高温超電導磁石用の光ファイバ温度センサの低温特性向上と実用性検証

摘要

To put the high-temperature superconducting (HTS) magnets to practical use, it is effective to detect a sign of failures by the temperature monitoring. It should be noted that the quench protection of HTS magnets is difficult. Temperature is distributed over HTS magnets when they are cooled by conduction cooling. Therefore, reliable multipoint temperature monitoring method is necessary. Unlike the conventional resistance temperature sensor, the optical temperature sensor can measure multipoint temperatures with a single fiber. The optical fiber is not affected by the fluctuation of the magnetic field and has low heat intrusions. A Fiber Bragg Grating (FBG) is a type of the optical fiber temperature sensor, but its accuracy of the measurement decreases at cryogenic temperature. Therefore, it has been proposed to coat the FBG sensor with a metal or a resin to increase the accuracy at cryogenic temperature. Zinc is suitable for the coating material, because it has a high thermal expansion rate and its coating process is simple. Three types of zinc coating methods were evaluated in this research: sputtering zinc, electroplating zinc after sputtering titanium and copper, electroplating zinc after electroless nickel plating. The FBG sensors coated by these zinc coating methods were compared with respect to the sensitivity at cryogenic temperature, the durability against vibration and thermal shock, and the repeatability of the cooling cycle. We report these results.%新たに開発した電気めっき方式にて製作した極低温用光ファイバセンサに実際の使用環境を模擬した熱変動，振動を加える耐久性試験を行った。センサは性能の劣化や損傷などの異常を生じることなく使用環境に対する耐久性を確認した。また，コスト低減の観点から電気めっき方式を改良した複合めっき方式の製法について検討を行った。酸性エッチング処理を用いた複合めっき方式にて，実際に光ファイバ温度センサを試作し，温度計測が可能であることを確認した。ただしこの方式にはコーティング厚さが不均一になりやすい課題があるため，今回採用した溶液配合をもとに，濃度および浸漬時間を調整して改善に取り組み，本製法での耐久性の評価を行う予定である。