92-’99 The broad spectrum of internally resonant MHD m=1 tearing modes on rational surfaces can easily couple with harmonics of an error field Two main sources of error fields in the passive Aluminium stabilizing shell: –2 poloidal insulating gaps –2 toroidal insulating gaps Axisymmetric equilibrium coils poloidal gap local control coils with m=0 Pre-programmed Equilibrium m=1,n=0 Equilibrium feedback local field error minimization short circuited gap

23 rd SOFT September /31 Tearing modes in RFX - ’92-’99 RFX always exhibited high amplitude m=1 tearing modes: –phase locked with respect to each other –locked with respect to the wall

23 rd SOFT September /31 m=1 mode control through m=0 mode coupling in RFX 1,9 Toroidal position 1,8 1,7 1,10 1,11 1,12 Controlling the currents on the toroidal winding sectors (0,1 mode) the control the m=1 tearing mode position has been obtained Also a slight reduction of mode amplitude has been evidenced

23 rd SOFT September /31 The modified RFX It has been conceived to extend the non-axisymmetric control of the MHD modes by introducing a direct action of external harmonic m=1 magnetic fields The capability to produce m=0 modes has been improved by the new toroidal system power supply to control the toroidal field independently on each of the 12 winding sectors Further significant improvements: –Axisymmetric equilibrium control –Poloidal gap field error minimized –Toroidal gap field error minimized –First wall power handling capability –Vessel wall protection –Plasma breakdown

23 rd SOFT September /31 The modified RFX new vacuum vessel ports for ISIS feedthroughs toroidal coil new toroidal support structure shell clamping bands shell equatorial gap shortcircuits vessel-shell insulated spacers vacuum vessel 3 mm copper shell saddle coil system

23 rd SOFT September /31 The stabilizing shell The first basic choice has been to install a passive stabilising shell as close as possible to the plasma having a  (1,0) = ms: –to allow a passive stabilisation for instabilities acting on a time scale faster than the operational frequency of the power supplies/winding systems (~20 ms) –corresponding to a passive stabilization of the characteristic internal resonant modes of ~10-20 ms for m=1, n=7 to n=18 –the shell will be nearly completely penetrated for the m=1,n=1,5 RWMs during the shot Welded gap

23 rd SOFT September /31 The stabilizing shell: passive error field minimization 23°overlapped poloidal gap short-circuited equatorial gap Field error through the poloidal gap overlapped poloidal gap Butt joint gap

23 rd SOFT September /31 The saddle coils The second design choice regards the shape and the discretization of the radial field control coils: –the presence of an equatorial gap used also as an opening surface to have access to the vessel -> only saddle coils are compatible –the saddle coils must be designed without any gap in between, to avoid undesired sources of high spectrum error fields and source of sidebands Turns60 (4 layers x 15 turns) Section3.6 mm 2 I nom 400 A (0.3s) V nom 650 V MaterialCopper Insulationgrade 2 Dacron glass tape, epoxy pre-impregnated, final vacuum impregnation with epoxy resin

23 rd SOFT September /31 The saddle coils: sidebands Toroidal and poloidal sidebands at the plasma edge for a single m=1, n=8 harmonic produced n sb = n 1,8 ± k. N t k = ±1, ±2, …. N t = 48

23 rd SOFT September /31 Saddle coil power supply Each saddle coil is fed with its own switching dc/dc power supply, which performs independent control of the current H-bridge converter topology with standard voltage components (IGBT ) Total power: 50 MW Output voltage650 V Output current400 A DC link voltage700 V Switching frequency10 kHz Control lawDouble Pulse Width Modulation Duty cycle (ON/Cycle)0.5 s / 600 s Time: 20 ms/div Reference (480 A/div) Current (480 A/div) Voltage (750 V/div)

23 rd SOFT September /31 Toroidal field power supply The system is foreseen to be used also to generate rotating m=0, n=1-5 modes superimposed to the bias reversed B 

23 rd SOFT September /31 Vessel braking torque and driving m=0, n=1 torque m=1, n=8 braking torque Normalized to 1 mT of mode amplitude lower amplitude is expected in the modified RFX Old RFX Rotation frequency limit 50 Hz 20 Hz 10 Hz5 Hz old RFX The new PS will allow an increased m=0, n=1

23 rd SOFT September /31 saddle probes Magnetic measurements: out-vessel probes B tor -B pol biaxial pick up coils “Ad hoc” designed for non-axisymmetric control The system comprises 192 (4x48) measures of, B t, B p Bandwidth few kHz (vessel shielding effect)

23 rd SOFT September /31 Magnetic measurements: In-vessel probes Designed to measure high frequency, high n components of B t 96 (48 x 2) measures of B t Bandwidth close to 1 MHz

23 rd SOFT September /31 Control system: computer based, distributed system The system includes seven VMEbus stations equipped with single board computers, all connected through one real-time network: –Three stations (processors) dedicated to the real time data acquisition –Four stations (controllers) drive the control power amplifiers. The performance was measured: –latency time worst case 300  s

23 rd SOFT September /31 Control system: MHD mode control scheme It consists of a lumped parameter electromagnetic model of the Saddle Coil (SC) system integrated with a linear model of the evolution of RWMs in a RFP plasma - 0 +K Applied voltages Actuator: SC X’= Ax + Bu y = Cx + Du Coil currents Dynamic & FFT Field harmonics Generated by SC X’= Fx + Gu y = Hx + Pu Plasma dynamic model Reference Measured field harmonics

23 rd SOFT September /31 Control system: MHD mode control scheme applied to T2R Recently in T2R a saddle coils system has been installed: –Total SC= 64 –Poloidal = 4 –Toroidal = 16 –Not covering completely the plasma surface The RFX MHD mode control system has been tested The RWMs multi mode control has been demonstrated NO FEEDBACK Feedback on n=+5,+6 Feedback on n=+5,+6,+7,+8

23 rd SOFT September /31 Control strategies MHD mode control –stabilisation of RWMs having m=1, n=2-5 –interaction with internally resonant tearing modes, to either mitigate or excite their amplitudes or control their phases “Virtual ideal shell” close to the plasma. “Wise virtual shell” is similar to the “virtual ideal shell”, but the components of the radial magnetic field are minimised, except for the equilibrium m=1,n=0 component Phase control of m=0, n=1-5 modes. Action on the dynamic current unbalance on the toroidal winding sectors to produce m=0, n=1-5 rotating modes able to drag the m=1 phase & wall locked modes

23 rd SOFT September /31 Conclusions The new RFX device is the most versatile experiment to test the interaction of external harmonic fields with MHD modes The experiments will allow to investigate: –the RWMs stabilization and tearing mode interaction –the error field control, including the effect of the sidebands All of these features are of common interest for: –Present tokamak and RFP experiments –For the implementation of similar systems in ITER