This paper presents a low-voltage 3V single supply analog lock-in amplifier (LIA) for processing small AC sensor signals buried in noise, including those presenting a relative phase with respect to the reference signal. Reference and bias sensor signals are provided by a quadrature oscillator. Experimental results confirm the capability of the proposed lock-in amplifier to effectively recover information from signal to noise ratios below 0.025, with an error below 9%.

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...

The blocking of fine-capillary tubes used as flow impedances in 4 He evaporation cryostats to achieve temperatures below 4.2 K is generally attributed to nitrogen or air impurities entering these tubes from the main bath. The failure of even the most rigorous low-temperature laboratory best practices aimed at eliminating the problem by maintaining the cleanliness of the helium bath and preventing impurities from entering the capillary tubes suggests that a different cause is responsible for the inexplicable reduction of impedance flow. Many low-temperature research laboratories around the world have suffered this nuisance at a considerable financial cost due to the fact that the affected systems have to be warmed to room temperature in order to recover their normal low-temperature operation performance. Here, we propose an underlying physical mechanism responsible for the blockages based upon the freezing of molecular H 2 traces present in the liquid-helium bath. Solid H 2 accumulates at the impedance low-pressure side, and, after some time, it produces a total impedance blockage. The presence of H 2 traces is unavoidable due its occurrence in the natural gas wells where helium is harvested, forcing gas suppliers to specify a lower bound for impurity levels at about 100 ppb even in high-grade helium. In this paper, we present a simple apparatus to detect hydrogen traces present in liquid helium and easily check the quality of the liquid. Finally, we propose a solution to eliminate the hydrogen impurities in small-and large-scale helium recovery plants. The solution has been implemented in several laboratories that previously experienced a chronic occurrence of blocking, eliminating similar occurrences for more than one year.

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Liquid helium is the coldest fluid that exists in nature. By virtue of this fact, any unwanted substance present in liquid helium, that is, any impurity, will be "frozen" and will be in solid form. In practice, these solid impurities can be easily eliminated to obtain "optically clean" liquid. However, even "optically clean" filtered liquid helium may contain a non-negligible quantity of molecular hydrogen. These minute traces of molecular hydrogen are the causes of a known problem worldwide: the blockage of capillary tubes in helium evaporation cryostats. This problem seriously affects a wide range of cryogenic equipment used in low-temperature physics research at a considerable operational cost increase. In this chapter, we propose an underlying mechanism for this effect and provide a definitive solution by means of production of hydrogen-free liquid helium, that is, not only "optically clean" liquid helium but completely "clean" liquid helium. Moreover, basic superfluidity research studies could benefit from the availability of "clean" liquid helium.