Ultimate current resolution of a cryogenic current comparator

The sensitivity and current resolution of an overlapping tube cryogenic current comparator (CCC) are calculated in terms of an equivalent electrical circuit model. The key features for optimum design are given, and theoretical predictions are compared with experiments. We propose that optimum current resolution will be achieved when the overlapping tube is used as single turn sensing coil directly connected to a SQUID, which must have its input coil inductance matched to that of the overlapping tube. The possibility of including a strip wound ferromagnetic core is also analyzed. It is shown that a "fractional turn loop" sensing coil may enhance the CCC sensitivity above that of the SQUID.