M. Thürk

Low Noise Cold Head of a Four-Valve Pulse Tube Refrigerator

We have designed and constructed a split type of a four-valve pulse tube refrigerator (FVPTR) in order to cool high-Tc Superconducting Quantum Interference Devices1 (SQUIDs). A SQUID, in particular, places rigorous upper limits on the tolerable levels of magnetic interference, vibrations and temperature fluctuations. The pressure wave in the system is generated by means of a commercial He-compressor in combination with a rotary valve. In order to protect the cold head from disturbances generated by the valve gear and the compressor, it is useful to separate those parts spatially. Reduction of interference signals from the compressor and the rotary valve is accomplished using flexible tubes between the rotary valve and the cold head. Most parts of the cold head, including the regenerator, were made of non-metallic, electrically insulating, and non-magnetic materials to minimize eddy currents and local magnetic fields, which interfere the sensor directly.

Intrinsic Behaviour of a Four Valve Pulse Tube Refrigerator

Publisher Summary The pulse tube refrigerator containing no moving parts in the cold section is more attractive for higher reliability, simpler construction and lower vibrations of mechanical and electromechanical origin than any other regenerator type refrigerator. This chapter reveals the effects of the oscillating flow phenomena of working gas inside the pulse tube on the thermal behavior of a four valve pulse tube refrigerator (FVPTR) in order to gain a better understanding of the real working conditions. It measures the pressure and temperature oscillations at various points. The oscillation behavior of the gas pressure is quite similar at all positions through the pulse tube. However, the one of the gas temperature changes with the location along the pulse tube dramatically. The amplitude of the temperature oscillation near the center of the pulse tube is larger than that at the ends. The phase of temperature oscillations in comparison to the pressure oscillations shifts over the length of the pulse tube. The observed data of the temperature and pressure behavior were used to identify typical loss mechanism of the FVPTR.

Thermodynamic analysis of an ideal four-valve pulse tube refrigerator

Abstract By applying the first law of thermodynamics for open systems to subvolumes of a four-valve pulse tube refrigerator, we have calculated its ideal cold generating process. The coefficient of performance increases for a decrease in the ratio of high to low pressure. In the limit of a pressure ratio of 1 the FVPTR reaches the Carnot-COP. The pressure ratio is limited to a maximum of 3.17 for the use of rare gases leading to a maximum specific performance of 1.47 W MPa −1 cm −3 Hz −1 with respect to low pressure, pulse tube volume and operating frequency. Using diatomic gases the pressure ratio and the specific performance are 2.64 and 1.12 W MPa −1 cm −3 Hz −1 respectively.

Observation and control of temperature instabilities in a four-valve pulse tube refrigerator

Abstract The use of a pulse tube cryocooler in an application requires temperature stability at the cold end. In our four-valve pulse tube refrigerator we have observed long-term temperature instabilities lasting some days and short-term instabilities lasting some hours or even minutes. Investigations have shown that the latter anomaly is caused by the dc-flow. The negative influence on the stability is due to an additional mass flow (dc-flow) to the cold end of the pulse tube, which results in a parasitic heat input. In this paper we present an actively controlled dc-flow suppression device, which uses a temperature gradient in the regenerator as a control parameter. This device enables us to eliminate the temperature instabilities.

Hot end loss at pulse tube refrigerators

Most pulse tube refrigerators need an additional heat exchanger at the hot end of the working space in contrast to other refrigerators with regenerators. There is a periodic, alternate gas flow which passes through this hot heat exchanger. Due to this fact the hot heat exchanger has to be considered not only with a desired recuperative effect but also with a regenerative effect. This regenerative effect of the hot end heat exchanger acts as a loss. In this contribution the so-called hot end loss is described by means of an orifice pulse tube refrigerator and is compared with that of a four valve pulse tube refrigerator. Experimental investigations on a four valve pulse tube refrigerator verify this loss. An orifice pulse tube refrigerator without hot end loss is presented.

