Alumina shunt for precooling a cryogen-free ⁴He or ³He refrigerator

Highlights

• Construction of a thermal shunt from sintered alumina to precool a 1 K ³He or ⁴He refrigeration stage in a cryogen-free cryostat.

• Performance of the shunt (cooldown curves of a ⁴He 1 K stage).

• Thermal conductivity of sintered alumina from 300 K to 3 K.

Abstract

In this technical report a cryogen-free 1 K cryostat is described where the pot of the ⁴He refrigeration unit is precooled by the 2nd stage of a pulse tube cryocooler (PTC) from room temperature to T ∼ 3 K via a shunt made from sintered alumina (SA); the total mass of the 1 K stage is 3.5 kg. SA has high thermal conductivity at high temperatures; but below ∼50 K the thermal conductivity drops rapidly, almost following a T³-law. This makes SA an interesting candidate for the construction of a thermal shunt, especially as the heat capacity of metals drops by several orders of magnitude in the temperature range from 300 K to 3 K. At the base temperature of the PTC, the heat conduction of the shunt is so small that the heat leak into the 1 K stage is negligible.

Introduction

Cryogen-free cryostats have become increasingly popular with low temperature experimentalists in recent years mainly due to their ease of use. Usually, cryogen-free refrigerators have only one vacuum space and no inner vacuum can. Therefore, a 1 K stage or a dilution refrigeration stage cannot be precooled by exchange gas in this type of cryostat.

Instead, these cooling stages have to be thermally anchored to the 2nd stage of the PTC on cooldowns by one of the following: a gas heat switch [1], [2], a separate gas precool circuit [3], a mechanical switch [4] or by the pumping line and the mechanical supports of the 1 K-stage (usually stainless steel) [5].

It has been pointed out before that materials showing a T³-behavior at low temperatures are suited to fabricate heat “switches” [10]. Here, experiments are reported on where the “switch” between the PTC and the pot of a 1 K stage is made from a cylinder of SA (Al₂O₃).

In Fig. 1, experimental data of the thermal conductivity κ of SA are shown. The κ-curve shows a maximum at ∼80 K, but drops off steeply at lower temperatures, almost like T³. At the base temperature of the PTC of ∼3 K, κ has dropped by about two orders of magnitude from its maximum. In this figure, new heat conductivity measurements on our own SA sample are included for comparison with older data of other authors. Furthermore, the heat capacity of copper has been included in this figure; it drops by almost 5 orders of magnitude from 300 K to 1 K. This drop is essential for cooling metals efficiently by materials like SA. The SA shunt is a purely passive device and very easy to make and equally easy to use.