Conclusions of phase 1 & transition to phase 2 of MultiPinch Franco Alladio Luca Boncagni, Andrea Grosso, Alessandro Lampasi, Giuseppe Maffia, Alessandro Mancuso retired, Paolo Micozzi, Benedetto Tilia, Vincenzo Zanza, FTU diagnostic pool, Mechanical Engineering Group WiP – 24 Aprile 2015

PROTO-SPHERA immediate aim A spherical tokamak where the metal centerpost is replaced by a plasma centerpost: will the plasma centerpost be able to produce and drive the plasma ring? anode and cathode needed for setting up the configuration Introduction

A simply connected configuration particularly suitable for magnetic fusion space propulsion: charged fusion products would emerge in jets through triple X-point: high efficiency for thrust Sidney ICPP 2001 orbits of promptly lost charged fusion products Possible far-future aim… Introduction

PROTO-SPHERA Part 1b: annular cathode, annular anode** PROTO-SPHERA Part 1a: annular cathode, cylindrical anode Phase 1 Simple cylindrical element as anode: aim of first experiments was plasma breakdown* **risk of annular anode is plasma arc anchoring Introduction * anode to cathode power supply is limited to 350 V

Cathode heated ~ 350 cycles: no problems yet on cathode spirals Plasma shot 209 Plasma current ~ 250 A thereafter the 2 kA current takes an anomalous path …later shown Plasma H α light 10 ms later metal discharge white light Cylindrical cathode results Break-down easily obtained at Hydrogen pressure p H2 ~ \10 -2 mbar

First problem caused by cylindrical anode: improper back discharge from the cylindrical anode due to field configuration in upper part of the machine: the plasma searches the not-yet-in place annular anode Plasma shot 170, anode camera Cylindrical cathode results

magnetic field configuration was modified in the upper part of the machine… …but discharges could appear from cylindrical anode to busbar entrances on top of machine Plasma shot 216, anode camera Second problem caused by cylindrical anode: Cylindrical cathode results Busbar entrance

V anode ~ +100 V V cathode ~ -100 V V PF4low ~ -50 V V PF2-3low ~ 0 V V PF4up ~ +70 V V PF2-3up ~ +30 V V vessel = 0 V (ground) Plasma shot 170, voltages of floating PF coils casings (plasma double layers) Conducting vessel at ground, PF coil casings insulated from vessel, anode and cathode …a gradual increase of the floating voltages from cathode to anode along the height Cylindrical cathode results

V anode ~ +100 V V cathode ~ -100 V V PF4low ~ -50 V V PF2-3low ~ +50V V PF4up ~ +30 V V PF2-3up ~ +70 V V vessel =0 V (ground) Plasma shot 228, use of lower PF2-3 as a “triode grid” Lower PF2-3 coils casing connected with 10 Ω to the anode (Vincenzo Zanza proposal) …sharper increase of the floating voltages from cathode to anode near the cathode Cylindrical cathode results

Plasma shot 228 Plasma breakdown 10 ms later, 500 A 20 ms later, 1500 A Anode camera Plasma camera Cathode camera Cylindrical cathode results

Plasma shot 228 Cylindrical cathode results note long lasting glow-discharge <10 A, capacitively driven after power supply is switched-off

Third problem caused by cylindrical anode: Plasma starts on its proper path, through both PF2 throttles then a metal discharge appears between the cylindrical anode and its containing PF2 upper coil casing The cylindrical gap between the throttle of the PF2 upper coil casing and the cylindrical anode is very small ( ~ 5 mm): at any machine closing the upper lid had to be finely aligned in order to avoid a short circuit between the two PF2 Anode Cylindrical cathode results

Plasma shot 226, the two current paths can coexist for ~ 0.1 s Cylindrical cathode results

Anomalous current path from 1) cylindrical anode to PF2 2) PF2 to the 6 vessel supports 3) along vessel 4) vessel to back of cathode plasma discharge from front of cathode is replaced by plasma discharge from back of cathode

Problems with electrical power supply: No major problem up to now, controls of normal and anomalous situations (protections) work smoothly Only a plastic lever broke with no reason (poor plastic quality) in an ABB disconnecting switch, a few €cents car fuse and the clever hand of Giuseppe Maffia cured it… Minor problems after a protection intervention …the next shot (only) starts at wrong time …but it is not conceivable to carry on working without a maintenance contract with EEI producer, any problem would result in 6-12 month delay to experiments… …Cost of contract ~ 15 k€ A reasonable management of the machine would require this contract in operation 6 month from now (autumn 2015) Cylindrical cathode results

