Operational Scenarios in RFX-mod

1. Operational Scenarios in RFX-mod Roberto Cavazzana with the support of the whole RFX-Team

2. Summary • The recipe needed to a obtain a good Reversed Field Pinch plasma is a balance of different ingredients • Here we will discuss a few… • RFP basics • How the RFP works • Startup of RFP plasma • … at high current • RFP equilibrium control • The main control knob of the RFP • Why? • How?

3. The textbook RFP • The Taylor description is the basic tool to introduce and explain the basic RFP equilibria. • An elegant math tool to obtain basic description of RFP equilibria • Boundary conditions: • A given toroidal flux • A given plasma current the natural control parameter is: • The F parameter: is the “output” of this description

4. Real RFP plasma S. Martini et al., PPCF 41,A315 (1999) • The toroidal magnetic flux is not a conserved quantity • RFP does not need a flux conserver to be established • nevertheless a close proximity shell is necessary to keep MHD modes stabilized and controllable • The total toroidal flux is mainly generated by the RFP internal current distribution, being the outcome of the internal self organization processes

5. RFP startupotions - tradtionalapproach J.C. Sprott, Phys. Fluids 28, 2266 (1988) F > 0 Low q Pinch F < 0 RFP • Looking only at the total magnetic energy balance, in principle there could be three types of startup. • ∆Φp: total available transformer flux • Ramped • Matched • Aided • This approach does not take into account the constrain of evolving the system through allowed equilibria

6. Aided startup simulation – no dissipation • Model: • purelyinductivebehaviourof the plasma (no dissipation) • RFX-modgeometricalparameterswith toroidal coils short circuited F > 0 Low q Pinch F < 0 RFP ? What does it happen when dissipation comes into play?

7. Attemptofmatchedstartup Time [s] Low q Pinch RFP Reversal • Toroidal flux dissipation and small gain or even reduction of plasma current • Increase of the resistive loop voltage • i.e. poloidal flux consumption

8. Matchedstartup Not exactly matched, but looks promising Works pretty well at low plasma current (up to 600 kA on RFX) At higher current things change..

9. Density pump-out Shortly after the ionization Hydrogen particles are suddenly adsorbed by the First Wall Affects mainly machines with Graphite covered FW In the case of high current operation one needs an empty Graphite to avoid sudden release of H2/D2 due to FW heating during the discharge

10. Ramped start-up The workaround is to start with a low bias toroidal field, so to reduce the reversal time and obtain the RFP before the density is exhausted. This is the is the same scheme proposed for Titan design (70 mT), based on the experience of ZT-40 On RFX-mod 40 – 60 mT.

11. Rampedstartup on RFX-mod Matched startup for comparison

12. Finalcurrentlowerthan the RFX-mod 2MA design point F > 0 Low q Pinch F < 0 RFP RFX has been originally designed to reach 2MA through a matched start-up scheme starting from an initial toroidal field of 0.5 T Starting from a lower toroidal field, the final current is lower.

13. Reconfiguration of Power supply L. Novello Fusion Eng Des 86, 1993 (2011) Time [s] Using the ramped startup, only a small toroidal field is needed. So ¾ of the toroidal power supply units have been disconnected and connected to the ohmic coils. The additional poloidal flux available allows to reach the 2 MA current.

14. Real RFP equilibrium control • Boundary conditions: • The current flowing into toroidal coils Toroidal Field at the Edge • The applied loop voltage Plasma Current • The control parameter of real plasma is: • The toroidal flux cancels out and becomes a free parameter of the configuration • The F parameter can be used as the practical control variable for the configuration …

15. 1/7 Axi symmetric q RFP equilibrium control: q profile and dynamo • For a given RFP plasma, the edge Bt(a) regulates the reversal parameter F and thus the boundary condition of the q(r) profile:

16. n=7 RFP equilibrium control: QSH ansSHAx states • For a given RFP plasma, the edge Bt(a) regulates the reversal parameter F and the formation of the QSH and SHAx states:

17. Basicscheme: Open LoopF-Control M. Cavinato Fus Eng Des, 83, 220 (2008) • Open loop F-control: • The reference current for the toroidal power supply is calculated in real time: Itor Fref · <Bt> • Based on stationary equilibrium conditions • Extremely simple implementation • Unable to deal with abrupt plasma current change: • dIp/dt -> d<Bt>/dt = Vpoloidal • variable load: inadequate response of power supply

18. AdvancedF-controlscheme M. Barp Fusion Eng. Des. 86 1000 (2011) • Combines • feed-forward • feedback • Takes into account • the vessel reaction • the toroidal flux variations caused by the change of plasma current

19. Conclusions • A meaningful study of the RFP properties requires a precise control of the toroidal field at the edge, since at different value of F paramater the internal structure and the property of the RFP changes significantly. • The main control parameter for the RFP is the q at the edge. • The RFP plasma formation is relatively easy at low current (up to 500 kA), but becomes increasingly difficult at higher currents

20. What’s left out… • Plasma current control / Shape and Position control • Transient operations • Pulse Poloidal Current Drive, • Oscillating Poloidal Current Drive • Oscillating Field Current Drive • Self Similar Current Decay • MHD control • Tearing mode control • measurement error limit • m=0 low n mode control • First Wall / density control • Graphite • Wall Conditioning

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22. J.A. Phillips, Nuclear Fusion28, 1241 (1988) Volume 86, Issues 6–8, October 2011, Pages 1000–1004 Closed loop control of reversal parameter in RFX-mod Marco Barp, , Roberto Cavazzana, Giuseppe Marchiori, Anton Soppelsa, Loris Zanotto Volume 86, Issues 6–8, October 2011, Pages 1393–1397 Enhancement of the power supply systems in RFX-mod towards 2 MA plasma current Luca Novello, , Andrea Zamengo, Alberto Ferro, Loris Zanotto, Marco Barp, Roberto Cavazzana, Claudio Finotti, Mauro Recchia, Elena Gaio Control of plasma parameters by the externally applied toroidal field in RFX-modOriginal Research Article Fusion Engineering and Design, Volume 83, Issues 2–3, April 2008, Pages 220-223 M. Cavinato, R. Cavazzana, A. Luchetta, G. Manduchi, G. Marchiori, A. Soppelsa, M. Spolaore, C. Taliercio