High-resolution current measurement system using a high-Tc superconductor sampler

We are developing a prototype system for measuring high-frequency current flowing through a room-temperature sample without contact. It is based on a high-T_c superconductor sampler that can measure current waveforms at picosecond and microampere resolution. The sampler chip is housed in a vacuum chamber and is cooled down to its operating temperature by a GM-pulse tube cryocooler. Current flowing through the sample, which is placed just below and outside the vacuum chamber, generates a magnetic field. The sampler can measure this current because a superconducting loop, which includes the comparator junction of the sampler, picks up the magnetic field. Sinusoidal current flowing through a 50-ohm microstrip line was successfully observed up to 7 GHz by the system. Frequency dependence of measurement sensitivity is a problem to overcome for applying it to practical uses