**5. Clean helium recovery plant**

Up to this point, we have described the impedance blocking problem, and we have shown how a small amount of H2 (*yH*<sup>2</sup> <10−10) is enough to produce the blocking of fine capillary tubes used to achieve temperatures below 4.2 K in helium-pumped cryostats. We have seen that hydrogen is naturally present in raw helium sources. Therefore, the production of hydrogenfree "Clean" helium is necessary to reliably operate cryostats with small impedances for long periods without interruptions. In the following paragraphs, we present a helium recovery plant capable of producing "Clean" helium.

We propose a helium purification and liquefaction system layout using small-scale helium liquefiers based on closed-cycle refrigerators (cryocoolers). The commercial Advanced technology liquefiers (ATLs) [31, 32] have a liquefaction rate of 30 L/Day with a performance of 0.16 (L/h)/kW, close to the performance of industrial size Collins liquefiers (0.5–1.2 (L/h)/kW) [33]. This technology adapts the liquefaction rate to the consumption, it is modular and scalable and it covers needs of consumption from a few liters per day up to liquefaction rates of the Collins industrial technology >240 L/Day.

The purification stage of the "Clean helium" recovery plant proposed is based on a combination of two purification techniques:

