14th IEA - RFP Workshop Consorzio RFX, Padova, ITALY, April 26th-28th 2010

1. 14th IEA - RFP Workshop Consorzio RFX, Padova, ITALY, April 26th-28th 2010 Resistive Wall Modes control under different coil configurations and implications for JT-60SA M. Baruzzo T. Bolzonella, G. Manduchi, G. Marchiori, M. Takechi 1 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

2. Introduction • Effective active RWM control is critical to access and maintain high bNplasmas in advanced tokamak operations. • RWM control in future devices will be an issue, taken into account that passive stabilization by plasma rotation and kinetic effects might not be present. • Present sector coils design for future devices often implies very small coils (0.38% of total surface each for JT60-SA) and a low coverage of the plasma surface. No present experiment attempted RWM control with similar active coils. • Innovative experiments on RWM control have been planned and recently performed on RFX-mod device in collaboration between Consorzio RFX and JAEA. • RWM in RFX-mod drive is the stationary, equilibrium current profile, i.e. their growth rate is robustly constant over the plasma discharge; • RWM in RFX-mod can be stabilized only by active control: passive stabilization effects (rotation, kinetics, ...) play a negligible role; • RFX-mod has well known and flexible control system. • RFX-mod can provide answers for outstanding RWM control issues 2 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

3. Outline • Introduction • Outline • RFX-mod and JT60-SA active coils configurations • RFX-mod active control reconfiguration: • Implementation • single RWM control • multiple RWMs control • Conclusions and future work 3 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

4. RFX-mod MHD control system RFX-mod control system is made by 192 active saddle coils, each independently fed. 100% coverage of the plasma surface. RFX-mod active control system is designed to address tearing mode control, it is over-dimensioned for RWMs control RFX-mod “quantum” for active control: Df=7.5°; Dq=90°. Area covered=0.52% 4 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

5. JT60-SA active RWM control Stabilising plate "External" sector coils Each “external” sector coil covers 0.38% of the total surface; the total active area is then less about 6.8%. “Internal" sector coils (alternative design) 5 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

6. Active system reconfiguration The main idea is to test on the same plasma and on the same device different active control configurations. NB: it is a software reconfiguration, active on selected harmonics only! 6 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

7. Software reconfiguration scheme + i,j coil space m,n mode space ON/OFF, Average, Other 7 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

8. Different configurations explored 48 (f) x 4 (q): 100% 48 (f) x 1 (q): 25% 16 (f) x 1 (q): 8.3% 12 (f) x 1 (q): 6.25% 8 (f) x 1 (q): 4.2% 3 (f) x 4 (q): 6.25% 8 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

9. (1,-6) RWM control, 48x1 coils Previous experiments in the 48x1 configuration already showed that by increasing the proportional gain it is possible to fully stabilize the most unstable RWM. 9 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

10. Vacuum pulse, reduced toroidal array (1,-6) 8x1 configuration (#28306), (1,-6) perturbation rotating at 20 Hz. Measured sidebands can be up to 3 times the main harmonic. Remember that aliased harmonics enter also in the (1,-6) amplitude, so that real (1,-6) amplitude is smaller than the one experimetally measured. 10 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

11. (1,-6) RWM control, reduced toroidal array In new experiments, for all the configurations explored it was possible to find a set of (Kp; Ki) values completely stabilizing the selected RWM (but with active control on sidebands and toroidal harmonics present). 11 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

12. Sideband effect on plasma Control starting from different finite amplitudes, 8x1 coils configuration. Fast transients due to finite amplitude of RWM (growth rate, r.t. sensor system sampling time, control system latency time, ...) can affect very seriously overall plasma performances. In the experiments shown only 2 sidebands are excluded from the control loop. What affects the discharge is not the RWM growth, but the sudden increase of sideband induced error fields. 12 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

13. Reconfiguredcontrol on n=-4,-5,-6 m=1 Black: no control (n=-6 m=1 controlled from 0.1s) Blue: control with 3x4 coils Red: control with 8x1 coils 13 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

14. Reconfiguredcontrol: limitations Blue and black: m=1,n=-6 reference free growth until 0.1s red:control with 12x1 coils The mode moves towards a minimum of feedback action, and then grows. A good coil periodicity has to be chosen 14 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

15. Conclusion We are well aware that easy translations are not possible, however these experiments in our opinion suggest that: 1. Reduced control configurations can cope with multiple unstable RWMs; 2. Unavoidable sidebands production is a main concern and its relative importance grows with smaller coils; 3. Large amplitude sidebands can be generated in fast transients, these sidebands are very dangerous for the discharge; 4. The control is effective provided that the there are not periodicities in common with MHD instabilities to be controlled; 15 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

16. Future work 1. Impact of sidebands will be quantitatively investigated 2. Future experiments could aim at the effect of different coil sizes (again possible by software): comparison for fixed % converage or fixed coil number 3. Similar experiments on RFX-mod in tokamak configuration have been proposed 4. Control using reduced set of measurements might be explored 5. Adimensional control parameters have to be identified, to bridge the magnetic control physics to future devices. RFX-mod JT60-SA ITER 16 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

17. Backup slides 17 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

18. RWMs in RFX-mod Time (s) Multimode control in RFX-mod. Comparison between a fully controlled 1 MA discharge (full black line) and a 0.6 MA discharge where unstable RWM were not controlled (blue, dashed line). The most unstable RWM is the m=1, n=-6 that terminate the discharge when its amplitude reaches almost 10 mT. Note the different amplitude scale of the (1,-5) and (1,-4) that, for the given magnetic equilibrium, have a growth rate lower than the (1,-6). 18 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010

19. (Some) RWM controlissues 1. What is the minimum number of active coils needed to stabilize RWMs? (or the minimum surface coverage; or the optimal coil distribution; or...) 2. Can the unstable mode deform itself when large portions of the plasma surface are left without active control? 3. What are the consequences of the different choices in terms of sideband generation? 4. How will these sidebands affect the plasma evolution? 5. How can we quantify the impact on the control system of possible fast transients in RWM amplitude? 19 M. Baruzzo – 14th IAE Workshop, Padova, April 26th-28th 2010