Investigation of energy transport within a pulse tube

A compact Four-Valve Pulse Tube Refrigerator (FVPTR) in U-tube configuration without a reservoir has been built. At present, the cooler provides a minimum temperature of 32 K and100 W of cooling power at 90 K with a nominal input power of 5.6 kW. Experiments were performed to study the special refrigeration mechanisms of the FVPTR. The highly instrumented system that includes gas temperature sensors hot wire anemometers and pressure sensors is used to assess the p-V work and enthalpy flow at the key locations in the pulse tube. The experiments have enabled us to verify the various analytical models of the FVPTR. Based on the first law of thermodynamics for open systems we have estimated the gross refrigeration power for this special type of pulse tube refrigerator. Furthermore our model takes typical loss processes into consideration to analyse the real FVPTR process. These calculations need some assumptions about the real flow behaviour and the time-dependent temperatures within the pulse tube. The accuracy of these assumptions will be checked by our experiments. By using these results a further technical improvement of our FVPTR should be possible.

Noise Reduction of Cryo-Refrigerators

Using miniature closed cycle refrigerators it is possible continuously to extend the operating temperature range of highly sensitive cryogenic sensors like SQUIDs or bolometers down to 10 K. However, it is necessary to reduce the noise influences of the operating system. There has been developed a noise reduced cryo-refrigerator operating in a temperature range higher than 50 K at mechanical and electromagnetic noise levels equal to those of dewar based systems with a temperature stability < 0.1 K depending on its electronic temperature control. At a sensor temperature of 75 K the noise reduced operating time reached more than 30 minutes. Using this refrigerator there have been measured the superconducting qualities of HTSC-Josephsonjunctions in the noise reduced and nonreduced modes for comparison. To improve the operating parameters the development of a cold collector is planned using the latent heat of phase crossovers of cryogenic gases in a closed system. First measurements on a prototype of a latent heat collector have been provided and operating times of about 100 minutes have been realized.

Investigation of a Single Stage Four-Valve Pulse Tube Refrigerator for High Cooling Power

We discuss the optimization of a pulse tube refrigerator for high cooling power. Our approach is to increase the system efficiency by analyzing and reducing the various loss mechanisms. Because stationary losses (such as radiation and thermal conduction in the system) as well as design principles for the regenerator are well understood, our main effort is focused on controlling the flow behaviour of the working gas at the various tube connections between the components. For time resolved measurements of the gas velocity and gas temperature we use hot wire anemometry and thermocouples respectively. The results of this analysis are used to improve the design especially of the cold head heat exchanger and the hot end setup of the pulse tube. Despite the consequent separation of the in-and outlet gas at the hot end of the pulse tube we find a strong hot end loss caused even by very simple flow parallelizing devices at the hot end of the pulse.

Properties of the commercially available magnetoresistive sensor KMZ10A in the temperature range between 50 and 290 K

We have performed experimental investigations in order to determine the sensitivity and the resistance of the KMZ10A magnetoresistive (MR) sensor in the temperature range between 50 and 290 K by using a cryocooler. The sensitivity increases for a decrease of temperature whereas the resistance decreases. At 50 K the MR sensor shows 1.33 times the sensitivity that it does at 290 K. The resistance at 50 K is 0.52 times the resistance at 290 K. Noise measurements performed in a cryoperm shielding at room temperature and at 77 K in liquid nitrogen have shown that the voltage fluctuation in the white noise range of the MR sensor is equal to that of an equivalent resistor leading to a resolution of 44 pT/√Hz at room temperature and 13 pT/√Hz at 77 K.

Demonstration of HTS microwave sub-systems with a pulse tube cryocooler

Abstract A special pulse tube cryocooler was designed and fabricated. Acceleration measurements show that the vibration of this cooler is much smaller than that of a Stirling machine. Two HTS microwave devices have been integrated with this cooler, which operate successfully as practical HTS sub-systems.