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AXISYMMETRIC CONTROLS IN RFX-mod: STATUS AND PERSPECTIVES

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AXISYMMETRIC CONTROLS IN RFX-mod: STATUS AND PERSPECTIVES

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  1. AXISYMMETRIC CONTROLS IN RFX-mod: STATUS AND PERSPECTIVES M. Barp, R. Cavazzana, G. Marchiori, A. Soppelsa, L. Zanotto M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  2. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  3. Feedback Control of Reversal Parameter M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  4. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  5. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  6. vq vq* Closed Control Loop Feedback controller scheme vq(t) affected by plasma reconnection The reversal parameter control can be reformulated as a control of toroidal edge field at the plasma radius M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  7. Results Reversal loss M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  8. Optimization of Toroidal Power Supply Control System M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  9. 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 + + + + Toroidal inverters + - M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  10. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  11. Feedback Control of Plasma Current M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  12. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  13. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  14. Automatic Ip ramping Plasma Current 29271 PCAT Voltage 29271 Vmag29271 Vmag estimate 29271 • 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 real-time system but a good estimation can be obtained Automatic Ip ramping M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  15. MHD Equilibrium Codes for Horizontal Equilibrium and Shape Control M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  16. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  17. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  18. 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 M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives

  19. 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 codefor horizontal equilibrium and plasma shapeto be used with shallow F is needed. M. Barp – 08 Feb. 2011 – Axisymmetric Controls in RFX-mod: Status and Perspectives