A gravity based U-shape natural circulation loop coupled with a cryocooler is proposed to serve as a self-sustaining thermal link at the temperature of liquid helium. Experiments are performed with a constant helium mass inside the system by keeping the loop closed. This thermal link is mainly composed of a condenser/phase separator cooled by the second stage of a 1.5 W-class cryocooler at 4.2 K. Connected to the condenser, a 23 cm high testing tube is heated along its entire length. Liquid helium flows through this tube to the condenser and is recondensed there. Helium in both normal and supercritical regions is encountered in the experiment and the boiling curve is obtained with the heat flux being controlled. The different boiling regimes and hysteresis during the recovery process are observed and analyzed. Heat transfer coefficient is determined in the nucleate boiling regime and compared with existing correlations. The correlation proposed by Kandlikar reproduces the heat transfer coefficient within 30% error in the regime of two-phase nucleate boiling. (C) 2013 Elsevier Ltd. All rights reserved.