A new approach to plasma profile control in ITER

1. A new approach to plasma profile control in ITER S.H. Kim1and J.B. Lister2 1 ITEROrganization, St Paul lez Durance, France 2 EPFL-CRPP, Lausanne, Switzerland Acknowledgement : EURATOM/CRPP/CEA, FNSRS and ITER/Monaco J.-F. Artaud, V. Basiukand F. Imbeaux (CEA) – CRONOS collaboration J.-M. Moret and O. Sauter (CRPP) – discussions D. Campbell, T.A. Casper, V. Chuyanov (ITER) – support for continuing this work

2. Current state of research • Experiments on several devices (JET, Tore-Supra, DIII-D and etc.) •  Demonstrated for some cases with limited conditions • Two time-scale model-based profile control technique (D. Moreau, NF 48) for real-time control in JET uses a profile response model deduced from the identification experiment • Concern on the range of applicability due to the evolution of the plasma state • Using simplified transport and source models to forecast plasma profile responses to actuator power changes in real-time (F. Felici, NF 51, etc)  Currently in an initial phase, such as an real-time identification of transport. Not yet fully developed/tested for active profile control

3. Active plasma profile control in ITER • Goal : robustreal-timeactive control of multiple plasma profiles using multiple actuators • Robust against the evolution of the plasma state • Fast and simple control algorithm for real-time application • Handling non-linearly coupled plasma profile evolutions • Handling saturation and quantization of actuator powers • Without requiring expensive experiments and simulations • Our new approach : • Real-time update of • Static plasma profile response models developed by • Simplifying the related physics with the dual assumption of • Linearity and Time-Invariance.

4. Te profile response model 1. The evolution of the electron pressure 2. Assuming (1) a stationary state, (2) no electronparticle flux and (3) zeroelectronheat convective speed 3. Furthermore, the auxiliary electron heat source is assumed as a product of the time-varying power and normalized radial profile shape. 4. Te profile response to the auxiliary power changes is

5. q profile response model 1. 1D averaged q and plasma currentdensity profiles Controlling Jpl (2nd deriv. of psi)  target q (1st deriv. of psi) 2. Directlyrelating the twoequations and assumingthe non-time-varyingbootstrapcurrentdensity and no radial current diffusion 3. Furthermore, assumingthatthe ohmiccurrentcounteractseachdrivencurrentin such a way of resulting in no edge q variation 4. q profile response to the auxiliary power changes is

6. Required actuator power changes 1. Combining the Te and q profile responses 2. Using matrix vector notation and multiplying by weights (we=1.0 & wq=1.0e5)and proportional gains (ge=1.0 & gq=0.5)of the control loops • The inverse matrix can be obtained using the SVD technique 4. The saturation of the actuator powersis taken into account by modifying the SVD calculation

7. Simulation of ITER hybrid mode operation • CRONOS(J.-F. Artaud, NF50) used to test the new active control approach, re-using the simulation setting from DINA-CH/CRONOShybrid mode simulations (S.H. Kim, PPCF 51) • 12MA flat-top current with 33MW of NBI and 20MW of ICRH • A global transport model based on the energy confinement time scaling laws, KIAUTO, is used with an assumption of improved energy confinement (H98=1.2) • Slightly reversed or flat q profiles above 1.0 at SOF

8. Validation of the profile response models Additional 20MW EC (near on-axis) and 20MW of LH (far off-axis) • Teprofile response is over-estimated in the model, due to the lack of consideration on the confinement degradation and electron-ion equipartition heating power change • Radial plasma current diffusion and fast evolution of the bootstrap current are not considered • However, the models provide enough information for feedback control

9. Active control of Te profile • Actuators : EC, IC, LH & NB • Slow control (> 50s) with a control interval of 10s • EC power is not saturated  further control of Te profile

10. Active control of q profile • Actuators : EC, LH & NB • Slow control (>100s) with a control interval of 10s • All H&CD powers are saturated at around 900s  deviation from the target

11. Active control of Te and q profiles • Tecontrol startslater (@ 400s) than q control (@ 300s) to avoid a strongconflictbetween the twocontrols • The profiles werereally‘actively’ controlled

