Low-temperature research laboratories with typical liquid-helium consumption of the order of tens of liters per day have greatly benefited from the recent development of small-scale liquefiers. In general, these liquefiers are based on Gifford-McMahon or pulse-tube closed-cycle refrigerators with a nominal cooling power ranging from 1 to 1.5 W at 4.2 K. The liquefaction rate for these cryocooler-based liquefiers depends on the pressure at which the helium is liquefied, although the final user conditions of the produced liquid helium are always atmospheric pressure and boiling temperature (e.g., 4.2 K at 100 kPa). Here, we show a systematic study on this effect, in which an enhancement in excess of 70% in liquefaction rate is found experimentally for pressures near and above the critical point of helium (220 kPa). We propose that the underlying mechanism for the liquefaction enhancement is based on the increase in cryocooler cooling power with temperature and the decrease of the helium enthalpy with pressure.In recent years, there have been several dramatic liquidhelium shortages worldwide. These shortages, coupled with ever-increasing prices for a liter of liquid helium, have created operational challenges for a wide spectrum of scientific disciplines and technological areas. Liquid helium is, for example, necessary to operate superconducting magnets in medical research centers and hospitals, and is needed in many low-temperature physics, chemistry, and biology research laboratories, where helium recovery and liquefaction infrastructures do not exist [1]. The well-known commercial and industrial liquefaction plants derived from Collins technology [2] are available on the market and have been for many decades, but they are neither economically justified nor efficient when liquidhelium consumption is below 100 l=day. Therefore, there is an increasing need for efficient, small-scale helium liquefiers and helium associated recovery technology [3].During the last decade, several closed-cycle refrigeratorbased small-scale helium liquefiers [4] have been proposed and successfully commercialized as viable alternatives to the Collins liquefiers. In spite of their apparent simplicity, however, these liquefiers are not energy efficient, they suffer from helium losses, and are difficult to operate and transport to and from cryostats requiring helium refills.In an attempt to address these shortcomings, we develop a versatile, small-scale liquefier [5-7] based on technology that achieves better liquefaction rates with lower energy consumption. This paper shows how better liquefaction rates are achieved and describes some fundamental thermodynamics aspects that are behind the advantage of working near and above the critical point.These experiments are performed on a helium liquefier, model ATL160 from Quantum Design, that is schematically shown in Fig. 1. It incorporates a commercial twostage Gifford-McMahon (GM) cryocooler [8] that employs a 7-kW compressor, and has a nominal cooling power of 1.5 W at 4.2 K on the second stage...
