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, 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 shielded  it 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 =2  0 Vol /B T 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 =2  0 Vol /B pol 2 marks the distance from a force free-state ( j  B =0).  In an ST( B pol ~ B T )  A high   (40%) plasma  in 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 transport Micro-instability related to trapped particles Conventional Tokamak High Spherical Torus 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 I e : such a configuration has been obtained for the first time on the TS-3 experiment (Tokyo University) Screw Pinch with longitudinal field B Z toroidal field B  safety factor q Pinch = 2   Pinch B Z /L Pinch B  The non-linear phase of the instability converts toroidal flux in poloidal flux

TS-3 PROTO-SPHERA Comparison I e = 40 kA Pinch current I e = 60 kA I p = 50 kA ULART current I p = 240 kA A = 1.6 Aspect Ratio A ≤ 1.2  pulse = 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 q axis  1 and q     PROTO-SPHERA aims at sustaining the configuration more than  R =  0 a 2 /  (one resistive time) when TS-3 achieved ≤120  A (120 Alfvén times;  A =R axis /v A)  PROTO-SPHERA aims at obtaining a dimensionless Lundquist number S=  R /  A  10 5, whereas TS-3 had S=  R /  A  4400.

Cathode The Most Unconventional Item ProtoSphera CathodeModule for Sector :21Total Module : 126 Coils for module : 3Total Coils : 378Emission for coil : 170 A Total Current : AWorking Temp : 2400 °CMain Sectors 6

Anode Indium Cu Cooling W-Cu ElementINDIUMCu Melting Point156 ºC1050 ºC Vap. Pr 250 C< mmHg- Vap. Pr 400 C mmHg< mmHg Vap. Pr (700 C)10 -4 mmHg10 -8 mmHg Term. Cond82 W/(mol K)400 W/(mol K)  Thermal Bridge : Indium  Modules : 32  Sectors : 4

PROTO-PINCH (Electrodes’ Testbench) ProtoPinch experimental Studies  The arc breakdown.  The effects of arc current I e ≤1 kA, with q Pinch ≥ 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 I e return External coil. Feedback Gas Feed Valve Turbo Mol Vacuum Pump RotativeVa cuum 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 I e =600 A, B=1 kG. Pinch Length : 75 cm Stabilizing Field : 1.5 kG Safety Factor q Pinch ≥2 I Pinchmax = 700 A I DenCathmax = 6 A/cm 2 V pinch = 100 V V cathode = 20 V

Progress of PROTO-PINCH November ‘98 December ‘98 March ‘99 September ‘99 I e =10 A I e =70 A I e =300 A I e =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 H 2 (or He) puffed through it.  The Hydrogen pinch breakdown occurs in the filling pressure range p H = ÷ mbar, which is the same of a standard Tokamak discharge.  The pinch breakdown voltage is V e ≤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 I e =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÷ mbar).  Anode and cathode have withstood 400 discharges with the current and the power densities required for PROTO-SPHERA

Cathode Materials: Plates: Molybdenum Columns:Tantalum Insulator : Alumina. Coils : W-Th 2% Module Power = 8.4 Kw Module Current = 600 A Module Voltage = 14 V Wire Number = 4 Wire Diameter = 2 mm Wire Length = 40.0 cm Wire Surface = 25 cm 2 Wire Temp = °C Wire Em = 6 Amp/cm 2 Wire Weight = 22 gr Cathode 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  I e = A of plasma current needs AC heating I cath = A (rms.) at V cath =14.5 V (rms.) I e /I cath  1.  P cathode  8.5 kW allows for a power injection into the screw pinch plasma P e  kW.  No Damages after 400 discharges. Extrapolation to the PROTO-SPHERA cathode  AC current to heat the cathode : I cathode = 60 kA (rms.) at V cathode < 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 P cathode  850 kW.  The overall power injection into the PROTO-SPHERA plasma sheaths will be P e  50 MW (I e  60 kA at V e  80÷90 V)  The ohmic input P  Pinch = 4.7 MW  Helicity injection power required for sustaining the torus, P HI =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 AmpDisanchoring 300 Amp B=0.8 kG Cathode D.C  The gas feedback system keeps p H =constant.  No  Damages 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 Spectroscopy Measurements of intensity of the spectral lines of gas (H 2 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 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 H 2 ZoomH2H2 Å Very low level of impurities No HeII (4686 Å) : 1 eV <Te  3.5 eV Å Å He Å Å Å Å Å He zoom

Density Measurements Microwave Interferometer parameters  Generator Frequency : 140 GHz  Line average density per fringe : m -3 IeIe Microwave Detector Phase Shift Data Aquisition 140 GHz Oscillator Ip=120 A B = 1.25 kG : N = m -3 B = 0.83 kG : N = m -3 B = 0.42 kG : N = m -3 Microwave interferometer response He Discharges - D = mbar I pinch A B = 1.25 kG : N = m -3

New Coils & Creep Test Vessel Short Term Targets  Wind W-Th Coils  Test Conical Connections  Horizontal Creep Test at 2400 ºC  Test New Coil in Proto-Pinch Testbench Coil Connection – Conical Hole Null Field Coils Vacuum Creep Test Vessel Winding Tool Split Furnace : 400 ºC Conical New Coil Features  Max Exposed Surface  Null Field (Double Winding)  Optimal Weight Balance  No Front Plasma Connector I1I1 I2I2