Progress towards a hand portable pulse tube refrigerator for high Tc dc SQUID operation

Compared to Stirling or Gifford-Mac Mahon crycoolers, the pulse tube refrigerator (PTR) has the advantage of having no moving parts in the cold stage. This results in lower levels of vibration, which is more suitable for the design of highly sensitive superconducting magnetometers. We describe a PTR system in which the compressor is connected to a cold part with a semi-flexible one meter long tube, under an average helium gas pressure of 3 MPa. Using a frequency of 30 Hz and a high to low pressure ratio of 1.5, the lowest temperature achieved is 56 K. The cooling power is 200 mW at 77 K with a cooling down time of about of 45 min. A bare YBaCuO dc SQUID has been successfully operated at a temperature close to 77 K. The SQUID was mounted onto a copper block, attached to the cold head of the PTR.