First results of plasma centerpost in PROTO-SPHERA

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Presentation transcript:

First results of plasma centerpost in PROTO-SPHERA F. Alladio1, L. Boncagni, F. Causa, E. Giovannozzi, A. Grosso, A. Lampasi, G. Maffia, A. Mancusoretired, P. Micozzi, V. Piergotti, G. Rocchi, A. Sibio, B. Tilia, V. Zanzaretired Centro Ricerche ENEA, Frascati C.P. 65, Rome, Italy 1e-mail: franco.alladio@enea.it

Summary History of construction of PROTO-SPHERA Cylindrical anode experiments (2014) Annular anode experiments (2015) Absence of anode attachment due to electrostatic potential Tailoring of electrostatic potential by connecting PF coils casings, built as floating Tailoring of magnetic field through additional PF coils external to the vessel Conclusions

anode and cathode needed for setting up the configuration PROTO-SPHERA aim A spherical tokamak with metal centerpost replaced by a plasma centerpost: anode and cathode needed for setting up the configuration Present stage Next stage 2.5 m vessel Plasma current 10 kA in centerpost, no torus Plasma current 60 kA in centerpost, 240 kA in spherical torus Experiments began in 2014 on plasma centerpost only; aim is 10 kA, 1 s Years 1995 - 2002: Idea & design Years 1997 - 2000: Test of electrodes 1995 - 2nd ST Workshop Princeton in “Champagne bottle experiment” 2002 - International Workshop Frascati (late Derek Robinson, Prague EPS98)

Cylindrical vacuum vessel was START vacuum vessel, donated by Culham in 2004 PROTO-SPHERA was built (2006-2009) by ASG Superconductors of Genoa, cost 1 M€. Years 2004 - 2010: …from ideas …to detailed design …to hard metal

Aluminium cylindrical Only the PF coils necessary for setting up the plasma centerpost have been built annular anode 8 PF coils in series inside the machine Stainless steel up\down new extensions PF4up PF3up PF2up anode plasma current must run through both PF2 throttles Aluminium cylindrical START vessel PF2low PF3low cathode PF4low Stainless steel up\down new lids annular cathode vacuum vessel is GND potential PF coils casings built as floating, can be connected to potentials: anode +, cathode -, vessel 0

Electrical power supplies built by EEI of Vicenza, cost 0.7 M€ Anode PF coils Cathode To fire the plasma centerpost 3 Power supplies are required • Cathode heating, rotating 6-phase: 6 x(1.7 kA,25 V)rms • PF coils: 2kA, 350 V • Plasma centerpost: 10 kA, 350 V Electrical power supplies built by EEI of Vicenza, cost 0.7 M€ Years 2011 - 2014:

Being afraid of anode attachment, often observed in arc discharges 2014 cylindrical anode 2015 annular anode Being afraid of anode attachment, often observed in arc discharges (arc welding, plasma torches) the first rounds of experiments had a simple cylindrical anode break-down was easily achieved, apart from this the cylidrical anode was just a source of troubles cylindrical cathode results Hydrogen plasma break-down 170 V +10 ms, 500 A +20 ms, 1.5 kA anode camera May 2015 annular anode lowered on top of machine centerpost camera

up: hollow gas-puffed anode down: 3000º K heated cathode present stage cathode (18 x 3 emitters): aim 10 kA final stage cathode, 6 x present (108 x 3 emitters): aim 60 kA PROTO-SPHERA CR-ENEA Frascati “Caduceus”-like emitting spirals have survived ~ 1000 cycles

hollow annular anode performs • plasma goes through both PF2 throttles • plasma enters anode gas-puffing holes • no sign (I < 3.5 kA) of anode attachment • filling pressure 10-3 - 10-2 mbar Argon plasma: break-down 80 V annular anode plasma is never filamented anode camera cathode camera whereas annular catode plasma (sparse emitters) and even plasma centerpost are filamented

