September 26-29, 2005 Knoxville. Tennessee USA Progress and Plan of the PROTO-SPHERA Program Outline of the Talk 1)Introduction to PROTO-SPHERA 2)The PROTO-PINCH

September 26-29, 2005 Knoxville. Tennessee USA

Assoc iazione Eu ratom -ENEA sulla Fu si one

Progress and Plan of the PROTO-SPHERA Program

Outline of the Talk1) Introduction to PROTO-SPHERA

2) The PROTO-PINCH Experiment: results

3) Engineering Design and Analysis of PROTO-SPHERA

4) Multi-Pinch Experiment

5) Conclusions

Stamos Papastergiou, Franco Alladio, Alessandro Mancuso, Paolo Micozzi, CR-ENEA, CP 65, 00044 Frascati (Roma), Italy

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Aims at producing a Spherical Torus (ST) with a Screw Pinch (SP) replacing the centerpost

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Introduction to PROTO-SPHERA (Spherical Plasma for Helicity Relaxation Assessment)

Ip: toroidal current

Bt: toroidal field

Bp: poloidal field

Je: pinch current


Assoc iazione Eu ratom -ENEA sulla Fu si one 3

Engineering Design of

PROTO-SPHERA



Physics Design

Ie, Ip = 40, 50 kA pinch, ST currents Ie, Ip = 60, 240 kA

A=R/a = 1.6 aspect ratio A ≤ 1.3pulse = 80 s~1/100R pulse duration pulse ≥ 70 ms ~ 1 R

•PROTO-SPHERA aims at sustaining the plasma for more than R=0a

2/ (resistive time) to verify the stability of the configuration

4

Tokyo University - Japan



Main Breakdown featuresScrew pinch (SP)

formed by a hot cathode breakdown

Filling pressure pH~10-3÷10-2

mbar

Cathode filaments heated to 2600 °C

Ve~100 V applied on the

anode

Initial electrode arc current limited to Ie~8.5 kA

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•Test of PROTO-SPHERA Cathode & Anode

•Arc breakdown in configuration similar to PROTO-SPHERA

PROTO-PINCH Experiment - Results

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Anode

NoDamages after 1000 discharges

Material W 95% Cu5%.

Scheme : Puffed Hollow Anode

PWAnode = 2/3 (670 120) kW » 56 kW (module)

Asurface= 1.8 10-3 m2

Dpw = PW/ Asurface» 30 MW/m2

anode arc anchoring with Cathode DC heated (a)

No Anode anchoring with AC cathode heating (b)

a b

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Cathode Emissivity Coil Material : Pure W Wire Diameter = 2 mm Total Coils Number = 378 Single Coil emission = 167 A Tot126 module 3 X 167 A 63 kA Current Density 1 MA/m2

Conical Wire Terminal & Clamp

Cathode – Extrapolation to PROTO-SPHERA

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W

Mo



Gas Puff Ie

TotSurf = 0.2 m2W Cu5%

OFHC Hardened CU

Gas Puff

Anode Power Handling Capability:PROTO-SPHERATested = 9 MWPROTO-SPHERAMax = 6 MW

Anode Extrapolation to PROTO-SPHERA

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Engineering Design and Analysis of

PROTO-SPHERA

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Design Philosophy

- Simplicity, symmetry, conservatism.

- Viewing capability and good access to electrodes.

- All connections to PF coils from top and bottom flanges,

external to Machine.

- Bellows in coil feedthroughts for adjustment.

- Operation and disruption (but no fault) conditions considered.

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Design AimsOperationability, Reliability, Repairability of Machine.

Reliability of electrodes and avoidance of arc anchoring.

Minimise Technological Risks: provide appropriate elecrical

insulation, avoid local coil overheating ( >100°C ) due to hot

anode and cathode ( 12MJ, 2750°C ), minimise electromagnetic stresses.

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Operating conditions

Temperature 20°C

Vacuum < 1•10-7 mbar (predicted outgassing rate: ~3•10-5 mbar•l/s)

Baking

80-90°C

Adequate for water removal, coil protection; compatible to Viton O-rings

Possibility of N2 1 mbar contact gas

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Spherical Torus (ST) diameter 0.7 mLongitudinal Screw Pinch current 60 kAToroidal ST current 120÷240 kAPlasma pulse duration 1 sMinimum time between two pulses 5 min.Maximum heat loads on first wall componenets in divertor region 2 MW/m2 , for 1 msMaximum current density on the plasma-electrode interface 1 MA/m2

Machine Parameters

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Protection Componenets•Thermal and Heat Transfer Behaviour

Philosophy: collect the heat ( via radiation ) from the electrodes to black resilient componenet and then conduct or radiate it away in a controlled way. Hot spots are thus avoided.

Danger: the hot cathode (less the anode) radiates at 2750°C with ~3.5MW/m2 and even a 10mm thick coil casing cannot sustain this power density for more than 0.5s without exceeding 100°C

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Assoc iazione Eu ratom -ENEA sulla Fu si one Thermal and Heat transfer Behaviour

• Solution:

1. Use of Cu componenets to collect or reflect the heat away from coils. Conservative (but symmetrical) design for the anode. Thermal mass of these components results in operational temperature < 100°C. Water cooled! Optimum design; i.e. one circuit/component, D~10-15mm, v~2m/s, duty cycle 5min.

2. Use of thermocouples in no water cooled componenets to avoid >100°C with frequent pulses; every 5min.

3. Use of 10mm divertor AISI 304LN plates. 2 MW/m2 for 1 s gives T~120°C, th~320MPa

4. Use of optical diagnostics to view the electrodes which are relatively easily accessible.

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Electromagnetic Forces and Stresses

B=50T/s B=500G t≤1 ms

Subdivision of Cu and 304LN plates to achieve ≤100 MPa of eddy current stresses.

Unaccounted skin effects (t ~1ms) significant. Conservative prediction of eddy current effects.

Electromagnetic forces in the coils (~x10 weight) require to be sustained.

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Assoc iazione Eu ratom -ENEA sulla Fu si one Predicted and Permitted StressesASME III-NB 3221ASME III-NB 3221

Stainles SteelStainles Steel

Allowable Sm for 304LN 100°C : 200MPa [304L Sm~150MPa]

Predicted e-m stresses : 320MPa

Thermal (divertor) local stresses : 320 MPa (2 MW/m2 ,1s)

Criterion 1 : 100 <200x1.5

Criterion 2 : 100320 <200x3

CopperCopper

Minimum required allowable at 100°C> 70 MPa (100MPa<70x1.5)

Thus y >115 Mpa and u > 230 MPa at 20°C i.e. Cu hardenned

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Provisional single anode Cathode

~ START DimensionSimplified PF coils system fed with 600 A, 120 V

Does not need cooling

Aim: verification of the arc formation & stability

Multi-Pinch Experiment

STARTVacuum Vessel



START in Frascati

Arrival

Disassembling

May 2004

October 2004



Multi-Pinch Experiment: Phase I & Phase II



CONCLUSIONSThere is high degree of confidence that the design, construction and reliable operation of PROTO-SPHERA are feasible

The Multi-Pinch experiment is being constructed to prove the formation and stability of the PROTO-SPHERA initial arc

PROTO-SPHERA can be obtained simply adding components to Multi-Pinch

Adequate technological basis, particularly in relation to electrodes, for extrapolation to a bigger device is expected

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Documents

September 26-29, 2005 Knoxville. Tennessee USA Progress and Plan of the PROTO-SPHERA Program Outline of the Talk 1)Introduction to PROTO-SPHERA 2)The PROTO-PINCH