L. Wegener

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.

Final design and construction of the Wendelstein7-X coils

The Stellarator of the Wendelstein 7-X (W7-X) experiment contains a system of 50 non-planar and 20 planar superconducting coils. The coils were designed by the IPP. The coil manufacturing and inspection is shared between several European enterprises and consortiums. The coils consist of the winding pack embedded in a stainless steel casing and of the related instrumentation. Design details, tolerances and guarantee values and differences between the coils types are described in the contribution. The features of the superconductor are described separately. Finally, the contribution indicates measures adopted by the W7-X project to ensure the quality of the coil design and manufacturing.

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.

Overview of a new test facility for the W7X coils acceptance tests

In the frame of the W7X stellarator project, a cooperation agreement between the Max-Planck-Institut fur Plasmaphysik and CEA has been set-up in order to perform the acceptance tests of all the 70 superconducting coils that compose the W7X magnet system. The main purpose of these tests is to demonstrate that each coil can work at nominal operating conditions, with enough margin to ensure the coil safety during the stellerator operations. For that purpose, CEA has built a new test facility at Saclay. This paper presents a general overview of the test facility. It is mainly composed of two large cryostats (useful space of 5 m diameter and 4.2 in height), a cryogenic source to produce supercritical helium at 4.5 K and 6 bar with a power rating of 200 W, and an electrical power supply of 25 kA. Each cryostat can contain two coils. It is then possible to cool down two coils at the same time, and to warm up two others. But only one coil can be energized at the same time. As the assembly of the facility is now nearly completed, the first cryogenic tests with the prototype coil (DEMO) have started. The first conclusions of these tests and the facility performances will also be discussed in this paper.

Structural Analysis at the Transition From W7-X Construction to Operation

The Wendelstein 7-X modular advanced stellarator is in the commissioning phase at the Max Planck Institute for Plasma Physics in Greifswald, Germany. The focus of the numerical analysis has been shifted from support of the machine design and assembly toward preparation of the commissioning steps, assessment of possible field disturbances under operational loads, and exploration of operational limits. This paper emphasizes on the development, evolution, and realization of new analysis strategies, as well as on implemented numerical approaches for electromagnetic, thermal, and structural analyses. Remarkable results of the first comparisons with measurements from the extended mechanical instrumentation system are presented in detail and cover evacuation/flooding of the cryostat and plasma vessel, the first cooldown of the magnet system (MS), and both the superconducting coil groups and integral MS commissioning. Finally, some lessons learned during the transition phase are highlighted, which might be relevant for other large fusion machines.

Experiences From the Assembly of the Magnet System for Wendelstein 7-X

Wendelstein 7-X represents the continuation of fusion experiments of the stellarator type at the Max-Planck-Institute for Plasma Physics. The size of device (725 t, height of 5 m, diameter 16 m) and the superconductive magnet system distinguish W7-X from earlier stellarators at the Max-Planck Institute. The paper describes the technologies and methods used for the assembly of the magnet system and it compiles the experiences gained. The assembly of the W7-X facility will be accomplished in 2014.

W7-X DEMO coil cryogenic tests in the CEA/Saclay test facility

In the frame of the W7-X stellarator project, CEA cooperates with Max-Planck-Institut fur Plasmaphysik to perform the acceptance tests of all 70 superconducting coils of the W7-X magnet system. The test facility is now complete and its performance is being checked using the W7-X prototype coil. The main objective of the tests of the series coils is to demonstrate their proper function and to determine their margin of operation. Since many coils will be tested and compared to each other, it is important to measure the margin of operation in a reproducible way with sufficient accuracy. Either increasing the current, increasing the temperature or mixed operation can induce quenches. The test results on the prototype coil are analyzed with respect to the temperature and current margin and compared to the expected values calculated from superconducting strand data. The paper summarizes the results of these tests and the methods of evaluation.

Status of the construction of the W7-X magnet system

The stellarator of the Wendelstein 7-X (W7-X) experiment contains a superconducting magnet system comprises 70 coils and a central support structure. The magnet system has been designed with respect to the physics aims of the experiment. Based on these main features, the components have been ordered from European industry. The subsequently detailed design of the single components and recent prototype test results required the modification of some features. The final design of the components is described in the contribution.

Engineering Challenges of W7-X: Improvement of Numerical Modeling and Mechanical Monitoring After Commissioning and First Phase of Operation

Abstract The largest modular stellarator Wendelstein 7-X (W7-X) has successfully passed commissioning and first phase of operation in Greifswald, Germany. The limiter configurations of plasma with 2.5 T of magnetic induction on the plasma axis produce already considerable loads (MN) in the W7-X systems. The sophisticated W7-X superconducting magnet system with its non-linear support system is instrumented with an extensive set of mechanical and temperature sensors. Measurement results showed that magnet system behavior is in good correspondence with original predictions from numerical models. However, several areas require modeling improvements and/or proper adjustment of parameters to reflect “as-built” situation. Moreover, high temperature dependence of strain gauge signal accuracy in the range below 10 K requires its compensation in order to avoid fault alarms during monitoring. The work is considered as benchmarking of numerical models and as a preparation for upcoming more demanding phases with longer plasma pulses to guarantee safe and reliable W7-X operation with different divertor configurations. Both results of W7-X measurements and implemented improvements as well as lessons learned so far are also given.

Numerical modelling at the transition from W7-X construction to operation

The Wendelstein 7-X modular advanced stellarator is in the commissioning phase at the Max Planck Institute for Plasma Physics in Greifswald, Germany. The focus of the numerical analysis has been shifted from support of the machine design and assembly towards preparation of commissioning steps, assessment of possible field disturbances under operational loads, and exploration of operational limits. The paper emphasizes on the development, evolution and realization of new analysis strategies, as well as on implemented numerical approaches for electromagnetic, thermal and structural analyses. Remarkable results of first comparisons with measurements from the extended mechanical instrumentation system obtained during evacuation and flooding of the cryostat, the first cool-down of the magnet system and also during first part of the superconducting coil groups commissioning are presented in detail. Finally, some lessons learned during the transition phase are highlighted which might be relevant for other large fusion machines.