After the last year’s decision about the construction site, ITER (International Thermonuclear Experimental Reactor) prototype has entered its realization phase. During the last decade an extensive Research and Development program has been performed to demonstrate the feasibility of its magnet system. The major elements of this program have been the construction and test of real scale coils as well as solenoid prototypes. The testing of the model coils has provided valuable information to finalize the design of the magnetic system. However the behavior of Nb3Sn based cables was not as good as expected on the basis of the characteristics evaluated for the uncabled strands. This degradation in Nb3Sn performance seems to be due to various factors, among which the strain state of the filaments. It is worth to mention that Nb3Sn critical current depends upon the strain state, in addition to the applied magnetic field and temperature. For electrical and thermal stability in a superconducting (SC) strand the Nb3Sn compound is distributed into fine filaments (up to about 50 micrometers diameter) and embedded in a resistive matrix. Nb3Sn formation requires a solid state diffusion reaction at high temperature (900-950K), which causes transformation strains in each material during cool down to operating condition (4.2K). Moreover, the thermal strain is only part of the strain field, because of the complex layout of the wires inside the cable causes an additional strain. Finally, when the coil is energized, electromagnetic forces act as bending loads on the wires, which behave like continuous beams supported by the contacting wires in their neighbourhood. In this way a bending strain is added to the existing strain field. An accurate estimate of the strain state of the strand inside a coil experiencing magnetic loads is clearly not an easy task, a wide spectrum of coupled physical problems have to be considered for the prediction of Nb3Sn conductor performances. In this work we present a thermo-mechanical model based on homogenisation methods, suitably developed for the analysis of the SC fibrous composite with non-linear, temperature dependent components. We account also for local material yielding at the stage of microanalysis. The transformation strains due to cool down from the reaction temperature to the cable operating conditions are computed, as well as the following distribution of strain due to energisation. To recover the strain inside each single strand and in the Nb3Sn filaments, a suitable unsmearing technique is applied. The method is applied to the real case of the 3x3 and 3x3x5 cable sub-size samples tested at FZK in Germany.

Multiscale thermo-electro-mechanical analysis of superconducting coils for ITER (Keynote lecture)

SCHREFLER, BERNHARD;BOSO, DANIELA;
2007

Abstract

After the last year’s decision about the construction site, ITER (International Thermonuclear Experimental Reactor) prototype has entered its realization phase. During the last decade an extensive Research and Development program has been performed to demonstrate the feasibility of its magnet system. The major elements of this program have been the construction and test of real scale coils as well as solenoid prototypes. The testing of the model coils has provided valuable information to finalize the design of the magnetic system. However the behavior of Nb3Sn based cables was not as good as expected on the basis of the characteristics evaluated for the uncabled strands. This degradation in Nb3Sn performance seems to be due to various factors, among which the strain state of the filaments. It is worth to mention that Nb3Sn critical current depends upon the strain state, in addition to the applied magnetic field and temperature. For electrical and thermal stability in a superconducting (SC) strand the Nb3Sn compound is distributed into fine filaments (up to about 50 micrometers diameter) and embedded in a resistive matrix. Nb3Sn formation requires a solid state diffusion reaction at high temperature (900-950K), which causes transformation strains in each material during cool down to operating condition (4.2K). Moreover, the thermal strain is only part of the strain field, because of the complex layout of the wires inside the cable causes an additional strain. Finally, when the coil is energized, electromagnetic forces act as bending loads on the wires, which behave like continuous beams supported by the contacting wires in their neighbourhood. In this way a bending strain is added to the existing strain field. An accurate estimate of the strain state of the strand inside a coil experiencing magnetic loads is clearly not an easy task, a wide spectrum of coupled physical problems have to be considered for the prediction of Nb3Sn conductor performances. In this work we present a thermo-mechanical model based on homogenisation methods, suitably developed for the analysis of the SC fibrous composite with non-linear, temperature dependent components. We account also for local material yielding at the stage of microanalysis. The transformation strains due to cool down from the reaction temperature to the cable operating conditions are computed, as well as the following distribution of strain due to energisation. To recover the strain inside each single strand and in the Nb3Sn filaments, a suitable unsmearing technique is applied. The method is applied to the real case of the 3x3 and 3x3x5 cable sub-size samples tested at FZK in Germany.
2007
Tenth Engineering Mechanics Symposium
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/178059
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