J. Lingertat

Fiber-optic bragg grating sensors for structural health monitoring at cryogenic temperatures

While conventional resistance strain gages show increasing cross-sensitivities to temperature and magnetic field with decreasing temperature down to liquid helium, it has been found that fiber optic Bragg grating strain sensors show negligible thermo-optic and magneto-optic effects in cryogenic environment and allow, therefore, reliable strain measurements. These specific application advantages of optical fiber Bragg grating sensors at low temperatures, together with the electrical isolation and low electro-magnetic interference, low thermal conductivity to a large number of multiplexed sensors, make them attractive for structural health monitoring of super-conductive magnets, e.g., for super-conductive motors, magnetic levitation transport, nuclear fusion reactors, or for measurement of material parameters at low temperature, and, if using special sensor substrates, also for temperature measurements and hot spot detection on superconductors.

Design and test of the support elements of the W7-X superconducting magnets

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 superconducting magnet system has to fulfill demanding requirements regarding magnetic field, loads, manufacturing and assembly. The magnet support system consists of several types of structural components. The main one is the central support structure (CSS) to which the superconducting coils are connected through Central Support Elements (CSE). These are bolted interfaces that allow for flange opening to reduce loads on the components. The non-planar coils (NPC) are toroidially interconnected via lateral support elements (LSE) and narrow support elements (NSE). NSE are contact supports consisting of Al bronze pads that allow for sliding under large compressive loads between the coils. The planar coils (PC) are connected to the NPC through planar support elements (PSE). At the module and half-module separation planes Contact Elements (CTE) connect the neighbouring NPC. An integrated programme of design, FE analysis, experiments and assembly trials has been undertaken. The NSE experimental program provided confidence that the pads can cope with the requirements regarding loads and cycles. Weld trials provided procedures for installing the LSE whilst keeping shrinkage and distortion within tight limits. Tests have been carried out to provide insight on the functioning of the CSE, in particular of the bolts and high performance Superboltreg-nuts during pre-load. This paper gives an overview of the integrated program on the W7-X support elements.

Design of Narrow Support Elements for Non Planar Coils of Wendelstein 7-X

The stellarator Wendelstein 7-X is presently under construction and assembly in Greifswald, Germany. One of the main structural elements which have to take the electromagnetic forces of the superconducting coil system are the narrow support elements (NSE). They are placed between the non planar coils and have to take very high compressive forces while relative sliding and tilting of the coils must be allowed. The design has been optimised with regard to a proper load distribution among all support elements taking also into account manufacturing and assembly tolerances. The paper describes the design, analysis and tests which have been carried out for the NSEs

Stability Test of a Superconducting W7-X Coil With Respect to Mechanical Disturbances

The superconducting magnet system of the Wendelstein 7-X (W7-X) stellarator experiment consists of 50 non-planar and 20 planar coils which are supported by the central support structure and inter-coil structure elements. This highly loaded support system is prone to mechanical disturbances like stick-slip effects. On the other hand, the coils are built up from cable-in-conduit-conductors (CICC) whose strands are highly compressed by Lorentz forces during operation. Residual elastic energy release within a cable can be triggered by shock waves and corresponding frictional heat generation may occur. The released energy might come into the order of the conductor stability limit and possibly cause a quench. An experiment was performed to simulate the impact of such mechanical disturbances on W7-X coils with stability margins corresponding to different operation conditions. A non-planar coil installed within the magnet test facility was energized and then hit by a pendulum via a stainless steel transfer rod. The test has shown that mechanical disturbances expected in W7-X are not able to induce a quench in any of the foreseen W7-X operation scenarios.

Investigations of the electron temperature profiles at the WEGA stellarator

This paper presents a study of the hollow bulk electron temperature profiles measured in the low-temperature (2?15?eV) plasmas of the WEGA stellarator. For this the global power balance equation was solved for the bulk electrons. The observed hollow temperature profiles could be reproduced satisfactorily assuming that the bulk electrons are heated through energy transfer from fast electrons and that the parallel heat conduction in the scrape-off layer is determined by the potential drop in the sheath in the vicinity of the wall. These results imply that the cause of the hollow shape of bulk electron temperature profiles is the heating of them through collisions with fast electrons produced by the heating method.