1. ICC2004 Madison, Wisconsin The Multi-Pinch Experiment Outline PROTO-SPHERA purpose & aims Theoretical basis & analysis Multi-Pinch: a step towards PROTO-SPHERA Conclusions Franco Alladio, Alessandro Mancuso, Paolo Micozzi, 1 Stamos Papastergiou and 2 François Rogier CR-ENEA, CP 65, 00044 Frascati (Roma), Italy 1 Euratom-Enea, 2 ONERA Toulouse, France 1

2. ICC2004 Madison, Wisconsin PROTO-SPHERA I e, I p = 40, 50 kA pinch, ST currents I e, I p = 60, 240 kA A = 1.6 aspect ratio A ≤ 1.3  pulse = 80  s~110  A pulse duration  pulse ≥ 70 ms~1  R Goals: sustain the plasma (Helicity Injection SP  ST) for more than  R =  0 a 2 /  (resistive time) and compare  E with START 2 Aims at producing a spherical torus (ST) with a screw pinch (SP) replacing the centerpost 

3. ICC2004 Madison, Wisconsin PROTO-SPHERA & Spheromaks (TS-3) 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: q 0 ~0.94, q 95 ~2.6 at the edge 2)Strong jump of the surface averaged relaxation parameter =  0 between SP and ST  Helicity Injection (  ~35 m -1 inside SP,  ~10 m -1 inside ST) PROTO-SPHERA (R sph =0.35 m) has been designed in order to be as far as possible from the pure Spheromak  ST R sph ≤4.49 eigenvalue

4. ICC2004 Madison, Wisconsin Physics & Engineering Design ST diameter = 0.7 m Toroidal plasma current I p = 240 kA Aspect ratio A = 1.2 Elongation  = 2.35 Pinch current I e = 60 kA Engineering design complete (geometry, stresses and temperatures) 4

5. ICC2004 Madison, Wisconsin Main Breakdown features Screw pinch (SP) formed by a hot cathode breakdown  Filling pressure p H ~10 -3 ÷10 -2 mbar  Cathode filaments heated to 2600 °C  V e ~100 V applied on the anode  Electrode arc current limited to I e ~8.5 kA 5 Proto-Pinch Testbench (I e ~ 700 A) 

6. ICC2004 Madison, Wisconsin 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(  ) & f(  ) 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. ICC2004 Madison, Wisconsin 7 All equilibria has been obtained with the same coils set PROTO-SPHERA Equilibria: I p =180 kA, I e = 60 kA, = 0.22

8. ICC2004 Madison, Wisconsin 8 PROTO-SPHERA Stability Results I ST /I e = 5 and  15% Unstable n=1,2,3 investigated for all equilibria Main constraint I ST /I e Stability obtained for:  = 21÷26%, I ST /I e = 0.5 - 1  = 14÷15%, I ST /I e = 2-4 With  T0 =2  0 vol / vol  T0 =28÷29%, I ST /I e = 0.5 - 1  T0 =72÷84%, I ST /I e = 2-4 The dominant instabilities are: up to I ST /I e ≈3 Spherical Torus instabilities I ST /I e > 3 Screw Pinch kink instabilities n=1

9. ICC2004 Madison, Wisconsin Results for axisymmetric stability (n=0) Instabilities growth rate for free boundary codes (vacuum contribution fully included) Mode numbers n=0, m=[-15,+15] 9 Pinch Magnetic Structure prevents n=0 Vertical ULART Drift PROTO-SPHERA n=0 stable for any scenarios up to R/a = 1.2  < 4 Ulart Magnetic Structure prevents Radial Pinch Drift

10. ICC2004 Madison, Wisconsin Theoretical work to be done 10  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. ICC2004 Madison, Wisconsin Possible evolution of the PROTO-SPHERA proposal 11  PROTO-SPHERA can be viewed as a simplification of the Chandrasekar-Kendall-Furth (CKF) configurations (stable up to  ~ 1) Screw Pinch replaces in part the surrounding discharge Divertor PF coils replace the secondary tori

12. ICC2004 Madison, Wisconsin 12 - 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) Multi-Pinch Experiment  START vacuum vessel (already in Frascati) Aims: Stable Screw Pinch plasma with the full dimension and mushroom shape, but reduced current I e (2.7 Vs. 8.5 kA) Phylosophy: Almost all parts should be reutilized in PROTO-SPHERA

13. ICC2004 Madison, Wisconsin 13 Multi-Pinch Experiment (first phase) Cathode plasma only is mushroom-shaped Power density on the “single” anode ~ PROTO-SPHERA

14. ICC2004 Madison, Wisconsin 14 Multi-Pinch Experiment (second phase) Anode & Cathode plasma mushroom-shaped Definitive PROTO-SPHERA Anode Possibility of arc anchoring  needs of saddle coils?

15. ICC2004 Madison, Wisconsin 15 Definitive PROTO-SPHERA Cathode, but only partially filled Multi-Pinch Cathode

16. ICC2004 Madison, Wisconsin Multi-Pinch Cathode 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) Conical terminal & clamp Wire Length = 400 mm 16  Mo support W wires 

17. ICC2004 Madison, Wisconsin 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 17