Design of an A.C. susceptometer based on a cryocooler

Abstract

This paper describes a simple and low-cost experimental setup intended for low temperature measurements under a time-dependent magnetic field. A typical application of this system is the determination of A.C. susceptibility and flux profiles of high- T c superconductors. The A.C. magnetic field is generated by a sheeted iron-cored electro-magnet. The device is based on a commercial closed-cycle helium refrigerator. A sample holder made of a polycrystalline silicon rod is shown to establish a good thermal contact between the sample and the cold head; it allows temperatures down to 20 K to be achieved. Experimental details are described and illustrative measurements on high- T c superconductors are included. The performances and limitations of this system are discussed.