Dry dilution refrigerator with 4He-1 K-loop

• Cryogen-free 3He/4He dilution refrigerators with 4He-1 K-loop are described.
• Refrigeration powers and temperatures of the 1 K-loop are presented.
• Construction details and performance of the dilution refrigerators are described.
• A somewhat speculative outlook on possible future improvements is given.

In this article we summarize experimental work on cryogen-free 3He/4He dilution refrigerators which, in addition to the dilution refrigeration circuit, are equipped with a 4He-1 K-stage. This type of DR becomes worth considering when high cooling capacities are needed at T ∼ 1 K to cool cold amplifiers and heat sink cables. In our application, the motivation for the construction of this type of cryostat was to do experiments on superconducting quantum circuits for quantum information technology and quantum simulations. In other work, DRs with 1 K-stage were proposed for astro-physical cryostats. For neutron scattering research, a top-loading cryogen-free DR with 1 K-stage was built which was equipped with a standard commercial dilution refrigeration insert.Cooling powers of up to 100 mW have been reached with our 1 K-stage, but higher refrigeration powers were achieved with more powerful pulse tube cryocoolers and higher 4He circulation rates in the 1 K-loop. Several different versions of a 1 K-loop have been tested in combination with a dilution refrigeration circuit.The lowest temperature of our DR was 4.3 mK.