plasma formation in mast by using the double null merging ... · m. gryaznevich 2, j. hicks , m....
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Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
Associazione Euratom-ENEA sulla Fusione
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. 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 Italy2Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB UK
3 Kurchatov Institute, Institute of Nuclear Fusion, Moscow, Russia
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Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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In a Spherical Tokamak A=R/a~1, so very few space is left for the
central solenoid (wound around the central rod)
MAST
Only a small inductive flux can be stored
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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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
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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A possible solution is to use the flux of the poloidal field coilsin 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
MAST
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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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
Problems in CTF design with M/C :
in-vessel coils increase radial build,
may generate impurities,
need protection from neutrons
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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No in-vessel coils in CTF with DNM
These problems can be removedby 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
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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P3P2
Experimental set-up in MAST for DNM
• DisconnectedCentral Solenoid
• Solenoid Feederon P2
• Capacitor Bankon P3
P4
P5
• Eddy Currents in passive componentshas been extimeted by ANSYS code
• Currents in the PF feeders computedby “McArdle” code
DNM Simulation
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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Δψind+ Δψres=0.02+ 1+ CE( )⋅ μ 0R 0I p Lext + li 2[ ]Flux balance of formationand merging with20 mWb lost flux &Ejima coefficientCE=0.7
Ip iteratively computed
Simulation performed with a free boundaryEquilibrium Code with multiple contact points
P3
P2
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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Equilibrium modelling gives more currentthan in experiments
Without central solenoid Ip~340 kA,lasting ~0.3 s
Plasma is hot Te(0)~0.5 keVand dense ne(0)~9•1019
m-3
Equilibrium modelling of P4 current
In experiment P4current needsearlier rise
Good NBI target
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Difference between M/C and DNM:
• in M/C Ip x R proportional to IP3in DNM Ipl × R does not depend on IP3
• in M/C Wtot proportional to IP32
in DNM Wtot does not depend on IP3
BV ramp-up effectis clearly seen:in red shot BV increases,plasma is bigger,Ip current increases(~1.2 MW of NBI added)
Ip
IP4+IP5
Rext
(30% variation on IP3)
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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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
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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 Mn
e(rext), n=1,3,5,7describing 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
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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#13198 EQUILIBRIUM 4.4 msIterative (spherical geometry) Grad-Shafranov solver:(over)determine amplitude of 2 or 3 (given)functional dependences of p(ψ) and Idia
2(ψ)
Hollow pressure profile: p(ψ)~(ψ-ψedge)-0.5
-(exponent < 0 to get p(ψ) > 0)Idia
2(ψ)~(ψ-ψedge)0.5
Bpol fit Flux fit
rin
rext
Boundary: largest ψ among contact points (none on P3)
After equilibrium convergence
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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4.4ms 6.4ms 8.0ms#13212 - no equatorial plane plasma is produced
3.6ms 4.4ms 5.6ms ‘merging’ 7.0ms 9.0ms
#13198 Equilibrium reconstruction
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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9.0ms 14.0ms 16.0ms 20.0ms 28.0ms
#13198 Evolution of discharge after formation
Pink contours and shadeFrascati ODINsph code• Flux & Bpol measurements used• Only up/down symmetric plasma and eddy currents
Blue & black contours EFIT• Bpol and outer edge from Hα
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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Comments upon Magnetic ReconstructionResults 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 higherthan one at the X-point, moreover the quality of the null is poor
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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What to do to improve DNM experiments on MAST?
Try to form plasmoids on X-pointbetween 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 improvenull multipolarity and push plasmoids
Is it still possible to obtain break-down?
Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
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Conclusions•DNM experiments on MAST produce hot/dense final plasma
that is a good targed for NBCD in view of fully non-inductivestart-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 occoursaround P3: the differencies with the standard M/C could bedue to the effect of P2 current ramp-down
•Probably only major modification of the MAST hardwarecould allow for proper DNM start-up, but the price to payis to loose the possibility of using M/C
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Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th International Workshop on Spherical TorusSt. Petersburg, 3 to 6 October 2005
Associazione Euratom-ENEA sulla Fusione 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 Mn
e(rext), n=1,3,5,7describing eddy currents due to plasmaupon 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]
Magnetostatic assumption:jφ=constant within plasma range [rin, rext]
rin
rext