4.5-T低溫永磁移頻磁鐵研究

此研究是關於將永久磁鐵排列成平面Halbach陣列結構來設計一磁鐵間隙5 mm，工作溫度在77 K，磁場高達4.5 T的低溫永磁移頻磁鐵。移頻磁鐵是一種高磁場的插件磁鐵，其是由一高磁場的中心磁極與兩個分別安排在中心磁極的上下游，磁場較弱的補償磁極所組成。補償磁極的功能是用來消除電子束經過移頻磁鐵後的橫向偏移，使電子束在離開移頻磁鐵時能與入射前的狀態相同。移頻磁鐵所具有的高磁場能將同步加速器光源的臨界光子能量藍移，以提供光源用戶更高能量的光子進行實驗。設計時，採用了目前最新的稀土永久磁鐵鐠鐵硼與飽和磁場2.2 T的釩鋼作為材料。當PrFeB在低溫77 K時，其殘磁與本質矯頑場能分別高達1.67 T與6200 kA / m，其用來設計低溫永磁移頻磁鐵磁鐵能夠分別大幅提升中心磁場與承受磁路中所必定存在的反向磁場。這裡所設計的低溫永磁移頻磁鐵尺寸分別為高205 mm、長240 mm與寬120 mm。相比之下，其成本要比超導移頻磁鐵來得經濟許多。磁場的模擬與分析是使用有限元素模擬軟體OPERA，模擬的結果中心磁場能達4.54 T，且透過在中心磁極的墊補，中心磁場在橫向40 mm的範圍內均勻度能小於千分之一。電子束的橫向偏移亦可藉由補償磁極墊補來優化，其在橫向位置± 15 mm內入射後所產生的橫向角度偏移與橫向調移大小可優化至分別小於20 μrad以5 × 10-3。低溫永磁移頻磁鐵的磁路中存在反向磁場，在常溫的狀態下組裝時，鐵芯倒角附近的永磁可能會有不可逆的去磁效應。當在鐵芯倒角處與永磁間加入間隙，可以使得該區域的去磁效應減弱。在溫度的影響方面，考慮低溫永磁移頻磁鐵的整體溫度分別處在77 K與85 K的兩種情況，可以估計溫度在77 K至85 K範圍內的中心軸上垂直磁場對縱向的一次積分值的變率約為- 10 G．cm．K-1。而若在橫向上存在溫度梯度，則會產生四極場的成份，此會造成橫向調移大小增加。4.5 T的低溫永磁移頻磁鐵能夠將台灣光子源的臨界光子能量從7.1 keV藍移至27 keV，將可提供高光通量密度的硬X光進行實驗。

關鍵字

移頻磁鐵；插件磁鐵；高磁場；低溫永久磁鐵；鐠鐵硼

並列摘要

A 4.5-T cryogenic permanent wavelength shifter based on a flat Halbach array with 5-mm gap at 77 K has been designed. A wavelength shifter is a kind of high-field insertion device. It is composed of one high-field central pole and two relatively weak-field compensating poles arranged at up-stream and down-stream, respectively. The compensating poles are used to eliminate the deflection angle of an electron beam. A wavelength shifter can extend the critical photon energy to higher region in a synchrotron light source. In this work, the wavelength shifter consists of a PrFeB permanent magnet and vanadium permendur pole of which saturation field is about 2.2 T. The PrFeB has extraordinary performance at 77 K. Its residual flux density and intrinsic coercive field intensity are 1.67 T and 6200 kA / m, respectively. It can enhance the central field and avoid the permanent magnet blocks to become irreversibly demagnetized. The overall dimensions of the magnet array are 205 mm in height, 240 mm in length and 120 mm wide. Compared with a superconducting magnet, the cryogenic permanent wavelength shifter is more economic. The cryogenic permanent wavelength shifter is simulated by finite element method with simulation software, OPERA. It shows the magnitude of central field can attain 4.54 T, and the uniformity of the central field is less than 0.1 % in 40 mm with shimming on the central pole. The transverse angle deflection and the transverse tune shift of the electron beam with shimming on both compensating poles have been optimized to be less than 20 μrad and , respectively. An intrinsic reverse field existing in the magnetic circuit will probably partially demagnetize the PrFeB-blocks during their installation at near 295 K. The large reverse field can be facilitated by chamfering the iron poles near the strongly demagnetized regions. In addition, the temperature effect is also be considered. When the overall cryogenic permanent wavelength shifter at 77 K and 85 K, respectively, the integral field rate with respect to temperature is - 10 G．cm．K-1. And temperature gradient on transverse direction will cause a quadrupole field so that the tune shift will increase. This 4.5-T cryogenic wavelength shifter can shift the critical photon energy to 27 keV from 7.1 keV in Taiwan photon source. It will provide high flux density hard X-rays for experiments.

並列關鍵字

Wavelength shifter ； Insertion device ； High-field ； Cryogenic permanent magnet ； PrFeB

參考文獻

[1] C. S. Hwang, B. Wang, R. Wahrer, H. H. Chen, C. H. Chang, F. Y. Lin, et al., "Design and construction performance of a compact cryogen-free superconducting wavelength shifter," IEEE Transactions on Applied Superconductivity, vol. 12, pp. 686-690, 2002.

[2] K. Halbach, "Design of permanent multipole magnets with oriented rare earth cobalt material," Nuclear Instruments and Methods, vol. 169, pp. 1-10, 2/1/ 1980.

[3] J. Mallinson, "One-sided fluxes -- A magnetic curiosity?," IEEE Transactions on Magnetics, vol. 9, pp. 678-682, 1973.

[4] J. Chavanne, P. Van Vaerenbergh, and P. Elleaume, "A 3 T asymmetric permanent magnet wiggler," Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 421, pp. 352-360, 1/21/ 1999.

[5] H. Wiedemann, Particle Accelerator Physics, 3rd ed.: Springer-Verlag Berlin Heidelberg, 2007.

國際替代計量

4.5-T低溫永磁移頻磁鐵研究

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