Cryogenic Magnetic System Research Articles

Further Development of the W7-X Magnet System Finite Element Global Model in Preparation for Enhanced Operation Phase

The Wendelstein 7-X (W7-X) stellarator equipped with a large cryogenic magnet system (MS) has been enhanced for the long pulse operation at the Max-Planck-Institute for Plasma Physics in Greifswald, Germany. In parallel with the process, the MS global finite element model (GM) has been developed further to consider two interrelated issues: i) the effect of winding pack (WP) embedding (EB) process and ii) planar coil case pins and bolts plastification. The procedure of implementing the EB effect in ANSYS is based on the death/birth feature of particular elements, special fixation algorithm and careful check of the unavoidable artificial stress level. The approach to get a refined prediction of plastic strain/ stress levels for multiple pins and bolts uses the detailed local pin/bolt models to check the GM stiffness representation and to deliver the target values using an interpolation procedure. If the main focus during first two experimental campaigns, for roughly 13 months, was on the static structural strength of the MS, the issue of cyclic behavior is addressed in the updated GM post-processing procedure. The post-processing routine predicts how many electromagnetic cycles and cool down-warm up cycles with particular loading patterns could be safely withstood with required margins by the system components. This paper is focusing on the structural cyclic behavior modelling of the W7-X magnet system components. Several related issues are addressed, such as: 1) Specific features of winding pack embedding modelling, 2) Reasonably simplified modeling of multiple pins and bolts in the GM, 3) Preliminary assessment of coil case pin failure. In addition, lessons learned so far regarding the gradual modifications of the GM are also summarized briefly.

Monitoring of W7-X cryostat commissioning with cryostat system FE model

The Wendelstein 7-X stellarator started its first operational phase in October 2015 at the Max-Planck-Institute for Plasma Physics in Greifswald with the goal to verify that a stellarator magnetic confinement concept is a viable option for a fusion power plant.The main components of the W7-X cryostat system are the plasma vessel (PV), outer vessel (OV), 254 ports, thermal insulation, vessel supports and the machine base. The main task of the cryostat system is to provide an insulating vacuum for the cryogenic magnet system, UHV conditions within the PV and to provide external access to the PV through ports for diagnostic-, supply- and heating systems.The updated finite element (FE) Global Model of the Cryostat System (GMCS) has continued to be used for predicting and assessing the behavior of W7-X as measured during its commissioning and operational phase. The measurements with strain, temperature and displacement sensors as positioned on the OV, Ports, PV and its supports form the basis of the cryostat system monitoring. After successful evacuation of the OV [1] commissioning continued and in 2015 the PV has been evacuated and baked for the first time. The measurements show good correspondence with the predictions of the GMCS and allowed for continuation of the commissioning. This paper gives an overview of analyses performed with the GMCS in support of cryostat commissioning and operation. In addition, the assessment performed for optional PV position adjustment is presented. A PV adjustment might be required in case plasma operation reveals problems with the plasma heat load distribution on critical in-vessel components.

Coupled FEM–DBEM approach on multiple crack growth in cryogenic magnet system of nuclear fusion experiment 'Wendelstein 7-X'

At the Max Planck Institute for plasma physics in Greifswald, Germany, the world's largest nuclear fusion experiment of modular stellarator type Wendelstein 7-X has started plasma operation. The hot hydrogen plasma is confined in a plasma vessel by an electromagnetic field generated by 50 non-planar and 20 planar superconducting coils. The superconducting coils are encased in cast stainless steel housings. The coils are bolted onto a central support ring and welded together by so called lateral support elements (LSEs). In this paper, a procedure, based on a global–local finite element method (FEM)–dual boundary element method (DBEM) approach, is developed to simulate the propagation of multiple cracks detected in LSEs and undergoing a fatigue load spectrum. The global stress analysis on the superconducting coils is performed by FEM whereas the sub-modelling approach is adopted to solve the crack propagation in the DBEM environment. The boundary conditions applied on the DBEM submodel are the displacements calculated by the FEM global analysis, in correspondence of the cut surfaces (there are no body forces nor external loads applied on the submodel volume). Two cracks are simultaneously introduced, and a linear elastic fracture mechanics analysis is performed. Results in terms of cracks growth rates and evolving crack shapes are provided, and the residual life of the component is forecast.

Coupled FEM–DBEM approach on multiple crack growth in cryogenic magnet system of nuclear fusion experiment ‘Wendelstein 7‐X’

AbstractAt the Max Planck Institute for plasma physics in Greifswald, Germany, the world's largest nuclear fusion experiment of modular stellarator type Wendelstein 7‐X has started plasma operation. The hot hydrogen plasma is confined in a plasma vessel by an electromagnetic field generated by 50 non‐planar and 20 planar superconducting coils. The superconducting coils are encased in cast stainless steel housings. The coils are bolted onto a central support ring and welded together by so called lateral support elements (LSEs). In this paper, a procedure, based on a global–local finite element method (FEM)–dual boundary element method (DBEM) approach, is developed to simulate the propagation of multiple cracks detected in LSEs and undergoing a fatigue load spectrum. The global stress analysis on the superconducting coils is performed by FEM whereas the sub‐modelling approach is adopted to solve the crack propagation in the DBEM environment. The boundary conditions applied on the DBEM submodel are the displacements calculated by the FEM global analysis, in correspondence of the cut surfaces (there are no body forces nor external loads applied on the submodel volume). Two cracks are simultaneously introduced, and a linear elastic fracture mechanics analysis is performed. Results in terms of cracks growth rates and evolving crack shapes are provided, and the residual life of the component is forecast.