Damages to anode busbars damages to anode busbars have been cured top lid of the machine routinely removed, supports for top lid built by Mech.Eng. Group Cylindrical cathode results

…but damages to cathode busbars …however damages to cathode busbars cannot be cured, the tools for opening the lower lid of the machine have been designed but not yet built A reasonable management of the machine would require these tools to be ready 6 month from now (autumn 2015) Phase-2 restart will have to operate with grounded cathode, as the resistance between cathode and vessel has dropped at the end of phase-1 at a few tens of Ω… Cathode filaments look brand new… Cylindrical cathode results

Conclusions of cylindrical anode operation: 1)Plasma breakdown has been achieved at ~ V (maximum available voltage 350 V) 2)Successive problems (plasma current & duration) arise from cylindrical provisional cathode Decisions: 1)Install final annular anode 2)Allow connections of vessel to PF coils casings down to “0 Ω resistance” Fight with real problems relevant to PROTO-SPHERA and not with provisional problems… Annular anode as pre-mounted in ASG (Genua) Aim of MultiPinch with annular anode: Plasma discharges up to 8.5 kA lasting a few 100s ms Annular anode restart

Tools for opening the lower lid of the machine designed by Alessandro Mancuso and Paolo Rossi Annular anode restart

V anode ~ +200 V V cathode ~ 0 V (ground) V PF4low +50 V V PF2-3low ~ +70V V PF4up ~ +100 V V PF2-3up ~ +100 V V vessel =0 V (ground) Plasma shot 209 (with cathode common set to ground) Luckily enough the operation with the cathode set to ground has been as successful as the operation with the cathode floating, so the phase-2 restart will have grounded cathode 1) Grounded cathode operation Further possible problems and their cure

Plasma shot 228 X-point 2) Detrimental X-point effect? Further possible problems and their cure

the external PF coils would also eliminate the detrimental X-point effect 4 additional external PF coils (Paolo Micozzi proposal) If in presence of the annular anode, the lines of force have to be removed from the cylindrical part of the conducting vessel & to be concentrated on the lids, which will have to be insulated: it would be simple to add 4 external PF coils (home-made from spare connection cables) …and to feed them in series with the internal PF coils (PF coils power supply is more than sufficient)

3) Parasitic plasma path, outside PF2 throttles Plasma shot 148, An earlier discharge showing parasitic plasma path on the outside, along vacuum vessel Proposed cure for parasitic current path: build & install divertor supports … only after assessment of potentials has been obtained! Inox divertor supports Further possible problems and their cure Plasma H α light

Plasma shot 221, occurrences of anchoring: …but rotating cathode field rotates hotspots Resolution of present camera (10 ms per frame) not sufficient in future we shall have to borrow FTU fast camera when operating 4) Arc anchoring Further possible problems and their cure

Conclusions: Cylindrical anode results: Break-down obtained at Hydrogen pressure p H2 ~ \10 -2 mbar, V required Plasma starts on proper path –through PF2 throttles Reasonable distribution of double layer potentials on floating PF coil cases Problems specific of cylindrical anode limit plasma current to 1.5 kA & duration to 20 ms Improper back discharges from cylindrical anode if magnetic field was unmodified Anode busbar discharges if magnetic field was modified Proper plasma discharge terminated by discharge from anode to PF2 (5 mm gap!) Preliminary control of double layer potentials on floating PF coil casing quite helpful No major problem on power supply (work properly in normal and protection cases) Anode busbar damages can be cured (top lid of machine removed, supports in place) Cathode busbar damages cannot be cured, as tools for bottom lid removal not yet built Fight with real problems relevant to PROTO-SPHERA and not with provisional problems… Annular anode is being installed for restart of experiments, aim 8.5 kA, duration 100’s ms: Allow control of double layer potentials on floating PF coil casings down to “0 Ω resistance” Maintenance contract with power supply producer (EEI) Build tools for bottom lid removal Grounded cathode operation is mandatory, til bottom lid removal tools allow busbar cure Detrimental X-point effect\Shaping of magnetic field could require 4 external PF coils Parasitic plasma path (outside PF2 throttles) could require Inox divertor supports