1. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 Plasma formation in MAST by using the double null merging technique Outline Start-Up Techniques for Spherical Tori Equilibrium Simulation of the Double Null Merging (DNM) in MAST Experimental Results of the DNM in MAST Magnetic Reconstruction during the DNM Future Perspectives P. Micozzi 1, F. Alladio 1, P. Costa 1, A. Mancuso 1, A. Sykes 2, G. Cunningham 2, M. Gryaznevich 2, J. Hicks 2, M. Hood 2, G. McArdle 2, F. Volpe 2, Y. Dnestrovskij 3 1 Associazione Euratom-ENEA sulla Fusione, C.P. 65 Frascati, Roma, 00044 Italy 2 Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB UK 3 Kurchatov Institute, Institute of Nuclear Fusion, Moscow, Russia 1

2. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 2 In a Spherical Tokamak A=R/a~1, so very few space is left for the central solenoid (wound around the central rod) Only a small inductive flux can be stored

3. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 3 In a ST based CTF (or Power Plant), the central solenoid would be bombarded by neutrons (no space for internal protection): needs of different start-up techniques Steady State ST Reactors must rely upon non-inductive CD (e.g. NBCD) and high Bootstrap + Diamagnetic fraction to sustain the toroidal current

4. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 4 A possible solution is to use the flux of the poloidal field coils in order to obtain start-up & initial build-up of I p without central solenoid The Merging/Compression (M/C) scheme (developed in START and successfully used in MAST) inductively forms plasma toroids around a coil internal to the vacuum vessel (P3) and then merges them

5. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 5 Up to 400 kA of I p without cental solenoid (decay time ~ 200 ms) Up to 500 kA of I p with solenoid Hot final plasma, due to reconnection Problems in CTF design with M/C : in-vessel coils increase radial build, may generate impurities, need protection from neutrons

6. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 6 No in-vessel coils in CTF with DNM These problems can be removed by Double Null Merging (DNM) scheme * : Break-down is obtained in a low-order null between two coils external to the vacuum vessel * Y. Ono 20th IAEA 2004 IC/P6-44

7. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 7 P3 P2 Experimental set-up in MAST for DNM Disconnected Central Solenoid Solenoid Feeder on P2 Capacitor Bank on P3 P4 P5 Eddy Currents in passive components has been extimeted by ANSYS code Currents in the PF feeders computed by “McArdle” code DNM Simulation

8. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 8 Flux balance of formation and merging with 20 mWb lost flux & Ejima coefficient C E =0.7 I p iteratively computed Simulation performed with a free boundary Equilibrium Code with multiple contact points P3 P2

9. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 9 Equilibrium modelling gives more current than in experiments Without central solenoid I p ~340 kA, lasting ~0.3 s Equilibrium modelling of P4 current In experiment P4 current needs earlier rise Good NBI target

10. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 10 Difference between M/C and DNM: in M/C I p x R proportional to I P3 in DNM I pl  R does not depend on I P3 in M/C W tot proportional to I P3 2 in DNM W tot does not depend on I P3 BV ramp-up effect is clearly seen: in red shot BV increases, plasma is bigger, I p current increases (~1.2 MW of NBI added) IpIp I P4 +I P5 R ext (30% variation on I P3 )

11. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 11 High speed CC: some evidence of plasma ring between P3 and P2 (e.g. #13206) P2 P3 P3 support Fast camera images: visible light suggests merging faster than in equilibrium modelling

12. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 12 Magnetic Reconstruction of the DNM initial phase (plasma and passive currents effects must be distinguished) #13201 (ZERO) 4.4 ms Passive effects induced by PF currents and plasma Zero shot (no plasma)  passive effects of PF currents: measured PF coil currents are subtracted (casings included) least-square fit on all magnetic probes (~100) determines: Currents in passive elements (7 couples) (cross-check with ANSYS in progress) Spherical external multipoles M n e (r ext ), n=1,3,5,7 describing eddy currents upon vacuum vessel Effect of plasma on passive currents is then similarly analyzed in ( Plasma shots - Zero shot )  Input data for equilibrium solution

13. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 13 #13198 EQUILIBRIUM 4.4 ms Iterative (spherical geometry) Grad-Shafranov solver: (over)determine amplitude of 2 or 3 (given) functional dependences of p(  ) and I dia 2 (  ) Hollow pressure profile:p(  )~(  -  edge ) -0.5 -(exponent 0) I dia 2 (  )~(  -  edge ) 0.5 Bpol fitFlux fit r in r ext Boundary : largest  among contact points (none on P3) After equilibrium convergence

14. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 14 4.4ms6.4ms8.0ms #13212 - no equatorial plane plasma is produced 3.6ms4.4ms5.6ms ‘merging’7.0ms 9.0ms #13198Equilibrium reconstruction

15. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 15 9.0ms 14.0ms 16.0ms20.0ms 28.0ms #13198Evolution of discharge after formation Pink contours and shade Frascati ODINsph code Flux & Bpol measurements used Only up/down symmetric plasma and eddy currents Blue & black contours EFIT Bpol and outer edge from H 

16. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 16 Comments upon Magnetic Reconstruction Results are preliminar (only few shots analysed, no comparison with M/C), but: Plasma seems to form around P3, and not at the X-point between P2 & P3 It is not clear if a secondary break-down happens on the midplane (this could explain the I p dependence with BV and not with I P3 ) The ratio between I P2-P3 and I P4 is critical (for “wrong” values plasmoids do not merge, e.g. #13212) One can guess that the presence of coils inside the vacuum chamber does not allow for proper DNM  Vloop close to P3 (~100 V) much higher than one at the X-point, moreover the quality of the null is poor

17. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 17 What to do to improve DNM experiments on MAST? Try to form plasmoids on X-point between P3 & P5, then push with P4 Risk to still form plasmoids around P3 Insert a toroidal limiter surrounding P3, and/or add a further coil to improve null multipolarity and push plasmoids Is it still possible to obtain break-down?

18. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 Conclusions DNM experiments on MAST produce hot/dense final plasma that is a good targed for NBCD in view of fully non-inductive start-up & sustainement of the discharge (no central solenoid) There are remarkable differencies between M/C and DNM: in particular, final I p does not depend upon I P3 in DNM, but only on I P4. Moreover clear BV-ramp effects are observed but Preliminar results of magnetic reconstruction seem to suggest that - in present MAST configuration - break-down still occours around P3: the differencies with the standard M/C could be due to the effect of P2 current ramp-down Probably only major modification of the MAST hardware could allow for proper DNM start-up, but the price to pay is to loose the possibility of using M/C 18

19. Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical Torus St. Petersburg, 3 to 6 October 2005 Res1 13198 MAGNETOSTATIC 4.4 ms Flux fitBpol fit PLASMA SHOT: Zero shot is subtracted, Currents in passive elements (7 couples) due to plasma, added to same currents of zero shot Spherical external multipoles M n e (r ext ), n=1,3,5,7 describing eddy currents due to plasma upon vacuum vessel, added to same of zero shot Spherical internal multipoles M n i (r ext ), n=1,3,5,7 Spherical external multipoles M n e (r in ), n=1,3,5,7 describing plasma current within [r in, r ext ] Magnetostatic assumption: j  =constant within plasma range [r in, r ext ] r in r ext