Non-planar Coils Research Articles

Power supplies for the WENDELSTEIN 7-X stellarator

The magnetic confinement of the plasma in WENDELSTEIN 7-X (W7-X) will be determined by 50 superconducting non-planar coils and 20 superconducting planar coils. The coils are grouped in five periodic modules with ten coils each connected in series. In order to be able to vary the magnetic configuration and hence increase the experimental flexibility the five groups of non-planar coils and two groups of planar coils will be powered individually resulting in seven independent power supplies. To fine tune the magnetic configuration, shift the plasma axis and to modify the plasma edge an additional set of ten normal conducting control coils will be mounted. Other aims are to correct small field errors and to distribute the heating power on the target plates over a wider range. In order to correct field errors a direct current is necessary, whereas an alternating current is essential for the second aim. The paper describes the design of the two types of power supplies for W7-X as well as the results of the first tests of components.

The superconducting busbar system of Wendelstein 7-X

The superconducting magnet system of the stellarator Wendelstein 7-X (W7-X) consists of 50 non-planar and 20 planar coils grouped in five periodic modules. Ten coils of a given type of non-planar and planar coils will always be connected electrically in series with nominal currents ranging up to 18 kA. Because of the 5-fold symmetry five busbar systems are to be routed. Electrical connection of the busbar system will require 184 disconnectable joints with a resistance below 5 nΩ. The paper describes the design features of the busbar systems and their installation in the stellarator. Requirements for the design and qualification of the disconnectable joints will be pointed out.

Suspension of the W7-X coils

A large steady state plasma fusion experimental device WENDELSTEIN-7X (W7-X) of the stellarator type is presently being built at the Max-Planck-Institute for Plasmaphysics (IPP). The aim of the experiment is to prove the reactor validity of the stellarator research line in addition to the tokamak research line. The magnet coil system of the main and ancillary coils is carried by a central support structure and kept in a position strongly predefined in space. The central support structure together with the additional lateral stiffening elements between the coil housings represent a complex 3D-framework system. This system is responsible to balance the considerable electromagnetic forces which are typically in the range of 1–4 MN at a nonplanar field coil and 20 MN per period as residual force in the centripetal direction. This paper shows FE-results of the main components inside the structure. Especially the central support elements, so-called extensions, which connect the coil housings with the support ring, are loaded by very high forces and moments.

Fabrication of the superconducting coils for Wendelstein 7-X

The Max Planck Institute of Plasmaphysics is building up the stellarator fusion experiment Wendelstein 7-X (W7-X) at the branch institute in Greifswald. W7-X continues the line of stellarator experiments at IPP. To allow for steady state operation W7-X has a superconducting coil system with 50 non-planar and 20 planar coils. The coil system is grouped in five equal modules, each consisting of two mirror symmetric half modules. The half modules are assembled from five different non-planar coils, two planar coils and a sector of the coil support structure. All cryogenic parts are enclosed in a cryostat to protect them from ambient temperature. The magnet system was ordered from the European industry. The production of superconductor, winding packs and encasings are under way. The main focus of this contribution aims on the fabrication state of the coil system.

Buckling analysis of the complete W7-X plasma vessel

A buckling analysis of the complete W7-X plasma vessel (inner vacuum vessel) has been performed. The magnet system of the W7-X stellarator consists of five modules each containing five types of non-planar and two types of additional planar coils ‘18th SOFT, Karlsruhe, Germany, 1994 (1995) 937’. Two vessel systems outside and inside the toroidal assembly of the different coils confine the vacuum for cryogenic operation. The shape of the plasma vessel is defined by the inside surface of the non-planar coils. Subject of the analysis is the buckling behavior of the complete plasma vessel including the individual nozzles for diagnostics. The results of some first modes are shown together with a static evaluation due to different operating cases. Applications with options to get more detailed informations are described additionally.

