Results of PROTO-PINCH Testbench for the PROTO-SPHERA experiment F. Alladio, L.A. Grosso, A. Mancuso , S. Mantovani, P. Micozzi , G. Apruzzese, L. Bettinali, P. Buratti, R. De Angelis, G. Gatti, G. Monari, M. Pillon, A. Sibio, B. Tilia, O. Tudisco Associazione Euratom-ENEA sulla Fusione, CR Frascati, C.P. 65, 00044 Frascati (Rome), Italy
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Results of PROTO-PINCH Testbench for the PROTO-SPHERA experiment
Results of PROTO-PINCH Testbench for the PROTO-SPHERA experiment. F. Alladio, L.A. Grosso, A. Mancuso , S . Mantovani, P. Micozzi , G. Apruzzese, L. Bettinali, P. Buratti, R. De Angelis, G. Gatti, G. Monari, M. Pillon, A. Sibio, B. Tilia, O. Tudisco - PowerPoint PPT Presentation
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Results of PROTO-PINCH Testbench
for the PROTO-SPHERA experiment
F. Alladio, L.A. Grosso, A. Mancuso, S. Mantovani, P. Micozzi , G. Apruzzese, L. Bettinali, P. Buratti, R. De Angelis, G. Gatti,
G. Monari, M. Pillon, A. Sibio, B. Tilia, O. TudiscoAssociazione Euratom-ENEA sulla Fusione, CR Frascati, C.P. 65, 00044 Frascati (Rome), Italy
Some ULART Features
Reasons to push towards the Ultra Low Aspect Ratio Torus (ULART, A ≤ 1.3)
the critical central conductor cannot be shieldedit is bombarded by neutrons (cannot be a superconductor)it should be periodically replaced But the ULART does not leave enough space for an ohmic transformer and requires noninductive current drive
Higher MHD stability and high average total beta values: T=20<p>Vol/BT
2(T=40% with axis=70% on START)
High T
START : relatively high energy confinement times and density limits with H-mode in NBI X-point discharges
The pol=20<p>Vol/Bpol2
marks the distance from a force free-state ( jB =0). In an ST( Bpol ~ BT )
A high (40%) plasmain an ST is much nearer to a force-free configuration than a low (4%) plasma in a Tokamak
Aims of PROTO-SPHERA(Spherical Plasma for Helicity Relaxation)
Test an ULART, with central conductor substituted by a current carrying plasma
(arc discharge) Force-free Screw Pinch fed by two "polar caps"
electrodes placed along the expansions of the ULART internal divertor regions "Bumpy Z-Pinch" or "Flux-Core Spheromak“ [Jensen and Chu, J. Pl. Phys., '81], [Turner, Phys. Fl., '84], [Taylor, NF, '89]
Schema of the Spherical Torus + Screw Pinch
Helicity Injection Feed
Magnetic line of forces
Field line in Bad Curvature region Geodesic Curvature Neoclassical transportMicro-instability related to trapped particles
Conventional Tokamak
High
High
High
High
Spherical Torus
Low
Low
Low
Low
PROTO-SPHERA Plasma Formation
The idea is to form an ULART by producing a Screw Pinch that will be destabilized by increasing the longitudinal arc current Ie: such a configuration has been obtained for the first time on the TS-3 experiment (Tokyo University) Screw Pinch with longitudinal field
BZ toroidal field Bsafety factor qPinch = 2 Pinch BZ/LPinch B
The non-linear phase of the instability converts toroidal flux in poloidal flux
TS-3 PROTO-SPHERA
Comparison Ie = 40 kA Pinch current Ie = 60 kAIp = 50 kA ULART current Ip = 240 kAA = 1.6 Aspect Ratio A ≤ 1.2pulse = 80 s 120 Alfvén Pulse duration pulse ≥ 50 ms 1 R
Main Differences with TS-3 PROTO-SPHERA has larger plasma elongation: 2.3, to get qaxis 1 and q 2.5-3PROTO-SPHERA aims at sustaining the configuration more than R=0a2/ (one resistive time) when TS-3 achieved ≤120•A (120 Alfvén times; A=Raxis/vA)
PROTO-SPHERA aims at obtaining a dimensionless Lundquist number S=R/A 105, whereas TS-3 had S=R/A 4400.
Cathode The Most Unconventional Item
Main Sectors 6Working Temp : 2400 °CTotal Current : 63882 A
Emission for coil : 170 ATotal Coils : 378Coils for module : 3
Total Module : 126Module for Sector :21ProtoSphera Cathode
Anode
Indium
CuCooling
W-Cu
Element INDIUM Cu
Melting Point 156 ºC 1050 ºC
Vap. Pr 250 C <10-12 mmHg -
Vap. Pr 400 C 10-10 mmHg <10-12 mmHg
Vap. Pr (700 C) 10-4 mmHg 10-8 mmHg
Term. Cond 82 W/(mol K) 400 W/(mol K)
Thermal Bridge : Indium Modules : 32 Sectors : 4
PROTO-PINCH (Electrodes’ Testbench)
ProtoPinch experimental StudiesThe arc breakdown.
The effects of arc current Ie≤1 kA, with qPinch≥ 2.