12. Are these test simulations enough? • Q. Any validation experiments? A. Not yet. We would like to draw attention on this approach from experimentalists and control experts. Q. What happens if real plasma responses are different with those shown in the simulations using a global transport model? A. Let’s do another test using aphysics basedlocal transport model, GLF23

13. Profile response to LH application (GLF23) • The GLF23 transport model (0.2< ρtor <0.95) has been used for CRONOS simulations and profile responses with 20MW of additional EC or LH are studied • Application of 20MW EC : similar but lower temperature profile response than the modelled one • Application of 20MWLH : off-axis source currents degrade the energy confinement through the evolution of magnetic shear profile (J. Citrin, NF 50)

14. Are these models not valid anymore? • Q. De we need to improve the electron temperature profile response model ? • The energy confinement dependence on the evolution of the magnetic shear profile is not yet confirmed by experiments. Real plasma profile response might be better. Therefore, this simulation can be regarded as a pessimistic (and realistic) case. • Q. What happens if the partially opposite profile response is real? • A. Let’s do test simulations without any modifications.

15. Active control of Te profile (GLF23) • 2 cases, with and without LH, are compared • The electron temperature profiles were controlled • When LH is applied, about 2 times of LH power (20MW) were consumed • At 900s, • Ptot with LHCD ~ 55MW • Ptot w/o LHCD ~ 12MW

16. Active control of q profile (GLF23) LHCD was indispensable for controlling q profile Central q value was initially increased due to the EC power requested to minimize the errors on q values outside ρtor = 0.2 • At 900s, • Ptot with LHCD ~ 88MW • Ptot w/o LHCD ~ 53MW 2 2

17. Active control of Te and q profiles (GLF23) • Tecontrol startslater (@ 400s) than q control (@ 300s) The profiles were controlled even in the presence of partially opposite profile response to the models

18. Summary and Conclusions A new, simple, fast and robust approach to the real-time active control of plasma profile for ITER has been studied The profiles were controllable even in the presence of partially opposite profile responses to the models. A more sophisticated but less robust control technique, such as the model-based technique, can be coupled to complement each other. This approach would be useful to support recently initiated ITER PCS project and IPC working group activity.

19. Additional slides

20. Auxiliary power changes (control of Te & q ) 2 1 3 1. NB power is initially reduced due to the strong control demand on the central q values 2. Actuator powers (EC, LH, NB) are quickly saturated at their upper limits 3. IC power is still available for the control of Te

21. Active control of Te profile (flat Xe) Flat Xewith a constant value has been used in evaluating the Te profile response model The Te profile response model is not sensitive to the estimation of Xe profile Lower Xe (higher Ce,index(ρ)) assumption results in a slower control

22. NBI power quantization • Discrete NBI power change only when the control demand is larger than a quantized power. • The quantized NBI power was either 4.125MW or 8.25MW • A larger quantized power results in a slower control

23. Profile control with NBI power quantization Both Te and q profiles were well-controlled with the NBI power quantization

24. L-H mode transition (additional slide) Feedback control is switched off at about 406s, and the auxiliary powers set to prescribed minimum values  H-L transition. When the plasma is in L-mode (t=466s), the feedback control is switched on with the target Te profile at H-mode  L-H transtion Te profile experienced overshooting and then it was stabilized.

25. Control demand (GLF23) Increasing EC(red) is more effective for reducing q outside rho>0.3, while deceasing NB(blue) will only reduce central q. IC(green) is more effective in increasing central Te while maintaining the temperature at the other locations.

26. Frequently asked questions (additional slide) • No experiments ? • Not yet. This approach is presented in this conference for the first time, aiming at drawing attention of experts on experiments. • No ITB formation or L-H/H-L confinement mode transitions? • Not yet. Real-time active profile control techniques and modeling capability of plasma transport are not yet advanced that much. • These fast transients are usually generated in a pre-programmed manner in present-day experiments by tailoring the plasma profiles. • Two time-scales ? • No. Fast control of kinetic profiles appears not a strong requirement for achieving and maintaining the target profiles which are linked to improved confinement regimes. • It appears to be better to separately handle the fast MHD control issues • However, this can be complemented using a more sophisticated control technique.