plasma formation in mast by using the double null merging ... · m. gryaznevich 2, j. hicks , m....

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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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



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



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



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



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



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



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



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



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)



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



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



#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



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



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



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



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?


Associazione Euratom-ENEA sulla Fusione

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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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

plasma formation in mast by using the double null merging ... · m. gryaznevich 2, j. hicks , m....

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