Transcript Document

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Associazione Euratom-ENEA sulla Fu sione
The Multi-Pinch Experiment
Franco Alladio, Alessandro Mancuso, Paolo Micozzi, 1Stamos Papastergiou and 2François Rogier
CR-ENEA, CP 65, 00044 Frascati (Roma), Italy
1Euratom-Enea ,2ONERA Toulouse, France
Outline
•
PROTO-SPHERA purpose & aims
•
Theoretical basis & analysis
•
Multi-Pinch: a step towards PROTO-SPHERA
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Conclusions
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PROTO-SPHERA
Aims at producing a
spherical torus (ST)
with a screw pinch
(SP) replacing the
centerpost

Ie, Ip = 40, 50 kA
pinch, ST currents Ie, Ip = 60, 240 kA
A
= 1.6
aspect ratio
A ≤ 1.3
tpulse = 80 ms~110 tA pulse duration
tpulse ≥ 70 ms~1 tR
Goals: sustain the plasma (Helicity Injection SPST) for more than tR=m0a2/h
(resistive time) and compare tE with START
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PROTO-SPHERA & Spheromaks (TS-3)
Two important requirements are different for the PROTO-SPHERA equilibria
with respect to Flux-Core-Spheromaks like TS-3:
1)
2)
Tokamak-like safety factor profile of ST: q0~0.94, q95~2.6 at the edge
Strong jump of the surface averaged relaxation parameter <m>=m0<j•B/B2>
between SP and ST  Helicity Injection (m~35 m-1 inside SP, m~10 m-1 inside ST)
PROTO-SPHERA (Rsph=0.35 m) has been designed in order to be
as far as possible from the pure Spheromak mSTRsph≤4.49 eigenvalue
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Physics & Engineering
Design
ST diameter = 0.7 m
Toroidal plasma current
Ip= 240 kA
Aspect ratio A = 1.2
Elongation k = 2.35
Pinch current
Ie= 60 kA
Engineering design
complete
(geometry, stresses and
temperatures)
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Associazione Euratom-ENEA sulla Fu sione
Main Breakdown features
Proto-Pinch
Testbench
(Ie~ 700 A)
Screw pinch (SP)
formed by a hot cathode breakdown

 Filling pressure pH~10-3÷10-2 mbar
 Cathode filaments heated to 2600 °C
 Ve~100 V applied on the anode
 Electrode arc current limited to Ie~8.5 kA
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Associazione Euratom-ENEA sulla Fu sione
Theoretical analysis of the PROTO-SPHERA configuration

Ideal MHD Equilibria:
• formation phase fully analyzed
• equilibrium resilience checked by varing the functional forms of p(y) & f(y)
both on the SP and on the ST
Ideal MHD Stability:
• New free boundary code developed  plasmas extending up to the
symmetry axis (R=0) with regular (|B|≠0) & singular (|B|=0) X-points
• Toroidal mode numbers n=0 (axisymmetric), 1, 2, 3 investigated
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PROTO-SPHERA Equilibria: Ip=180 kA, Ie= 60 kA,
= 0.22
All equilibria has been obtained with the same coils set
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n=1
PROTO-SPHERA Stability Results
n=1,2,3 investigated for all equilibria
Main constraint IST/Ie
Stability obtained for:
• b = 21÷26% , IST/Ie= 0.5 - 1
• b = 14÷15% , IST/Ie= 2-4
With bT0=2m0<p>vol/< BT 0>vol
• bT0=28÷29% , IST/Ie= 0.5 - 1
• bT0=72÷84% , IST/Ie= 2-4
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The dominant instabilities are:
• up to IST/Ie≈3 Spherical Torus
instabilities
• IST/Ie> 3 Screw Pinch kink instabilities
IST/Ie = 5 and b15%
Unstable
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Associazione Euratom-ENEA sulla Fu sione
Results for axisymmetric stability (n=0)
Pinch Magnetic Structure
prevents n=0
Vertical ULART Drift
PROTO-SPHERA
n=0 stable for any
scenarios up to
R/a = 1.2
k<4
Instabilities growth rate for free
boundary codes
(vacuum contribution fully included)
Mode numbers
n=0, m=[-15,+15]
Ulart Magnetic Structure
prevents
Radial Pinch Drift
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Associazione Euratom-ENEA sulla Fu sione
Theoretical work to be done

Investigation about the resistive MHD instabilities,
in particular at the Screw Pinch / Spherical Torus interface
Evaluation of the Helicity Injection efficency
both from a macroscopic as well as a microscopic point of view
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Possible evolution of the
PROTO-SPHERA proposal

PROTO-SPHERA can be viewed as a simplification of the
Chandrasekar-Kendall-Furth (CKF) configurations
(stable up to b ~ 1)
Screw Pinch replaces in part
the surrounding discharge
Divertor PF coils replace
the secondary tori
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Multi-Pinch Experiment
Aims:
Stable Screw Pinch plasma with the
full dimension and mushroom shape,
but reduced current Ie (2.7 Vs. 8.5 kA)
Phylosophy:
Almost all parts should be reutilized
in PROTO-SPHERA
 START vacuum vessel
(already in Frascati)
- Provisional simplified PF coils system with constant
current (partially recoverable for PROTO-SPHERA)
- Fed with 0.6 kA, 120 V (no water cooling)
(1.9 kA, 350 V in PROTO-SPHERA)
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Associazione Euratom-ENEA sulla Fu sione
Multi-Pinch Experiment
(first phase)
Cathode plasma only is mushroom-shaped
Power density on the “single”
anode ~ PROTO-SPHERA
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Associazione Euratom-ENEA sulla Fu sione
Multi-Pinch Experiment
(second phase)
Anode & Cathode plasma mushroom-shaped
Definitive PROTO-SPHERA
Anode
Possibility of arc anchoring  needs of saddle coils?
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Multi-Pinch Cathode
Definitive PROTO-SPHERA Cathode, but only partially filled
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Multi-Pinch Cathode
W wires

 Mo support
Wire Length = 400 mm
Conical
terminal &
clamp
•Multi-Pinch test is
proposed with a limited
number of cathode coils
(18 Vs. 378)
•Coils will drive a total
limited current
(2.7 kA vs. 60 kA) inside the
pinch, simplifying the power
supply for the Multi-Pinch
•Each coil will be capable of
delivering the design
current (150 A)
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Conclusions
•PROTO-SPHERA aims to explore the feasibility of a ST formed
around a SP, with respect to the possibility to sustain it through
Helicity Injection on resistive time scale
•An exhaustive range of PROTO-SPHERA scenarios has been
investigated (resilience of the equilibria Vs. internal profile &
ideal MHD stability boundaries)
•The Multi-Pinch experiment will investigate the feasibility of a
stable mushroom-shaped Screw Pinch
•Multi-Pinch will be built inside the START vacuum vessel,
PROTO-SPHERA can be obtained with a modular
implementation of this experiment
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