Design and performances of a shielded liquid helium insert with an optically heated variable temperature stage for the study of low frequency magnetic noise in HTc superconductors

We have designed and built a liquid helium immersion insert which is magnet ically shielded from the ambient magnetic field fluctuations by the use of a superconducting lead can. The insert has a variable temperature stage mad e of sapphire which is optically heated by the light guided from an externa l lamp via a glass rod. The temperature regulation is achieved by the use o f a commercial PID controller: the temperature of the sapphire ranges from 50 K to 100 K with peak to peak fluctuations less than 20 mK over a 3 minut es observation period. The magnetic noise inside the insert has been measur ed with a low T-c dc-SQUID and the white noise level above 200 Hz is less t han 4 fT/root Hz. Furthermore, the magnetic attenuation factor due to the s uperconducting lead can is greater than 26 500 for frequencies above 15 Hz. This cryogenic insert is used for studying low frequency vortex noise in h igh T-c superconducting thin films at different temperatures and the field penetration in the films as a function of the applied magnetic field during its transition state.