Abstract
Fiber-optic thermometry has the potential to provide rapid and reliable quench detection for emerging large-scale, high-field superconducting magnets fabricated with high-temperature-superconductor (HTS) cables. Developing non-voltage-based quench detection schemes, such as fiber Bragg grating (FBG) technology, are particularly important for applications such as magnetic fusion devices where a high degree of induced electromagnetic noise impose significant challenges on traditional voltage-based quench detection methods. To this end, two fiber optic quench detection techniques—FBG and ultra-long FBG (ULFBG)—were incorporated into two vacuum pressure impregnated, insulated, partially transposed, extruded, and roll-formed (VIPER) high-current HTS cables and tested in the SULTAN facility, which provides high-fidelity operating conditions to large-scale superconducting magnets. During surface heater induced quench-like events under a variety of operating conditions, FBG and ULFBG demonstrated strong signal-to-noise ratios (SNRs) ranging from 4 to 32 and measured single-digit temperature excursions; both the SNR and temperature sensitivity increase with temperature. Fiber thermal response times ranged between effectively instantaneous to a few seconds depending on the operating temperature. Strain sensitivity dominates the thermal sensitivity in the conditions achievable at SULTAN; however, measurements at higher quench evolution temperatures, coupled to future work to increase the thermal-to-strain signal, show promise for quench detection capability in full-scale magnets where temperature and strain may occur simultaneously. Overall, FBG and ULFBG were proven capable to quickly and reliably detect small temperature disturbances which induced quench initiation events for high current VIPER HTS conductors in realistic operating conditions, motivating further work to develop FBG and ULFGB quench detection systems for full-scale HTS magnets.
Highlights
A rapid and reliable quench detection is vital for high current superconducting magnet systems to prevent irreversible damage to a magnet following a quench event, which is an abrupt, localized transition from the superconducting state during an off-normal event
Fiber optic thermometry is an alternative approach to quench detection that is immune to the electromagnetic noise present in all voltage-based schemes
In experimental tests at SULTAN on high current HTS VIPER cables, fiber Bragg grating (FBG) and ultra-long FBG (ULFBG) fiber technologies demonstrated several characteristics necessary for detecting quench in HTS cables: first, they demonstrated better performance in identifying an early-stage quench growth before thermal runaway occurred compared to voltage taps; second, signal-to-noise ratios (SNRs) increases with larger quench energies, accelerates when the conductor is operating in lower quench stability regimes, and are at least 2–3 times higher than co-wound voltage tap systems
Summary
A rapid and reliable quench detection is vital for high current superconducting magnet systems to prevent irreversible damage to a magnet following a quench event, which is an abrupt, localized transition from the superconducting state during an off-normal event This is important for superconducting magnets with large stored magnetic energy that operate in complex environments, where off-normal events can trigger a magnet to quench and release the stored energy. FBGs are sensing elements which can be photo-inscribed into a silica fiber doped with germanium by exposing the fiber to a UV laser pattern This exposure induces a periodic modulation of the refractive index of the core of the fiber over a certain length. A change in these parameters leads to a shift in the Bragg wavelength due to the effect they induce on both the refractive index neff (T,ε) and the grating period Λ(T,ε) (see figure 1). Multiple FBG, each with a unique grating period, can be inscribed at regular intervals along a single fiber to facilitate position-dependent temperature monitoring along the fiber’s length
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