ReviewDry dilution refrigerator with 4He-1 K-loop
Introduction
Dilution refrigerators (DR) are the work horses for scientists doing research at milli-kelvin temperatures [1]. We estimate that over 100 new commercial DRs have been manufactured annually worldwide in recent years. Whereas DRs have been pre-cooled by liquid helium cryostats in the past, modern DRs are pre-cooled by pulse tube cryocoolers (PTC). Since our first publication on this subject [2], pulse tube precooled DRs have been commercialized, and today several cryo-engineering firms offer them for sale in different sizes and with useful options for experimentalists. One important feature of cryogen-free DRs is that they offer lots of experimental space in the region of the mixing chamber. The diameter of its mounting plate can be increased up to ∼50 cm and higher as there are only radiation shields and a vacuum jacket with these cryostats and no helium dewar. This version of DR is easy to operate, cost efficient, and can be completely computer controlled.
Besides the traditional areas of research like materials research, neutron scattering or astro-physics, quantum information technology has been a field of great interest in recent years where many DRs are employed. For this application we have constructed a cryogen-free test cryostat where in addition to the dilution circuit a separate 4He loop with a base temperature of about 1 K was installed to increase the cooling power in this temperature range. This loop reaches cooling powers of up to 100 mW in our cryostat which is a factor of 5–10 higher than the cooling power of the still of the DR which usually has to be used for cooling purposes at T ∼ 1 K. Even higher refrigeration powers have been reported for a 1 K-stage which was mounted in a top-loading cryostat for neutron scattering research; here, a cooling power of 230 mW at a temperature of 1.9 K was quoted [3].
Some of the experimental results described in the following have already been presented at various cryo-engineering conferences.
Section snippets
Proposed concepts for cryogen-free DR with 1 K-stage
Cryogen-free DRs with 1 K-stage have been proposed before by Hollister and Woodkraft [4], mainly to cool instrumentation for astronomical and particle physics applications. Three different designs were suggested by these authors to cool a condenser where the 3He flow of the DR is liquefied and where additional refrigeration power is available to cool amplifiers and heat sink cables.
One design uses a pair of charcoal adsorption pumps which are alternately cooled by the second stage of a PTC to T ∼ 4
Configuration 1: DR and 1K-stage separate
In Fig. 1, a cross section of our cryostat is given; a pulse tube cryocooler (PTC) precools the 3He flow of the dilution refrigeration unit and the 4He flow of the 1 K-loop. We chose a small PTC (0.5 W at 4 K), mainly to keep mechanical and acoustic vibrations low, but also to conserve electrical power [5]. The downside was that cooldowns from room temperature took longer with the small PTC (∼20 h). Temperatures of the PTC were measured with a Cernox resistor [6] and PT100 resistors. In contrast to
Top loading dry DR with 1 K-stage
A very different approach to a DR with 1 K-stage has been reported by Kirichek et al. [3], [19]. For neutron scattering applications, they combined a 50 mm top-loading cryogen-free cryostat with a 4He loop where a base temperature of 1.35 K is reached. Samples can be lowered into the cryostat with a sample stick where exchange gas provides the thermal contact to the cryostat insert. Instead of a sample holder, a commercial dilution refrigeration insert can be installed.
The cryostat is cooled by a
Summary and outlook
We have described cryogen-free DRs with separate 4He-loops whose base temperatures were in the vicinity of 1 K; depending on the temperature of their collecting vessel, the refrigeration power was of the order of 100 mW. Two different versions of our cryostat and a top loading cryogen-free 1 K cryostat with a commercial DR insert are described. In the latest version of our DR, the dilution loop was run through a hx in the vessel of the 1 K-loop so that the condensation of the helium gas stream of
Acknowledgements
The author thanks Oxford Instruments for support and B.S. Chandrasekhar and A. Marx for their interest in this work. K. Neumaier made available several calibrated RuOx thermometers for the dilution refrigeration unit.
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