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The Multi-Pinch Experiment

This paper presents the theoretical analysis and experimental results of the Multi-Pinch Experiment, which aims to explore the feasibility of producing a spherical torus (ST) with a screw pinch (SP) replacing the centerpost. The study focuses on the PROTO-SPHERA configuration and its equilibria, stability, and magnetic structure. The ultimate goal is to develop a viable alternative to existing fusion reactor designs.

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The Multi-Pinch Experiment

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  1. 1 The Multi-Pinch Experiment Franco Alladio,Alessandro Mancuso, Paolo Micozzi, 1Stamos Papastergiou and 2François Rogier CR-ENEA, CP 65, 00044 Frascati (Roma), Italy1Euratom-Enea ,2ONERA Toulouse, France • Outline • PROTO-SPHERA purpose & aims • Theoretical basis & analysis • Multi-Pinch: a step towards PROTO-SPHERA • Conclusions

  2. 2 PROTO-SPHERA Aimsat producing a spherical torus (ST) with a screw pinch (SP) replacing the centerpost  Ie, Ip = 40, 50 kA pinch, ST currentsIe, Ip = 60, 240 kAA = 1.6 aspect ratioA ≤ 1.3tpulse = 80 ms~110 tApulse durationtpulse ≥ 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

  3. 3 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)Tokamak-like safety factor profile of ST: q0~0.94, q95~2.6 at the edge 2)Strong jump of the surface averaged relaxation parameter <>=0<j•B/B2> between SP and ST  Helicity Injection (~35 m-1 inside SP, ~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 STRsph≤4.49 eigenvalue

  4. 4 Physics & Engineering Design ST diameter = 0.7 mToroidal plasma current Ip= 240 kA  Aspect ratio A = 1.2Elongation k = 2.35  Pinch current Ie= 60 kA Engineering design complete (geometry, stresses and temperatures)

  5. 5 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 

  6. 6 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

  7. 7 PROTO-SPHERA Equilibria:Ip=180 kA, Ie= 60 kA, = 0.22 All equilibria has been obtained with the same coils set

  8. 8 n=1 PROTO-SPHERA Stability Results • n=1,2,3 investigated for all equilibria • Main constraintIST/Ie • Stability obtained for: • b =21÷26% , IST/Ie= 0.5 - 1 • b= 14÷15% , IST/Ie= 2-4 • With bT0=2m0<p>vol/< >vol • bT0=28÷29% , IST/Ie= 0.5 - 1 • bT0=72÷84% , IST/Ie= 2-4 • 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

  9. 9 Pinch Magnetic Structure prevents n=0 Vertical ULART Drift Results for axisymmetric stability (n=0) PROTO-SPHERAn=0 stable for any scenarios up to R/a = 1.2k < 4 Ulart Magnetic Structure preventsRadial Pinch Drift Instabilities growth rate for free boundary codes (vacuum contribution fully included) Mode numbersn=0, m=[-15,+15]

  10. 10 Theoretical work tobedone  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

  11. 11 Possible evolution of thePROTO-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

  12. 12 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)

  13. 13 Multi-Pinch Experiment (first phase) Cathode plasma only is mushroom-shaped Power density on the “single” anode ~ PROTO-SPHERA

  14. 14 Multi-Pinch Experiment (second phase) Anode & Cathode plasma mushroom-shaped Definitive PROTO-SPHERA Anode Possibility of arc anchoring  needs of saddle coils?

  15. 15 Multi-Pinch Cathode Definitive PROTO-SPHERA Cathode, but only partially filled

  16. 16 Wire Length = 400 mm Conical terminal & clamp Multi-Pinch Cathode W wires  • 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)  Mo support

  17. 17 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 theSTARTvacuum vessel, PROTO-SPHERA can be obtained with a modular implementation of this experiment

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