1. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 PROTO-SPHERA Experiment: Time scenarios & Power Supplies requirements F. Alladio, A. Mancuso, P. Micozzi, F. Rogier* Associazione Euratom-ENEA sulla Fusione, CR Frascati C.P. 65, Rome, Italy * ONERA-CERT / DTIM / M2SN 2, av. Edouard Belin - BP 4025 – 31055, Toulouse, France

2. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 2 2 Spherical Tokamaks allow to obtain: High plasma current I p (and high ) with low B T Plasma  much higher than Conventional Tokamaks More compact devices But, for a reactor/CTF extrapolation: No space for central solenoid (Current Drive requirement more severe) No neutrons shield for central stack (no superconductor/high dissipation) Intriguing possibility ⇒ substitute central rod with Screw Pinch plasma (I TF → I e ) Potentially two problems solved: Simply connected configuration (no conductors inside) I p driven by I e (Helicity Injection from SP to ST) Flux Core Spheromak (FCS) Theory: Taylor & Turner, Nucl. Fusion 29, 219 (1989) Experiment: TS-3; N. Amemiya, et al., JPSJ 63, 1552 (1993)

3. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 New configuration proposed: PROTO-SPHERA “Flux Core Spherical Tokamak” (FCST), rather than FCS Disk-shaped electrode driven Screw Pinch plasma (SP) Prolated low aspect ratio ST (A=R/a≥1.2,  =b/a~2.3) to get a Tokamak-like safety factor (q 0 ≥1, q edge ~3) SP electrode current I e =60 kA ST toroidal currentI p =120÷240 kA ST diameterR sph =0.7 m ⇓ Stability should be improved and helicity drive may be less disruptive than in conventional Flux-Core-Spheromak 3 But Flux Core Spheromaks are: injected by plasma guns formed by ~10 kV voltage on electrodes high pressure prefilled with ST safety factor q≤1

4. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 4 PROTO-SPHERA formation follows TS-3 scheme (SP kink instability) T0 I e =8.5 kA I e 8.5 → 60 kA T3 I p =30 kA A=1.8 T4 I p =60 kA A=1.5 T5 I p =120 kA A=1.3 T6 I p =180 kA A=1.25 TF I p =240 kA A=1.2 Tunnelling (ST formation) ST compression (I p /I e ↑, A ↓ ) I p /I e ratio crucial parameter (strong energy dissipation in SP) MHD equilibria computed both with monotonic (peaked pressure) as well as reversal safety factor profiles (flat pressure,  =J · B/B 2 parameterized) Some level of low n resistive instability needed (reconnections to inject helicity from SP to ST) but SP+ST must be ideally stable at any time slice ⇓ Ideal MHD analisys to assess I p /I e &  limits

5. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 5 Ideal MHD stability results for PROTO-SPHERA PROTO-SPHERA stable at full  21÷26% for I p /I e =0.5 & 1, down to 14÷16% for I p /I e =4 (depending upon profiles inside the ST) Comparison with the conventional Spherical Tokamak with central rod:  T0 =28÷29% for I p /I e =0.5 to  T0 =72÷84% for I p /I e =4 Spherical Torus dominates instabilitiy up to I p /I e ≈3; beyond this level of I p /I e, dominant instability is the SP kink (that gives rise to ST tilt motion) Spherical Torus elongation  plays a key role in increasing I p /I e Comparison with TS-3 (Tokyo University, 1993) experimental results: disk-shaped Screw Pinch plasma important for the configuration stability Ideal MHD stability of Flux Core Spherical Torus rather insensitive to internal ST profiles ⇒ configuration quite robust from an ideal point of view Resistive instabilities behaviour is the main experimental point of PROTO-SPHERA

6. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 6 PROTO-SPHERA poloidal field Coils Group A: ST compression coils (connected in series) Not installed on Multi-Pinch Group B: SP shaping coils (connected in series) Already installed on Multi-Pinch Four Power Supplies: 1)“Group A” P.S. (no Multi-Pinch) 2)“Group B” P.S. (already on Multi-Pinch) 3)“Cathode” P.S. (reduced on Multi-Pinch) 4)“Screw Pinch” P.S. (reduced on Multi-Pinch)

7. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 7 Waveforms Cathode power supply V out : 6 independent outputs; max 25 Vrms, phased by 60° Controlled in tension (±2%) I out max (each branch):10 kA rms (PROTO-SPHERA), 1.67 kA rms (Multi-Pinch) Inputs: linear ramp duration (15-30 s), final V out (for 1 s flat-top) Protections: on every secondary branch a max I out electronic fuse installed (threshold set locally); when it open V out must vanish in t~10 msec Multi-Pinch version commissioned to EEI (Vicenza, Italy) Group ‘B’ – pinch shaping coils power supply V in : 20 kVAC; ±10%, 50 Hz three-phase V out : 350 VDC (twelve-phase feedback) I out : 1.9 kA (DC) Load: R - 80 m Ω ; L – l0 mH Current rise/descent time < 100 msec Pulse duration (flat-top): 1 sec, 10 min repetition time Accuracy of I out (including ripple): ±2% Definitive version commissioned to EEI (Vicenza, Italy)

8. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 8 Waveforms Screw Pinch power supply (Multi-Pinch) V in : 20 kVAC; ±10%, 50 Hz three-phase V out : 350 VDC (twelve-phase feedback) I out : 10 kA (DC) Load: gas arc discharge with voltage ~150 V when arc is formed Current rise/descent time: < 25 msec (with a load inductance of about 1 μ H) Pulse duration (flat-top): 1 sec, 10 min repetition time Accuracy of I out (including ripple): ±2% Definitive version commissioned to EEI (Vicenza, Italy)

9. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 9 Layout Multi-Pinch (FTU Assembly Room)

10. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 10 Waveforms for PROTO-SPHERA discharge (Screw Pinch power supply) FRIEM (Milano, Italy) proposal – 2001 To be added to the EEI Screw Pinch feeder: One capacitor bank (2 kV, 128 kJ; max Ipeak= 100 kA; pulse duration = max 4 ms; 5 capacitor, 20 mF) to allow for the fast rise of Ie (0.5-1 ms) Two more feeders, 25 kA-200 Vdc each one Multi-Pinch

11. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 11 Waveforms for PROTO-SPHERA discharge (Group ‘A’ – compression coils - power supply) Analysis performed by ENEA in 2001 Group 'A' coil current ramp-up (two initial slopes 1000 kA/s and 70 kA/s) performed by two capacitor banks (charging voltage 16 kV and 2 kV respectively) switched on by thyristors. Inductance of the PF coils (in series): L 'A' = 14.2 mH. A 6 pulse-2 quadrant thyristor bridge charges the lower voltage capacitor bank and controls the flat top and the terminal part of the current rise. The thyristor amplifier voltage and current rates are 2 kV-1200 A, respectively, with a required load current ripple lower than 10%. Plasma shape feedback control at the flat-top operated by the coils 'A' amplifier: approximately 1200 A-100 Vdc.

12. Seminario UT FUSIONE Aula FT23, Centro Ricerche Frascati 25 Ottobre 2011 12 Proposed PROTO-SPHERA layout (old tokamak FT Hall)