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Recovering the expansion power in pulse tube cryocooler is of great utility in improving cooling efficiency. Using a second-stage cooler after a primary cooler to produce extra cooling power is an effective way especially when the cooling temperature is not very low. In the configuration, the two coolers are connected by a displacer which is used as a phase shifter. In this paper, experimental investigations were conducted to study this system. Firstly, the performance of the overall system and separated cooler was respectively presented. To better understand the displacer, phase relation, mechanical resistance and displacement were then clarified. In addition, the power consumption distribution of the cascade cryocooler was discussed. Finally, both numerical and experimental comparisons were made on the displacer-type and tube-type cryocooler. The experimental results show that the displacer-type cryocooler has superior performance due to the better phase-modulation capability and less power loss. With the input electric power of 1.9 kW and cooling temperature of 130 K, the overall system achieved a cooling power of 371 W and a relative Carnot efficiency of 24.5%.

Expansion work is generally wasted as heat in a pulse-tube cryocooler and thus represents an obstacle to obtaining higher Carnot efficiency. Recovery of this dissipated power is crucial to improvement of these cooling systems, particularly when the cooling temperature is not very low. In this paper, an efficient cascade cryocooler that is capable of recovering acoustic power is introduced. The cryocooler is composed of two coolers and a displacer unit. The displacer, which fulfills both phase modulation and power transmission roles, is sandwiched in the structure by the two coolers. This means that the expansion work from the first stage cooler can then be used by the second stage cooler. The expansion work of the second stage cooler is much lower than the total input work and it is thus not necessary to recover it. Analyses and experiments were conducted to verify the proposed configuration. At an input power of 1249 W, the cascade cryocooler achieved its highest overall relative Carnot efficiency of 37.2% and a cooling power of 371 W at 130 K. When compared with the performance of a traditional pulse-tube cryocooler, the cooling efficiency was improved by 32%.

A cryogen-free cryostat cooled by a closed cycle cryocooler is compact, can provide uninterrupted long-term operation (up to ten thousand hours) and is suited to temperatures from 3 K to 300 K. Its intrinsic temperature oscillation, however, limits its application in experiments requiring high thermal stability at low temperature (below 77 K). Passive suppression methods are effective but all suffer from drawbacks. We describe a novel, active suppression scheme more efficient than traditional proportional-integral (PI) control. The experimental results show that it can reduce the standard deviation of the temperature oscillation by a further 30% compared with PI feedback. To the best of our knowledge, this is the first time such active suppression of temperature oscillations has been implemented with the cryogen-free cryostat. The results also show, however, that an unwanted lower frequency thermal noise will be generated, which appears to be the limit of the method. Nevertheless, the approach could be used to improve the temperature stability in all cryogen-free cryostats.

A superconducting detector magnet and two sets of interaction region (IR) magnets are adopted in the upgrade project of Beijing Electron and Positron Collider (BEPC II). The operating currents of these magnets differ from 3.6 kA to 24 A. Totally 13 pairs of vapor cooled current leads (VCCL) were designed and fabricated for these magnets. After the successful test experiments, all the VCCL have been installed to BEPC 11 system and operate properly. This paper presents the detail of the design and the experimental behavior of these VCCL. (C) 2008 Elsevier Ltd. All rights reserved.