Development of a copper coating suitable for application on the nonplanar W7-X coils

Following the design changes on the non planar W7-X coils, the foreseen coating by spot welding of copper plates can no longer be applied due to the shape of the cases. Therefore BNN investigated alternative methods, for example electro-plating and thermal spraying as well as an adaptation of the copper plating, and compared the advantages and disadvantages of the different solutions. For all methods, industrial feasibility was the basis of the investigation. Since thermal spraying seemed very promising, the low temperature properties of thermal sprayed copper were evaluated. Based on the results, a cooling system was decided on, where most of the surface is covered with sprayed copper. Additionally, copper strips shall ensure that sufficient cooling power can be deposited in the coil cases.

Change of manufacturing technique for the W7–X nonplanar coil cases

The geometry of the coil cases of the nonplanar coil system for the Wendelstein 7-X (W7–X) experiment (Sapper, The superconducting magnet system for the W7–X stellarator. Proceedings 12th Topical Meeting on the Fusion Technology) was changed to a more complex shape compared to the DEMO Coil case (Kronhardt et al., 1998. Proceedings of the 20th SOFT (1998) 731–734). Therefore the manufacturing technique developed for the DEMO Coil case cannot be used for the series production of 50 coils. For the final design of the coil cases, investigations were performed to find a technique suitable for manufacturing the cases within the required geometrical tolerances and mechanical characteristics. In order to qualify the manufacturing technique a complete half case was cast and machined afterwards. The casting procedure was optimised with respect to the geometrical accuracy and the mechanical characteristics at 4.2 K. Measurements of the yield strength, the tensile strength, the elongation, and the Young's modulus were performed at room- and cryo-temperature (4 and 7 K). The influence of the heat treatment, the annealing temperature and the size of the casting on the mechanical values is shown. The requirements on the stainless steel are a yield strength of 800 MPa at 4 K and an elongation at fracture of >25%. The magnetic permeability has to be <1.01. Furthermore the welding properties of the case material were investigated. The development program showed that casting of complete case half shells is a feasible manufacturing technique for the series production of the Wendelstein 7-X nonplanar coil cases.

Structural analysis of the Helias reactor coil system

For the modular non-planar coil system of a Helias reactor (HSR) the distributions of the magnetic field and the coil forces are calculated by means of the EFFI code. In order to support the magnetic forces a scheme of mutual support of the coils is applied. The pancakes of the coil winding pack of each individual coil are embedded in a strong coil housing of stainless steel. Linear and nonlinear finite element computations are done with half a field period, using the ANSYS code and taking into account the stellarator symmetry. The mechanical stress and strain distributions inside the winding pack and the coil structure are analyzed.

Design and construction of WENDELSTEIN 7-X

Following the approval by EURATOM and the German government WENDELSTEIN 7-X (W7-X) is presently the largest fusion project under construction. W7-X is a helical advanced stellarator (HELIAS) which produces the confining magnet field only by magnet coils that enables steady-state operation. W7-X aims to demonstrate that the HELIAS configuration has the potential for a future power reactor. The successful application of new technologies for manufacturing prototypes and the positive results gained from tests allowed to design the machine in detail and to order major components. The geometry of the non-planar magnet coils has a considerable impact on the design of the machine in particular on the shape of the plasma vessel, positioning of the plasma-facing components and the size and orientation of the ports. The requirement for steady-state operation has consequences for many subsystems of W7-X. The magnet coils need to be superconducting and cooled to liquid helium temperature. Gyrotrons shall continuously provide 10 MW of ECR heating power. The divertor must be cooled to withstand heat loads of up to 10 MW/m 2. The schedule of W7-X is determined by the delivery dates of the non-planar coils, the plasma vessel and the outer vessel. Start of commissioning and scientific operation is scheduled for spring 2006.

