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
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011
2
Spherical Tokamaks allow to obtain:
• High plasma current Ip (and high <n>) with low BT
• Plasma b 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 (ITF → Ie)
Potentially two problems solved:
• Simply connected configuration (no conductors inside)
• Ip driven by Ie (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)
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
2
25 Ottobre 2011
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
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, k=b/a~2.3)
to get a Tokamak-like safety factor (q0≥1, qedge~3)
SP electrode current
Ie=60 kA
ST toroidal current
Ip=120÷240 kA
ST diameter
Rsph=0.7 m
⇓
Stability should be improved and helicity drive may be less
disruptive than in conventional Flux-Core-Spheromak
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011
4
PROTO-SPHERA formation follows TS-3 scheme (SP kink instability)
Tunnelling (ST formation)
T0
Ie=8.5 kA
Ie 8.5→60 kA
ST compression (Ip/Ie↑, A ↓ )
T3
Ip=30 kA
A=1.8
• Ip/Ie 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/B2 parameterized)
Seminario UT FUSIONE
T4
Ip=60 kA
A=1.5
T5
Ip=120 kA
A=1.3
T6
Ip=180 kA
A=1.25
TF
Ip=240 kA
A=1.2
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 Ip/Ie &  limits
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011
5
Ideal MHD stability results for PROTO-SPHERA
•PROTO-SPHERA
stable at full  21÷26% for Ip/Ie=0.5 & 1, down to 14÷16% for Ip/Ie=4
(depending upon profiles inside the ST)
Comparison with the conventional Spherical Tokamak with central rod:
T0=28÷29% for Ip/Ie=0.5 to T0=72÷84% for Ip/Ie=4
•Spherical
Torus dominates instabilitiy up to Ip/Ie≈3; beyond this level of Ip/Ie,
dominant instability is the SP kink (that gives rise to ST tilt motion)
• Spherical
Torus elongation  plays a key role in increasing Ip/Ie
• 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
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011
PROTO-SPHERA
poloidal field Coils
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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.
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
(reduced on Multi-Pinch)
25 Ottobre 2011
Waveforms
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Cathode power supply
Vout: 6 independent outputs; max 25 Vrms, phased by 60°
Controlled in tension (±2%)
Iout max (each branch): 10 kA rms (PROTO-SPHERA),
1.67 kA rms (Multi-Pinch)
Inputs: linear ramp duration (15-30 s),
final Vout (for 1 s flat-top)
Protections: on every secondary branch a max Iout
electronic fuse installed (threshold set locally);
when it open Vout must vanish in t~10 msec
Multi-Pinch version commissioned to EEI (Vicenza, Italy)
Group ‘B’ – pinch shaping coils
power supply
Vin: 20 kVAC; ±10%, 50 Hz three-phase
Vout: 350 VDC (twelve-phase feedback)
Iout: 1.9 kA (DC)
Load: R - 80 mΩ; L – l0 mH
Current rise/descent time < 100 msec
Pulse duration (flat-top): 1 sec, 10 min repetition time
Accuracy of Iout (including ripple): ±2%
Definitive version commissioned to EEI (Vicenza, Italy)
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011
Waveforms
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Screw Pinch power supply (Multi-Pinch)
Vin: 20 kVAC; ±10%, 50 Hz three-phase
Vout: 350 VDC (twelve-phase feedback)
Iout: 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 μH)
Pulse duration (flat-top): 1 sec, 10 min repetition time
Accuracy of Iout (including ripple): ±2%
Definitive version commissioned to EEI (Vicenza, Italy)
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011
Layout Multi-Pinch (FTU Assembly Room)
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
9
25 Ottobre 2011
Waveforms for PROTO-SPHERA discharge
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(Screw Pinch power supply)
Multi-Pinch
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
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011
Waveforms for PROTO-SPHERA discharge
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(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.
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011
Proposed PROTO-SPHERA layout
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(old tokamak FT Hall)
Seminario UT FUSIONE
Aula FT23, Centro Ricerche Frascati
25 Ottobre 2011