Features and analyses of W7-X cryostat system FE model

The Wendelstein 7-X stellarator is presently under construction at the Max-Planck-Institute for Plasma Physics in Greifswald with the goal to verify that a stellarator magnetic confinement concept is a viable option for a fusion power plant.The main components of the W7-X cryostat system are the plasma vessel (PV), outer vessel (OV), ports, thermal insulation, vessel supports and the machine base (MB). The main task of the cryostat system is to provide an insulating vacuum for the cryogenic magnet system while allowing external access to the PV through ports for diagnostic, supply and heating systems.The cryostat is subjected to different types of loads during assembly, maintenance and operation. This ranges from basic weight loads from all installed components to mechanical, vacuum and thermal loads. To predict the behavior of the cryostat in terms of deformations, stresses and support load distribution a finite element (FE) global model has been created called the Global Model of the Cryostat System (GMCS).A complete refurbishment of the GM CS has been done in the last 2 years to prepare the model for future applications. This involved a complete mesh update of the model, an improvement of many model features, an update of the applied operational loads and boundary conditions as well as the creation of automatic post processing procedures.Currently the GMCS is used to support several significant assembly and commissioning steps of W7-X that involve the cryostat system, e.g. the removal of temporary supports beneath the MB, transfer of the PV from temporary to the final supports and evacuation of the cryostat. In the upcoming months the model will be used for further support of the commissioning of W7-X which includes the first evacuation of the PV.

Nuclotron superconducting magnets and their improvement for use in the SIS100 heavy-ion synchrotron in the fair project

At the beginning of the 1970s, the stage of development and assimilation of magnetic and accelerator technologies based on superconductivity started in the leading scientific centers in world which were studying the fundamental problems of nuclear physics and the structure of matter. Superconductivity for developing magnets for the proton synchrotron with field amplitude much greater than the limit for ordinary magnets (B ~ 2 T) was harnessed at Fermi Lab (Batavia, USA). The magnetic system of the Tevatron with field amplitude in the working aperture to 4.7 T, placed inside the synchrotron tunnel at energy 500 GeV, made it possible to double the energy of the accelerator. The field in the Tevatron magnets is formed by a multi-turn winding, fabricated on precision equipment. The accuracy of its geometric dimensions determines the quality of the magnetic field in the aperture. Superconducting magnets of this type in their modern form were used in the designs of other accelerators and were termed cos θ magnets. The magnets are cooled successively; the cool-down time of the magnetic system to the working temperature is weeks. A precision cos θ magnet, cryostat with a vessel for liquid helium, heat screen, vacuum jacket, and anchoring of the magnet are integral parts of an extremely labor-intensive, metal-intensive, and expensive cryogenic-magnetic system. Magnets of a new type [3] were developed in the High-Energy Laboratory at the Joint Institute for Nuclear Research (OIYaI) to considerably simplify and reduce the cost of the construction of the superconducting magnet and its cryostating system for the Nuclotron ‐ a specialized synchrotron for relativistic nuclei [1, 2]. Their fundamental difference lay in the use of a magnetic with a window-frame cross-sectional geometry instead of the cos θ precision magnet and a specially developed tubular superconducting cable, intended for pulsed magnets with rapid ramping of the field, instead of the flat superconducting Rutherford cable. In the case of tubular cable, as opposed to Rutherford cable, there is no coil or frame electric insulation in the path of heat flow from the superconductor to the helium; this improves considerably the conditions for cryostating of the superconductor. In addition, the use of two-phase helium as the cryogenic coolant makes the conditions for cooling the superconductor in the tubular cable of the pulsed magnets of the accelerator unique, making it possible to operate with record high field ramping 4 T/sec and higher [4]. On the whole, the concept adopted made it possible to minimize the transverse cross section of the Nuclotron magnet, eliminate the helium vessel inside the cryostat, secure an operating regime with field rise and fall rate 4 T/sec, economize on materials and power supply, simplify the external infrastructure, and most importantly organize the fabrication and testing of the magnets on the basis of the production capacity of OIYaI. OIYaI Nuclotron ‐ SIS100 Prototype. The development of the Nuclotron, a superconducting, strongly focusing accelerator of relativistic heavy nuclei, was motivated by the problems of relativistic nuclear physics and quantum chromodynamics and investigations of the color degrees of freedom in atomic nuclei. To study these problems experimentally, it was necessary to have appropriate beams of accelerated particles, which could not be obtained with the proton synchrotron operating at the time without creating an essentially new accelerator complex. The choice of the maximum energy of the new

ESCAR - First superconducting synchrotron, storage ring

The ESCAR (Experimental Superconducting Accelerator Ring) project is an accelerator technology development for which the primary goal is to obtain design and operational experience with a complete accelerator and storage ring system utilizing superconducting magnets. Full scale design effort began in July 1974, and operation is planned in 1977. ESCAR will not only provide the experience of fabricating and operating an extensive cryogenic magnet system with a significant advancement beyond present experience, but it will also provide a comprehensive accelerator system from which can be learned the realities of the interaction between the new cryogenic elements and the many other components of an accelerator. The extent to which these components (e.g., rf, injection, vacuum, control systems, etc.) are compatible with the cryogenic features of the accelerator, and to which they force modifications of the design, will be determined. (auth)