A novel technique for temperature stabilization very near a phase transition: Results for He-II

A novel method for achieving high temperature stability at very small reduced temperatures near a phase transition is described. In this technique, fluctuations in a temperature dependent property of the system were used to regulate the system's temperature. This technique has been implemented in liquid helium just below the superfluid phase transition. In this case, the technique is based on second sound phase locking and uses the fact that the second sound velocity is strongly temperature dependent near the lambda transition. Phase fluctuations of a continuous second sound signal are detected and used to regulate the temperature of the system. The long term temperature stability as inferred from the stability of the second sound velocity is better than 3 X 10(-7) K, at a reduced temperature of less than 10(-5), making the technique particularly useful near the superfluid phase transition. (C) 1996 American Institute of Physics.