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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
Fusion Association, Culham Science Centre, Abingdon, OX14 3DB UK
3 Kurchatov Institute, Institute of Nuclear Fusion, Moscow, Russia
2Euratom/UKAEA
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
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
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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
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St. 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 Torus
St. Petersburg, 3 to 6 October 2005
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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
MAST
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
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
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Up to 400 kA of Ip without cental solenoid
(decay time ~ 200 ms)
Problems in CTF design with M/C :
in-vessel coils increase radial build,
Up to 500 kA of Ip with solenoid
Hot final plasma, due to reconnection
5
may generate impurities,
need protection from neutrons
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St. Petersburg, 3 to 6 October 2005
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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
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Experimental set-up in MAST for DNM
DNM Simulation
• Disconnected
Central Solenoid
• Solenoid Feeder
on P2
P2
P3
P4
P5
• Capacitor Bank
on P3
• Eddy Currents in passive components
has been extimeted by ANSYS code
• Currents in the PF feeders computed
by “McArdle” code
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
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Simulation performed with a free boundary
Equilibrium Code with multiple contact points
ind+ res=0.02+ 1+ CE   0 R 0I pLext  l i 2
Flux balance
P2
of formation
and merging with
P3
20 mWb lost flux &
Ejima coefficient
CE=0.7
Ip iteratively computed
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
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Equilibrium modelling gives more current
than in experiments
Without central solenoid Ip~340 kA,
lasting ~0.3 s
Plasma is hot Te(0)~0.5 keV
and dense ne(0)~9¥1019 m-3
Equilibrium modelling of P4 current
In experiment P4
current needs
earlier rise
Good NBI target
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
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Ip
BV ramp-up effect
is clearly seen:
in red shot BV increases,
IP4+IP5
plasma is bigger,
Ip current increases
Rext
(~1.2 MW of NBI added)
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 Wtot does not depend on IP3
(30% variation on IP3)
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
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P2
High speed CC: some evidence of plasma ring
P3
between P3 and P2 (e.g. #13206)
P3 support
Fast camera images:
visible light suggests merging faster
than in equilibrium modelling
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St. Petersburg, 3 to 6 October 2005
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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 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
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St. Petersburg, 3 to 6 October 2005
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#13198 EQUILIBRIUM 4.4 ms
Iterative (spherical geometry) Grad-Shafranov solver:
(over)determine amplitude of 2 or 3 (given)
functional dependences of p() and Idia2()
Hollow pressure profile:
p()~(-edge)-0.5
-(exponent < 0 to get p() > 0)
Idia2()~(-edge)0.5
Boundary: largest  among contact points (none on P3)
After equilibrium convergence
Bpol fit
Flux fit
rin
rext
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St. Petersburg, 3 to 6 October 2005
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3.6ms
4.4ms
#13198
5.6ms ‘merging’
7.0ms
9.0ms
Equilibrium reconstruction
#13212 - no equatorial plane plasma is produced
4.4ms
6.4ms
8.0ms
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
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#13198 Evolution of discharge after formation
9.0ms
14.0ms
Pink contours and shade
Frascati ODINsph code
• Flux & Bpol measurements used
• Only up/down symmetric
plasma and eddy currents
16.0ms
20.0ms
28.0ms
Blue & black contours EFIT
• Bpol and outer edge from Ha
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
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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
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
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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?
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
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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-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
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St. Petersburg, 3 to 6 October 2005
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Res1
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13198 MAGNETOSTATIC 4.4 ms
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]
Magnetostatic assumption:
jf=constant within plasma range [rin, rext]
Bpol fit
Flux fit
rin
rext
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