Optimum sensitivity of an externally shielded cryogenic current comparator

The sensitivity of an overlapped-tube cryogenic current comparator (CCC) has been calculated including the effect of a cylindrical external shield. Dimensions of the shield, tube and pick-up coil can be optimized to give maximum sensitivity. The sensitivity in the case of a fixed radius shield has been numerically investigated and a CCC based on an optimized design has been constructed. A single sensitivity measurement indicates that the cylindrical-shield model is accurate to within about 20% of the measured value. This is a significant improvement compared with a simplified ring model of the shield or neglecting the shield entirely.