Construction of a Cryogenic Magnet for Quasi-Steady High Magnetic Fields

1. 低温磁体的设计与制造(1)线圈：该磁体在液氮温度下工作，由一个外苦线圈和一个内多螺旋线圈组成。在设计磁体时，仔细考虑了电磁力和焦耳散热。(2)电源：将稳压高场磁体的大功率直流电源改造为脉冲操作电路，用于准稳压低温磁体。电流在1或2秒内增加到23ka，并保持在23ka 0-3秒。(3)低温恒温器：液氮从容量为9m3的大储罐中供给，并在低温恒温器内的磁铁线圈中循环。(1)室温激励：线圈的电阻在室温下很大，只有很小的电流可以通过线圈。在此条件下，测试了电源的特性和线圈的加热特性。没有意外的结果。(2)冷却至液氮温度：从300k冷却至77k，耗时10分钟。(3)液氮温度下的励磁：线圈电阻降低到室温时的1/8左右，从而使大电流通过线圈。虽然应该在液氮循环过程中进行激励，但本试验在激励之前停止了循环，通过循环液氮再次冷却被激励加热的线圈。磁体产生的磁场与设计值吻合较好。低温磁体尚未应用于高磁场下物理性质的测量。然而，需要强调的是，本研究的主要目的是阐明准稳态低温磁体的特性。

1. Design and fabrication of cryogenic magnet(1) Coil : This magnet is operated at liquid nitrogen temperature, consisting of an outer Bitter coil and an inner polyhelix coil. The electromagnetic force and the Joule heat dissipation were carefully taken into account for designing the magnet.(2) Power supply : A high-power DC source for steady high field magnets was modified with a pulse operation circuit so as to serve for the quasi-steady cryogenic magnet. The current is increased to 23 KA ror 1 or 2 seconds, and kept at 23 KA for 0-3 seconds.(3) Cryostat : Liquid nitrogen is supplied from a large reservoir of 9 m_3 capacity, and circulated through the magnet coils in a cryostat.2. Test of cryogenic magnet(1) Excitation at room temperature : The resistance of the coils is so large at room temperature that only a small current can be passed through the coils. Under this condition, however, the characteristics of the power supply and the heating of the coils were tested. No unexpected results were obtained.(2) Cooling down to liquid nitrogen temperature : It took 10 minutes to cool down the coils from 300 K to 77 K.(3) Excitation at liquid nitrogen temperature : The coil resistance is reduced to about 1/8 of that at room temperature so that a large current is passed through the coils. Although the excitation should be made during the circulation of liquid nitrogen, the circulation was stopped before the excitation in the present test : the coils heated by the excitation was cooled again by circulating liquid nitrogen. The field produced by the magnet was in good agreement with the designed value. The cryogenic magnet has not been applied for the measurement of physical properties at high fields. It should be emphasized, however, that the main purpose of this research is to clarify the characteristics of the quasi-steady cryogenic magnet.