Critical Design Issues of Wendelstein 7-X

M. Gasparotto

15th International Stellarator Workshop 2005, Madrid, Spain,

3rd -7th of October 2005

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Outline

Introduction

Design Tools: FE models

Critical Elements of the Mechnical Structure

Design Solutions and Test Campaigns

Conclusion

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The W7-X Machine

DivertorNPC

PC

Outer vessel

Machine Base

Central Support Ring

299 Ports

Plasma Vessel

6.8 TMax. magnetic field on the coils

3 TAverage magnetic field on plasma axis

425 tCold mass

725 tMachine mass

16 mMachine Diameter

4.5 mMachine height

30 m3Plasma volume

0.53 mMinor average plasma radius

5.5 mMajor plasma radius

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Critical Issues in the Design of the W7-X Device

The complicated 3D geometry.

The high number of required ports which limit the available space for the mechanical structure.

The high electromagnetic loads.

The high accuracy of the magnetic field configuration.

Magnet components (NPC, PC, central ring sector) in fabrication phase during the detailed design of the intercoil support structure.

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Magnet System

Non Planar Coil cross section

The magnet system includes 50 Non Planar Superconducting Coils, 20 Planar Superconducting Coils, the Central Ring, the Ring Supports and the IntercoilSupport Strucutre.

SS casingEmbedding

InsulationNb-Ti

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Cryostat System

The Cryostat System includes the Plasma Vessel (PV), the Outer Vessel (OV), 299 ports, the PV and OV supports and the Machine Base.

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Magnet System FE global model

Two FE models have beendeveloped:- ANSYS model (collaborationwith Efremov Institute – Russia)- ADINA model

The Coils are arranged torroidallyin 5 equal modules each oneconsisting of 2 semi-modules.

The FE model can be reduced to 1/10 of the whole machine (36°) introducing appropriate boundaryconditions derived from thesymmetries of the Magnet System

Maximum current in theNPC 18,4 MA turn

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FE Global Model results

Displacement (m) in High Iota plasmaconfiguration

0

0.0075

0.015

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FE Global Model result

Von Misesstress (MPa) in High Iota plasmaconfiguration

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0

250

450

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Cryostat FE Global Model

Machine Base

Plasma Vessel includingsupports and ports

Outer Vessel

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The complete cryostat FE Global Model

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Finite Element Global Model:Outer vessel, Plasma vessel, Machine Base

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Cryostat FE Global Model

Displacement (mm) due to the weight and the operational scenario

-3.5 mm

-5.8 mm

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Main results of the FE analyses

The global behaviour of the Cryostat and Magnet Systems with respect to displacements and stresses are within the limits;The loads on the Central Supports and on the Intercoil support structure are very high and detailed FE analysis is necessary to identify critical areas;A number of welds both on the Magnet and on the Cryostat Systems have to be reinforced.

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Mechanical Structure: Central Ring and intercoil support structure

Central Ring

Central Supports

NPC

Lateral Support

Narrow Supports

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Cross Section of a Typical Central Support Element Bolted Connection

Connection between the superconducting coilcasing and the Central Ring.

Superbolt nut

Superbolt washer

Sleeve

Interface Plate

Inconel Bolt (l=54.5 cm)

Extension

Shoulder

Wedge

Shim Plate

Central Support Element

Coil side Support sideCentral Ring

10 cm

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Central Support – detailed FE Analysis –collaboration with Warsaw University Poland

0

500

1123Central Support Finite Element Analysis Model: mesh (left), von-Mises stress (MPa) (right)

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Central Supports 3xM30 Mock-up

Full scale mock-up of a bolted connection, representing 1/3 of a complete CSE

Fn = 455 kN

Fs = 270 kN

Mb = Fs·l = 78.3 kNm

l = 0.29 m

FE model of the central support mock-up (ANSYS)

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3xM30 mock-up in the test machine

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Preliminary experimental results

After welding the wedges, the preload of the bolts fell from 500kN to 370 kN. Consequently, during assembly the bolts should be re-tightened;The mechanical limit was reached when the applied loads were equal to 1.2 times the nominal ones. Significant plastic deformations started in some critical areas;No significant change in the bolt preloads was observed after the tests;The different parts have been disassembled without problems, no significant deformations or damages on the surfaces were observed.

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Narrow Support Element –Detailed Design

Dust protectionWave spring

Sliding surfaceMoS2 coatedPad in Al-BronzePad frame

10 cm

Central Ring

Central Supports

NPC

Lateral Support

NarrowSupports

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Narrow Support – collaborationwith FZ Jülich

Plastic strain in partframe of Narrow Support pad after 8 loading-unloading cycles

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Narrow Support Design: Critical Issues

The normal loads acting on the NS elements are very sensitive to the gap thickness between the pad and the sliding surface at the start up of the experiment (4K and zero electrical current in the coils). Very accurate assembly procedure has to be developed in order to keep the assembly tolerance as low as possible.

The sliding and tilling motions have to happen smoothly to avoid sudden release of elastic energy (“stick and slip”) which could trigger a quench in the superconductor coils.

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Narrow Supports Tests - KRP

77K Vacuum friction test device for the narrow support pads

Narrow support pads transmitting forces between coils during magnet energization

Normal load < 1.5 MN

Sliding < 5 mm

Tilting < 1°

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Narrow support test results

Stick-slip is more sensitive in cryo vacuum tests than in RT tests.No stick-slip is present in cryo-vacuum tests after more than 4000 cycles, when the sliding surfaces (Al-bronze pad and SS counter-face) are coated by MoS2 and the roughness is kept as low as possible (Rz < 1µm)The friction factor between the Al-bronze pad and SS sliding surface in cryo-vacuum tests was lower than 0.08.

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Lateral Supports Tests

Welding tests at FZJ on the lateral support elements (LSE)

NPC

Lateral Support

Narrow Supports

Central SupportsCentral Ring

- Average parallel displacement: 0.5-1.5mm

- Tilting: lower than 0.2°

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Conclusion

The main critical issues in the W7-X design derive from the complexity of the 3D geometries, high dimensional accuracy to bereached, the limited available spaces and the high loads in the magnet system.

The optimisation of the design of the mechanical structure leads to a system of support elements based on:

CS connecting the coil casing to the central ring by a bolted connection; NS connecting the NPC in the inner region by sliding contacts;LS connecting the NPC in the outer regions by welded connections with the exception of those between semi modules and modules which are realised by bolted connections.

Extensive FE models and tests have been performed and are in progress to validate the adopted solutions.

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