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Dilution refrigerator system, Ymir

Dilution refrigerators are machines capable of reaching temperatures of a few mK; they are used for a variety of applications requiring ultra-low temperatures such as research in nanoelectronics, superconductivity and super-fluidity. They are available commercially at a price of around 200Keuros, depending on the model and vendor.

We have designed and fabricated a new model of dilution refrigerator (Ymir) using in-house expertise in cryogenics and manufacturing. The main advantage that distinguishes our machine from the commercial ones is that it combines the high cooling power of large dilution refrigerators with a short operation time and low helium consumption. Ymir is designed to be inserted directly into regular 5" transport Dewars, unlike commercial refrigerators, which require cooling of the Dewar at each cycle.

Several technical innovations have been also implemented. To optimise the space in the IVC, a single flat block of copper in which a relatively large hole has been drilled acts both as a pot and as a support plate. The still heater uses an original design: the heating resistor is outside the still, but the heat is transmitted directly to the He liquid through a coiled copper rod thermally isolated with Teflon and fixed in place by CuNi thin tubes. The merit of this design is that it avoids plastic-metal connections (a major source of leaks), thus the machine will be very robust. Finally, the refrigerator has five step heat exchangers in a configuration that allows for cold electronics and filters to be anchored in thermal contact but above the mixing chamber. The last step heat exchanger is the largest; it is placed in the middle of this configuration and it consists of four-sintered silver powder channels drilled in a copper cylinder.

The gas handling system consists of mechanical (both He sealed and normal) and diffusion pumps, as well as a pumping cabinet that we have built ourselves. The mixture is 1/4 3He/4He and is kept in a 80l tank at about 0.6 barr.

The refrigerator has been built and leak tested. As always in cryogenics, leak testing is the most tedious part of the fabrication process. At this stage, we have eliminated all the sources of leaks and we have performed a few successful cooling tests. The refrigerator is currently at the final development stage. What we need to do in the near future is fine-tuning the impedances and the mixture, and adding a Roots still pump.

We intend to use this refrigerator for quantum computing experiments. Due to its low fabrication and operation cost, this machine is perfectly suited for laboratories, which cannot afford to buy a commercial model.

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University of Jyvaskyla
Seminaarinkatu 15; P.O. Box 35
40351 Jyvaskyla
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