Test Anode material & conceptual design
Test Cathode material & conceptual design (1 module out of 100)
Phase Shift
140 GHz OSCILATOR
Mw Detector
DATA
Spectroscopy
Proto-Pinch Experimental Set-Up
Pyrex vacuum vessel
Microwave Interferometer
Visible Spectroscopy Diode Array
Cathode
8 conductors for the Ie return
External coil.
Feedback Gas Feed
Valve
Turbo Mol Vacuum
Pump
RotativeVacuum Pump
Anode
Cathode Thermocouples
Proto-Pinch Discharges
PROTO-PINCH has produced Hydrogen and Helium arcs in the form of screw pinch discharges.
Image of PROTO-PINCH Hydrogen plasma with Ie=600 A, B=1 kG.
Pinch Length : 75 cm
Stabilizing Field : 1.5 kG
Safety Factor qPinch≥2
IPinchmax = 700 A
IDenCathmax = 6 A/cm2
Vpinch = 100 V
Vcathode = 20 V
Progress of PROTO-PINCH
November ‘98 December ‘98 March ‘99 September ‘99 Ie=10 A Ie=70 A Ie=300 A Ie=670 A
Main results of the PROTO-PINCH Solution for the 5 cm diameter electrodes : a directly heated (AC) W-Th(2%) cathode and a Cu-W hollow anode, with H2 (or He) puffed through it. The Hydrogen pinch breakdown occurs in the filling pressure range pH=1•10-3÷1•10-2 mbar, which is the same of a standard Tokamak discharge. The pinch breakdown voltage is Ve≤100 V, which means that the insulation problems in PROTO-SPHERA should
be quite easy to deal with. The typical duration of a plasma pulse at Ie=600 A is
2÷5 s, limited by heating of Pyrex, rubber seals, etc… The arc plasma is very clean: a few barely measurable impurity lines appear in Hydrogen and in Helium discharges at lowest filling pressures (1÷2•10-3 mbar). Anode and cathode have withstood 400 discharges with the current and the power densities required for PROTO-SPHERA
Module Power = 8.4 KwModule Current = 600 AModule Voltage = 14 VWire Number = 4Wire Diameter = 2 mmWire Length = 40.0 cmWire Surface = 25 cm2
Wire Temp = 2200-2400 °CWire Em = 6 Amp/cm2
Wire Weight = 22 grCathode Treats and Recipes
AC current for heating the cathode, to spread the ion plasma current over the filaments. Wire preconditioning consists of flash heating at 2200 ºC for 1’, then 1 hour at 1800 ºC. The time required for heating up the cathode before the plasma shot is about 15 s.
Most relevant results concerning the cathode heating Ie=600-670 A of plasma current needs AC heating Icath=550-590 A (rms.)
at Vcath=14.5 V (rms.) Ie/Icath 1. Pcathode 8.5 kW allows for a power injection into the screw pinch plasma Pe 50-70 kW.No Damages after 400 discharges.
Extrapolation to the PROTO-SPHERA cathode AC current to heat the cathode : Icathode= 60 kA (rms.) at Vcathode< 20 V (rms.);The PROTO-SPHERA cathode will be composed of 126 (521 A each), modules similar to
the PROTO-PINCH cathode connected in parallel in six groups (six-phased power supply able to deliver 10 kA per phase).
The overall cathode heating power will be Pcathode 850 kW. The overall power injection into the PROTO-SPHERA plasma sheaths will be Pe 50 MW (Ie 60 kA at Ve 80÷90 V) The ohmic input P
Pinch= 4.7 MWHelicity injection power required for sustaining the torus, PHI=1.3 MW A pinch power supply able to deliver 60 kA at 300 V will be adequate for all requirements.
Anode
Anchoring Ip=200 Amp Disanchoring 300 Amp
B=0.8 kGB=0.8 kG Cathode D.CCathode D.C
The gas feedback system keeps pH=constant. NoDamages after 1000 discharges.Material W 90% Cu10%.
Schema : Pumped Divertor.
With cathode DC heated some anode arc anchoring.No anchoring with AC cathode heating.In PROTO-SPHERA saddle coils to contrast arc anchoring should be inserted near electrodes .
Visible SpectroscopyMeasurements of intensity of the spectral lines of gas (H2 and He ) and impurities
Visible spectroscopy of the Hydrogen plasma shows a not yet identified (4303 Å) line near the H at a count level of about 10-2 of the H
The plasma visible light is collected by a telescope and focused onto 1 mm diameter fiber optics
Grating 150 g/mm : linear dispersion 13 nm/mm
Detector : intensified diode array with 1024 pixels
Spectral Resolution : 3 A°/pixel
Time resolution : 250 ms
Enlarged
H2 ZoomH2
Å
Very low level of impurities No HeII (4686 Å) : 1 eV <Te 3.5 eV
Å
Å
He
ÅÅ ÅÅÅ
He zoom
Density Measurements
Microwave Interferometer
parameters
Generator Frequency : 140 GHz
Line average density per fringe : 3.4 1018 m-3
Ie
Microwave Detector
Phase Shift Data Aquisition
140 GHzOscillator
Ip=120 A B = 1.25 kG : N = 1.0 1019m-3
B = 0.83 kG : N = 7.0 1018 m-3
B = 0.42 kG : N = 3.4 1018 m-3
Microwave interferometer responseHe Discharges - D = 4.0 10-3 mbar