Basic research of cryogenic low contact resistance switches for persistent operation of superconducting coil

在大电流超导线圈的实验装置中，如大螺旋装置，需要能够保持线圈电流恒定。如果可能的话，持续电流操作显然优选于恒定电流控制操作。然而，由于目前还没有一种电流容量达到30 kA级的大电流恒流开关，因此恒流操作是不可能的。因为在超导技术的现阶段，还没有在断开状态下具有几千伏的介电强度和在闭合状态下具有零电阻和几十kA的电流容量的开关。然后选择了普通导电型机械开关的研制。研究结果如下：触点采用铜基材料，触点表面镀铅锡焊料。接触点由鼓形突起（曲率半径：20 mm，长度：20 mm）和平板形成。这些接触器在液氦浴中使用，并且彼此接触然后滑动。焊料镀层的厚度为0.15-0.5mm。接触力为70- 80 kg。在100 A、200 A、300 A时，接触电阻分别为0.01、0.1、0.3微欧，在小于100 A的范围内，接触电阻几乎为零。若用于1H级线圈回路，甩干时间常数为10年。

It is necessary to be able to keep coil currents constant in the experimental devices such as Large Helical Device which has large current superconducting coils. Persistent current operation is obviously preferable to constant current control operation, if possible. However, the persistent current operation is nowadays impossible because there is no such a high current persistent current switch as its current capacity of 30kA class. Because at the present stage of superconducting technology, there are no switches which have dielectric strength of several thousand volts in the open state and zero resistance and current capacity of several ten kA in the closed state. Then we chose development of normal conductive mechanical switches. Present results of are as follows ;The contact points are made of copper base and their contact surface are plating with Pb-Sn solder. The contact points are formed from both a drum shaped projection (radius of curvature : 20mm, length : 20mm) and a flat plate. These contactors are used in the liquid helium bath and contacted and then slides each other. The thickness of the solder plating is 0.15-0.5mm. The contacting force is 70-80kg. Obtained contact resistance are 0.01 micro ohm at 100A,0.1 micro ohm at 200A,0.3 micro ohm at 300A.It seems almost zero ohm in the range less than 100A.If 100 contacts are operated at parallel connection and each contactor is flowed 300A,then 30kA,0.003 micro ohm. If it is used in the circuit of 1H class coil, the dumping time constant is 10 years.