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MSC Seminars
Magneto-Thermal Instabilities in Nb3Sn conductors for High Field Accelerator Magnets: Where do we Stand in the Understanding of these Phenomena?
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Europe/Zurich
30/7-010 (CERN)
Description
Abstract:
Since the origin of superconducting magnets, magneto-thermal instabilities have been one of the major issues for magnet scientists. Indeed they can prematurely quench a magnet before it reaches its critical current. The main cause of these instabilities is the uneven distribution of the current within the superconductor: the current density J is either equal to the critical value Jc or it is zero. Starting from this distribution, a perturbation that infinitesimally reduces the Jc can cause current and magnetic field redistribution associated to Joule dissipation and consequently to a further reduction of the Jc. If this latest variation ΔJ is larger than the original one ΔJ0, an avalanche process, which leads to a sudden macroscopic redistribution of the magnetic field (flux-jump), can be established. Flux-jumps can completely quench the superconductivity.
In the 70’s 80’s and 90’s more and more sophisticated theories and stability criteria have been developed to manufacture stable conductors and/or to choose the proper conductor for a specific magnet. Generally these theories analyze the beginning of the instability and the stability criteria are based on not allowing the start of a flux-jump. This approach has been successful for NbTi conductors and magnets nevertheless it cannot be applied to high-Jc Nb3Sn conductors. Indeed state-of-the-art Nb3Sn conductors, which will be used in the LHC High Luminosity upgrade, cannot satisfy these stability criteria because of the high-Jc and large effective filament size. Flux-jumps cannot be excluded in high-Jc Nb3Sn conductor, on the other hand it is not necessary to complete eliminate flux-jumping in order to build a stable magnet. In the last 10 years a lot of experimental and theoretical work has been carried out in order to understand how flux jumps develop and when they lead to a quench.
In this seminar I will present the main concepts about magneto-thermal instabilities focusing on the work done in the last 10 years. The goal is to give an overview on the status of the comprehension of magneto-thermal instabilities.
