AIN ceramic thermal guide for cooling a SQUID and its effect on thermal magnetic noise

†We evaluated the thermal conductivity of AlN ceramic plates as a potential material of a thermal guide for cooling SQUID in cryocooler cooled SQUID systems. We found that the AlN ceramic plate does not show a good thermal conductivity at very low temperature. We tried to enhance its thermal conductivity by attaching a lot of thin insulated copper wires in parallel with each other to cover a back plane of the plate. With this enhancement we succeeded to operate a double relaxation oscillation SQUID (DROS) first-order gradiometer. Comparison of measurement results on white noise level between two cases where DROS was attached to a copper plate thermal guide and where it was attached to a modified AlN thermal guide shows almost 5 time reduction in noise floor with the latter plate.