PROTO-SPHERA%20Experiment:

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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
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Spherical Tokamaks allow to obtain High
plasma current Ip (and high ltngt) 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)
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But Flux Core Spheromaks are injected by
plasma guns formed by 10 kV voltage on
electrodes high pressure prefilled with ST
safety factor q1
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
(AR/a1.2, kb/a2.3) to get a Tokamak-like
safety factor (q01, qedge3) SP electrode
current Ie60 kA ST toroidal current Ip120240
kA ST diameter Rsph0.7 m ? Stability should
be improved and helicity drive may be less
disruptive than in conventional
Flux-Core-Spheromak
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PROTO-SPHERA formation follows TS-3 scheme (SP
kink instability)
Tunnelling (ST formation)
ST compression (Ip/Ie?, A ? )
T0 Ie8.5 kA
T3 Ip30 kA A1.8
T4 Ip60 kA A1.5
T5 Ip120 kA A1.3
T6 Ip180 kA A1.25
TF Ip240 kA A1.2
Ie 8.5?60 kA
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, µJB/B2 parameterized)
Some level of low n resistive instability
needed (reconnections to inject helicity from SP
to ST) but SPST must be ideally stable at any
time slice ? Ideal MHD analisys to assess Ip/Ie
ß limits
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Ideal MHD stability results for
PROTO-SPHERA PROTO-SPHERA stable at full ß
2126 for Ip/Ie0.5 1, down to 1416 for
Ip/Ie4 (depending upon profiles inside the ST)
Comparison with the conventional Spherical
Tokamak with central rod ßT02829 for
Ip/Ie0.5 to ßT07284 for Ip/Ie4 Spherical
Torus dominates instabilitiy up to Ip/Ie3
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
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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
• Group A P.S. (no Multi-Pinch)
• Group B P.S. (already on Multi-Pinch)
• Cathode P.S. (reduced on Multi-Pinch)
• Screw Pinch P.S. (reduced on Multi-Pinch)

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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 t10 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 mO L
l0 mH Current rise/descent time lt 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)
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Waveforms
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 lt
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)
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Layout Multi-Pinch (FTU Assembly Room)
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Waveforms for PROTO-SPHERA discharge (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
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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. Inductanc
e 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.
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Proposed PROTO-SPHERA layout (old tokamak FT Hall)