A nitrogen triple-point thermal storage unit for cooling a SQUID magnetometer

In order to achieve turnkey operation, we plan to use cryocoolers to cool a SQUID magnetometer system. To minimize the magnetical and mechanical interference from the coolers, we intend to switch them off during the actual measurements. Consequently, a thermal storage unit (TSU) is required with sufficient capacity at an appropriate temperature (<77 K). In a feasibility study, we consider a load of 0.5 W from the SQUID sensor unit and an operating time of 10 h. To account for an increased load caused by the TSU itself, an overall capacity of 15 Wh is aimed at. The nitrogen triple-point is chosen because of the large latent heat involved in the transition from solid to liquid and the corresponding well-suited temperature (63 K). Furthermore, any safety risks involved with the use of nitrogen are small compared to alternatives. To contain the nitrogen, highly porous alumina is used. A structure was made in which layers of copper and porous material alternate, thus establishing a good thermal contact between the nitrogen and the casing of the TSU. Experiments show an overall capacity of the system around 85% of the expected theoretical value. Suggestions for improvements are given so as to arrive at a TSU capacity of 15 Wh.