but the electric potential distribution is dominated by the plasma Electrostatic plasma effects vacuum vessel at GND PF coil casings can either be at floating potential or can be connected to: anode +, cathode -, vessel 0 through ~ 1 Ω resistors but the electric potential distribution is dominated by the plasma PF2\3up to anode (1 Ω)\PF4up cathode (10 Ω) VPF4up ~ 0 V Vanode ~ +45 V VPF2-3up ~ +25 V Vvessel = 0 V (ground) VPF2-3low ~ 0 V Vcathode ~ -35 V VPF4low ~ -15 V distribution of electric potential is not up-down symmetric: near both anode and cathode the uppermost PF coil casings is at GND PF2\3low to GND (1 Ω)\PF4low cathode (0.5 Ω)

plasma-induced electric potential near the annular anode : E-field is perpendicular to B-field …E x B azimuthal plasma rotation …starting from PF2up throttle vExB=(E/B) ~ 104 m/s ~ vdrift ~ vthi plasma-induced electric potential near the annular cathode: E-field is parallel to B-field …no E x B plasma rotation self-organization at work inside annular electrodes plasma…

equatorial X-point inside the vessel May 2015 annular anode experiment half of the plasma current: (1kA\2kA) was going around the required path due to equatorial X-point inside the vessel Argon plasma Hydrogen plasma Shot 78 discharges along field lines near X-point

The equatorial X –point has been removed from inside the vessel 4 external PF coils have been added (home-made from spare connection cables) …and fed in series with the internal PF coils (PF coils power supply has sufficient margin) 4 additional external PF coils plasma fired after 0.75 s of PF current to allow for skin currente diffusion in Al vessel and SS lids

October 2015 experiments with external PF: Argon discharge reaching 3 kA for 0.15 s the whole plasma current runs through both PF2 throttles Shot 205 plasma fired after 0.75 s of PF current (skin current time to remove X-point from inside vessel)

cathode is switched off < 0.1 s before plasma current October 2015, external PF Argon discharge sustained at 2 kA for ~ 0.3 s Plasma fired after 0.75 s of PF current Shot 197 however equatorial hot spots appear and a few hundreds Ampere are lost from main plasma

Plasma current outer paths for I > 3 kA Argon plasma aiming to 4 kA reaches 3.5 kA ~ 1.5 kA lost from main plasma in discharge from cathode down to lid\vessel Shot 208 plasma current lost on both PF2 throttles 10 V spikes appearing on floating voltages on lower PF coils are associated with plasma current outer path

to avoid voltage spikes on low PF coils Removal of equatorial hot spots through straighter grounding of low PF coils Argon discharge at 2 kA to avoid voltage spikes on low PF coils PF2\3low + PF4low connected to GND through much smaller (16 mΩ) resistors Shot 222 Equatorial hot spot eliminated, voltage spikes have been reduced to 1 V amplitude on PF2/3low only small sparks from PF2/3low remain in correspondence to 1V PF2/3low spikes

current flows only in external PF near equator Magnetic tayloring in order to have a better filling of electrodes when plasma current takes outer path cathode seems less emitting plasma current 3.5 kA plasma current 2kA, 1.5 kA lost Shot 208 current flows only in external PF near equator cathode seems better filled at all times Shot 227

Straighter grounding of lower PF coils and better filling of cathode flowing current only in the two external PF coils nearer to equator voltage spikes have been eliminated on PF2/3low, after 1 V amplitude jump even small sparks vanish voltage jump does not lose plasma current very quiet plasma at 2 kA after voltage jump Argon discharge at 2 kA Shot 227

Conclusions 20 years after 2nd ST Workshop – Princeton (1995) the “aggressive proposal” PROTO-SPHERA is producing its first centerpost plasmas …better late than never! …迟到总比不到好! Hollow anode surprise, no anode arc attachment (electrostatic potential, E x B plasma rotation) (1/3) of the due current (10 kA) achieved, 3 kA sustained in Argon plasma To achieve full current of 10 kA requires patient learning of how to tailor boundary conditions: • electrostatic potential by connecting PF coils casings, built floating with respect to the vessel • magnetic field by additional coils external to the vessel After Argon centerpost is tamed (80 V breakdown), start fight with Hydrogen centerpost (180 V) If full current of plasma centerpost(10 kA) is achieved and sustained with 54 sparcely spaced emitting filaments, then full current (60 kA) will be even better achieved with 324 filaments First stage of PROTO-SPHERA is only an appetizer for the production of the Spherical Torus …放龙入海! …give a chance to the dragon to show his endowments into the sea!