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Plasma formation in MAST by using the double null merging technique

1. Plasma formation in MAST by using the double null merging technique. 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

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Plasma formation in MAST by using the double null merging technique

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  1. 1 Plasma formation in MAST by using the double null merging technique P. Micozzi1, F. Alladio1, P. Costa1, A. Mancuso1,A. Sykes2, G. Cunningham2, M. Gryaznevich2, J. Hicks2, M. Hood2, G. McArdle2, F. Volpe2, Y. Dnestrovskij3 1Associazione Euratom-ENEA sulla Fusione, C.P. 65 Frascati, Roma, 00044 Italy 2Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB UK 3 Kurchatov Institute, Institute of Nuclear Fusion, Moscow, Russia • 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

  2. 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. 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. 4 A possible solution is to use the flux of the poloidal field coils in order to obtain start-up & initial build-up of Ip 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. 5 Problems in CTF design withM/C : in-vessel coils increase radial build, may generate impurities, need protection from neutrons Up to 400 kA of Ip without cental solenoid (decay time ~ 200 ms) Up to 500 kA of Ip with solenoid Hot final plasma, due to reconnection

  6. 6 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 No in-vessel coils in CTF with DNM * Y. Ono 20th IAEA 2004 IC/P6-44

  7. 7 Experimental set-up in MAST for DNM DNM Simulation • Disconnected Central Solenoid • Solenoid Feeder on P2 • Capacitor Bank on P3 P4 P2 P3 P5 • Eddy Currents in passive components has been extimeted by ANSYS code • Currents in the PF feeders computed by “McArdle” code

  8. 8 Simulation performed with a free boundary Equilibrium Code with multiple contact points P2 Flux balance of formation and merging with 20 mWb lost flux & Ejima coefficient CE=0.7 Ip iteratively computed P3

  9. 9 Equilibrium modelling gives more current than in experiments Without central solenoid Ip~340 kA, lasting ~0.3 s Equilibrium modelling of P4 current In experiment P4 current needs earlier rise Good NBI target

  10. 10 Ip BV ramp-up effect is clearly seen: in red shot BV increases, plasma is bigger, Ip current increases (~1.2 MW of NBI added) IP4+IP5 Rext • Difference between M/C and DNM: • in M/C Ip x R proportional to IP3 • in DNM Ipl R does not depend on IP3 • in M/C Wtot proportional to IP32 • in DNM Wtotdoes not depend on IP3 (30% variation on IP3)

  11. 11 High speed CC: some evidence ofplasma 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. 12 Magnetic Reconstruction of the DNM initial phase (plasma and passive currents effects must be distinguished) 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 Mne(rext), 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 #13201 (ZERO) 4.4 ms

  13. 13 #13198 EQUILIBRIUM 4.4 ms Iterative (spherical geometry) Grad-Shafranov solver: (over)determine amplitude of 2 or 3 (given) functional dependences of p(y) and Idia2(y) Hollow pressure profile: p(y)~(y-yedge)-0.5 -(exponent < 0 to getp(y) > 0) Idia2(y)~(y-yedge)0.5 Boundary: largest yamong contact points (none on P3) After equilibrium convergence Bpol fit Flux fit rin rext

  14. 14 9.0ms 3.6ms 4.4ms 5.6ms ‘merging’ 7.0ms #13198 Equilibrium reconstruction #13212 - no equatorial plane plasma is produced 4.4ms 6.4ms 8.0ms

  15. 15 #13198 Evolution of discharge after formation 14.0ms 28.0ms 9.0ms 16.0ms 20.0ms 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 Ha

  16. 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 Ip dependence with BV and not with IP3) • The ratio between IP2-P3 and IP4 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. 17 What to do to improve DNM experiments on MAST? Try to form plasmoids on X-point between P3 & P5, then push with P4 Insert a toroidal limiter surrounding P3, and/or add a further coil to improve null multipolarity and push plasmoids Risk to still form plasmoids around P3 Is it still possible to obtain break-down?

  18. 18 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 Ip does not depend upon IP3 in DNM, • but only on IP4. 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

  19. Res1 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 Mne(rext), n=1,3,5,7 describing eddy currents due to plasma upon vacuum vessel, added to same of zero shot • Spherical internal multipoles Mni(rext), n=1,3,5,7 • Spherical external multipoles Mne(rin), n=1,3,5,7 describing plasma current within[rin, rext] 13198 MAGNETOSTATIC 4.4 ms Magnetostatic assumption: jf=constantwithin plasma range[rin, rext] Bpol fit Flux fit rin rext

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