Self-Field Effects in Magneto-Thermal Instabilities for Nb-Sn Strands

Abstract

Abstract-Recent advancements in the critical current density ( ) of Nb 3 Sn conductors, coupled with a large effective filament size, have drawn attention to the problem of magneto-thermal instabilities. At low magnetic fields, the quench current of such high Nb 3 Sn strands is significantly lower than their critical current because of the above-mentioned instabilities. An adiabatic model to calculate the minimum current at which a strand can quench due to magneto-thermal instabilities is developed. The model is based on an 'integral' approach already used elsewhere [1]. The main difference with respect to the previous model is the addition of the self-field effect that allows to describe premature quenches of non-magnetized Nb 3 Sn strands and to better calculate the quench current of strongly magnetized strands. The model is in good agreement with experimental results at 4.2 K obtained at Fermilab using virgin Modified Jelly Roll (MJR) strands with a low Residual Resistivity Ratio (RRR) of the stabilizing copper. The prediction of the model at 1.9 K and the results of the tests carried out at CERN, at 4.2 K and 1.9 K, on a 0.8 mm Rod Re-Stack Process (RRP) strand with a low RRR value are discussed. At 1.9 K the test revealed an unexpected strand performance at low fields that might be a sign of a new stability regime.

Self Field Instability in High-${\rm J}_{\rm c}$${\rm Nb}_{3}{\rm Sn}$ Strands With High Copper Residual Resistivity Ratio

“…For fields lower than , the samples always have premature quenches due to magnetothermal instabilities. The premature quench current during V-I tests was measured at least 5 times at each magnetic field, covering the field range from to 0 T with steps of generally 1 T. The set of measurements at each magnetic field was preceded by one or more quenches of the sample to remove the strand magnetization [7]. During V-I measurements, premature quenches of not magnetized strand are provoked by the self-field instability [7] hence these measurements were done to determine the self-field stability current .…”

“…Although very promising, state of the art highstrands suffer magneto-thermal instabilities [1]- [7] that can severely limit the strand performance [7]- [11]. The magneto-thermal instability produces a fast magnetic flux motion within the conductor associated with Joule heating (flux-jump) that can prematurely quench the superconductor.…”

“…Recently, it has been shown that at 1.9 K the self field instability is the dominating mechanism that limits the performance of highstrands with a low RRR ( 10) [7]. It was also shown [7] that for RRP strand with low RRR: 1) the minimum quench current due to the self-field instability, that is the self-field stability current , is lower at 1.9 K than at 4.2 K; 2) at 1.9 K the can be considerably lower than the critical current at 12 T. The self-field instability might be the cause of the limited quench performance at 1.9 K of the latest magnets based on the RRP conductor built by the LARP collaboration [15], [16], [18].…”

Влияние теплоемкости и проводимости нормальной матрицы NbTi композитного сверхпроводника на устойчивость к скачкам магнитного потока

“…Их работы 2005-2010 гг. свидетельствуют о том, что проявления коллективных скачков магнитного потока собственного поля существенно снижают достигаемые значения токов в обмотках квадрупольных магнитов (40−70% от тока короткого образца -максимально достигаемого значения) [1].…”

“…Их работы 2005-2010 гг. свидетельствуют о том, что проявления коллективных скачков магнитного потока собственного поля существенно снижают достигаемые значения токов в обмотках квадрупольных магнитов (40−70% от тока короткого образца -максимально достигаемого значения) [1].Анализ устойчивости и его результаты существенно зависят от соотношений характерных времен тепловых и электромагнитных процессов в сверхпроводнике: маг-сверхпроводника, ρ -дифференциальное сопротивле-ние сверхпроводника, которое при достаточно низких уровнях электрических полей во много раз меньше, чем сопротивление стабилизирующей нормальной матрицы, C -объемная теплоемкость сверхпроводника, λ -коэффициент поперечной теплопроводности, h -коэф-фициент теплопередачи с поверхности.В жестких сверхпроводниках (без нормального метал-ла) τ m ≪ τ λ , а в многоволоконных композитных τ m ≫ τ λ . Теплообменное время может сильно изменяться в зави-симости от условий охлаждения.…”