Design goals and status of the WENDELSTEIN 7-X project

WENDELSTEIN 7-X (W7-X) is a helical advanced stellarator(HELIAS) which produces the necessary twist of the plasma columnonly by magnet coils which enable steady state operation. W7-Xaims to demonstrate that the HELIAS configuration is a viablealternative for a future power station. Major research anddevelopment activities have meanwhile been completed. Theseinclude the design of a machine sufficiently well advanced to allow the coilsystem to be manufactured, the power supplies to be ordered andthe cryostat to be specified. The geometry and configuration ofthe non-planar magnet coils have a considerable impact on thedesign of the machine, in particular on the shape of the plasmavessel, the positioning of the plasma-facing components and theports. The requirement for steady state operation hasconsequences for most of the subsystems of W7-X. The magnetcoils are superconducting and must be cooled to 3.8 K by ahelium refrigeration plant. The power supply for the magnetcoils features a reliable protection system dumping the storedmagnet energy in case of a rapid shutdown. The water-cooleddivertor must be designed for heat loads of up to 10 MW m-2. ECRH, ICRH and NBI will provide the necessary power to heat theplasma.

Buckling analysis and results post-processing of the W7-X inner vacuum vessel

The magnet of the WENDELSTEIN 7-X stellarator consists of five modules each containing five types of non-planar coils as presented in Ref. [1]. Two vessel systems outside and inside the toroidal magnet curvature confine the vacuum for cryogenic operation. The shape of the inner vacuum vessel is defined by the geometry between the surface of the plasma and the inside surface of the coils. The subject of this analysis is the test cryostat for W7-X. A buckling analysis has been performed to obtain the critical-load factors. The method used to perform the calculations is described together with some special demands for the post-processing.

An Advanced Method for Helical Coil Windings in Fusion Devices

In helical experiment devices, the accuracy of their nonplanar coils affects plasma confinement properties significantly; so coil winding accuracy is one of the crucial issues for their construction. An advanced method for helical coil windings in fusion devices was studied. This new method utilizes a geometrical characteristic of the trajectory of the helical coil, which enables us to improve the accuracy of coils, and is thus expected to improve the mechanical, electromagnetic and super-conducting properties of the coils. This paper presents the new concept of the winding method and practically shows the results of the R&D for the Large Helical Device (LHD). 8 refs., 7 figs.

Superconductor for the coils of the modular stellarator Wendelstein 7-X

The magnet system of the planned stellarator Wendelstein 7-X (W 7-X) consists of 50 nonplanar and 20 planar coils. Nonplanar coils require a winding process with compound bending (simultaneous bending in two planes). The use of a stiff conductor would lead to technical difficulties and requires very expensive tools for the winding of these coils. Therefore a conductor was industrially developed which is relatively soft and therefore of good workability during the winding process. Once the winding is completed, a heat treatment gives the conductor a final strength being high enough to withstand all mechanical loads which might be expected during the operation of W 7-X. The conductor consists of a four stage cable with 192 superconducting strands and a special Al-alloy jacket extruded around the cable. Properties of this conductor and test results attained up to now are given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Superconducting coil development for the Wendelstein 7-X stellarator

At the Max-Planck-Institut für Plasmaphsik (IPP), Garching, the Wendelstein 7-X stellarator (W 7-X) is in the stage of the beginning R&D phase. The experiment will be a large modular machine with nonplanar coils, following the Garching development line. It fits into the range of next step devices [G. Beidler et al., Fusion Technol. 17 (1990) 148–168]. The main technical parameters are: major radius: R 0 = 5.5 m, magnetic induction: B 0 = 3 T, stored magnetic energy: W m = 600 MJ, average plasma radius: r 0 = 0.53 m. The expected plasma parameters are: central temperatures: T i(0), T e(0)=2–5keV central electron density: n e(0) = 0.1−2 × 10 20 m −3, energy confinement time: τ E = 0.1−0.5 s, average beta value: 〈 β〉 ⩽ 0.05. The design has to allow steady-state plasma operation. Consequently the coil system is superconducting. An internally cooled cable-in-conduit conductor with copper stabilized NbTi strands will be used at 4 K (LHe). The paper presents an overview of the design features of the machine and describes in particular the conductor design, the coil arrangement with electrical, hydraulic and mechanical parameters as well as the sequence of prototype steps which are foreseen for establishing a well-developed series production of the magnet.

FE-COMPUTATIONS OF A NONPLANAR COIL WITH DIFFERENT SUPPORT CONSIDERATIONS

FE-COMPUTATIONS OF A NONPLANAR COIL WITH DIFFERENT SUPPORT CONSIDERATIONS