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

Cathode Materials:

Plates: Molybdenum Columns:Tantalum Insulator : Alumina. Coils : W-Th 2%

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

Ipinch 0-220 AB = 1.25 kG : N = 1.4 1019m-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 VesselWinding Tool

Split Furnace : 400 ºC

Conical

New Coil Features

Max Exposed Surface

Null Field (Double Winding)

Optimal Weight Balance

No Front Plasma Connector

I1

I2