3He/ 4He dilution refrigerator precooled by Gifford–McMahon cooler II. Measurements of the vibrational heat leak

Abstract

In this paper we describe improvements and changes to a 3He/ 4He dilution refrigerator (DR) which is precooled by a Gifford–McMahon (GM) refrigerator instead of a helium bath cryostat. The general layout of this “dry” DR has been described in detail before. First we report about the installation of two more step heat exchangers into the dilution unit to reduce its final temperature, which was thus lowered from 15 to 8.1 mK. To measure these temperatures we inserted a 3He melting-curve thermometer which is described briefly. The heat leak into the mixing chamber, predominantly caused by vibrations of the GM cooler, was measured with a second dilution unit which was equipped with a double mixing chamber (DMC); the method of measuring the heat leak with a DMC is explained. The vibrational heat leak was found to be 0.14 (±0.02) μW.