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AXISYMMETRIC CONTROLS IN RFX-mod: STATUS AND PERSPECTIVES M. Barp, R. Cavazzana, G. Marchiori, A. Soppelsa, L. Zanotto 1 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Outline • Feedback Control of Reversal Parameter • Optimization of Toroidal Power Supply Control System • Feedback Control of Plasma Current • MHD Equilibrium Codes for Horizontal Equilibrium and Shape Control 2 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Feedback Control of Reversal Parameter 3 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Feedback Control of Reversal Parameter • A reliable control of F was missing in RFX-mod: – F controlled in “open loop” – Control relied very much on the experience of the operator – Plasma events had strong impact on the control • Reasons for a reliable and precise control: – Best performance of RFX-mod at shallow F (-0.05≤F<0) – At high current and shallow F, small deviations of Bt(a) are tolerable – F positive loss of RFP configuration and very poor confinement 4 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives System Modeling General scheme and models • Model of the toroidal power supply (both power circuit and control system) • Model of the coupling between the toroidal windings and the plasma during the flat-top phase 5 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Closed Control Loop Feedback controller scheme vq (t) affected by plasma reconnection v* (t ) F Bt a Bt 0 a 2 * kvr* (t ) vq niT (t ) v (t ) / Rv Bt , a (t ) 0 2R0 vq* The reversal parameter control can be reformulated as a control of toroidal edge field at the plasma radius 6 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Results Reversal loss 7 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Optimization of Toroidal Power Supply Control System 8 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Optimization of Toroidal Power Supply Control System (1) • New feed-forward input to the internal controller of the toroidal PS to improve the dynamic performance of the system • Proportional and Integral gain optimization V ff K ff I ref Kp + - I tor 9 Ki s + + + + Toroidal inverters M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Optimization of Toroidal Power Supply Control System (2) • Small current unbalances in the toroidal coils give rise to wall mode locking • Significant m=0, n=1 mode/field error not adequately corrected • A proper controller necessary to drive independently the current in the 12 toroidal coils – reduction of field errors and current unbalances – m=0 mode control 10 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Feedback Control of Plasma Current 11 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Current Controller Limitations (1) • Plasma current controller implemented and tested • Tuning campaign not completed • Controller step response can be optimized • Feedback component is a lag compensator, which is affected by the actuators saturations • Experimental proposal has been submitted to tune the regulator 12 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Current Controller Limitations (2) • Plasma Current Controller (PCC) increases plasma current independently from plasma resistance • High energy deposition on a localized set of tiles • Debate whether the machine is protected “by design” against such occurrence or if the PCC should take any action • At present PCC switched off if a power threshold is exceeded 13 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Automatic Ip ramping Plasma Current 29271 PCAT Voltage 29271 Automatic Ip ramping Vmag 29271 Vmag estimate 29271 14 • Plasma current control based on operator experience • Plasma current ramps at the fastest rate without control • PCAT unit automatically switched on as soon as their voltage is higher than the voltage drop on the transfer resistor (vm) • vm not available in the realtime system but a good estimation can be obtained M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives MHD Equilibrium Codes for Horizontal Equilibrium and Shape Control 15 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Horizontal Equilibrium and Shape Control Codes (1) • Reliable MHD Equilibrium code to describe a wide range of plasma equilibria in RFX-mod is needed for engineering applications • Static/evolutionary MHD equilibrium – free boundary problem (direct problem): determination of the plasma boundary for an assigned set of external currents and JT(y) plasma density profile – fixed boundary problem (inverse problem): determination of position and amplitude of external currents for an assigned plasma boundary and JT(y) plasma density profile 16 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Horizontal Equilibrium and Shape Control Codes (2) • Engineering oriented versions (e.g. Proteus, MAXFEA) including realistic description of the conducting structures are essential: – Design magnetic winding systems – Derive linearized model of the plasma response – Test and to assess the performances of plasma magnetic control systems • Modified 2D non linear MHD equilibrium code MAXFEA used in RFX-mod to solve particular RFP equilibria 17 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Horizontal Equilibrium and Shape Control Codes (3) • Research activity for a renewed code or a different one to be adapted to RFP discharges, in particular to the shallow F equilibria. • Codes needed for: – Study of different plasma shape in RFP or Tokamak configuration in RFX-mod – Conceptual design of new RFP experimental devices 18 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives Conclusions • F-control system has been implemented and tested in 2010. It proved to be a very useful tool in particular at very high plasma current. • The optimization of the toroidal power supply control system for the improvement of the m=0 mode control and the reduction of field errors and currents unbalances has been considered. • Plasma current control has also been developed in 2010 but not fully tested. It is scheduled in 2011 a campaign to tune the regulator. A new controller for the automatic ramping of the plasma current is under consideration too. • The investigation for a new MHD equilibrium code for horizontal equilibrium and plasma shape to be used with shallow F is needed. 19 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives