Compact dilution refrigerator with a cryogenic circulation cycle of He3

A He³-He⁴ dilution refrigerator has been constructed which circulates He³ by using two alternately operating adsorption pumps which are in the same cryostat as the dilution refrigerator. The scheme described here organised the low temperature circulation cycle such that the construction and cryostat communications were essentially simplified, the dimensions of the arrangement were decreased which resulted in a circulation velocity of 10^?4 mole s^?1, which was sufficient to obtain a temperature in the order of 10 mK.     

An improved He3-He4 mixing chamber for single-cycle experiments.

A new type of mixing chamber is described with which any dilution refrigerator, capable of reaching 30 mK continuously, can now attain temperatures below 3 mK when operating in the single-cycle mode.In a preliminary experiment this mixing chamber used in a dilution refrigerator with an additional superfluid He⁴ circulation of 2 × 10^?3 moles s^?1 reached a temperature of 4 mK continuously.     

A cryostat for investigating inelastic scattering of neutrons on liquid He4 between 4.2 and 0.5 K

A cryostat is described which is designed for the research of inelastic scattering of neutrons on a large volume (3.5 l) of He⁴ between 4.2 and 0.5 K. Temperatures lower than 1.3 K are produced by means of pumping out vapour above liquid He³ (～ 330 cm³) with an adsorption pump (1 kg of coal CKT-2) located in the device itself. A minimum temperature is held during 140 h. The essential time for the cryostat start is about 10 h. The cryostat was used for the measurement of temperature dependence of the Bose-condensate density in He⁴.     

A dilution refrigerator with large cooling power

A dilution refrigerator with sintered copper heat exchangers and a He³ circulation rate of 1.4 × 10⁻⁴ − 3 × 10⁻⁴ mole s⁻¹ is described. The cooling power at 15 mK is 1μW. Construction of the cryostat and the pumping system is discussed in detail. Heat transfer in the exchangers and in the mixing chamber is discussed and measurements of thermal contact between the mixing chamber and an outside load are presented.     

Cryogenically pumped laboratory metallurgical installation

The paper is concerned with the laboratory metallurgical installation designed for heat treatment and zone melting of refractory metals without a crucible. A 10kW electron-beam gun serves as a heater. The installation is evacuated from the atmospheric pressure to the ultimate vacuum using cryopumps only. Particular attention is given to hydrogen pumping. Performances of helium adsorption and condensation cryopumps with an efficiency up to 10⁴/s^?1 for hydrogen have been studied experimentally. The consumption of liquid helium was found to be 40–50cm³ h^?1 for adsorption pumps and 7cm³h^?1 for condensation pumps. It is shown that the application of the condensation pump with a surface cooled down to 2.3 K seems to be most efficient.     

Practical design of heat exchangers for dilution refrigerators: part 2

A dilution refrigerator with a He³ circulation rate 80–730 μmols^?1 and a minimum temperature of 2.85 mK which is continuously maintained has been built. Discrete heat exchangers with pressed metal (Cu and Ag) powder are used. The heat exchangers are newly developed, based on the analysis of Radebaugh and Siegwarth. The machine is designed for many purposes such as the base of nuclear cooling and studies of general solid state physics. The design, construction and performance of this refrigerator are described.     

The cryopumping of water vapour in the continuum pressure region

Pumping speed measurements in the continuum pressure region, P>10^?3 torr, have been made for water vapour impinging on copper spheres and coils cooled to liquid nitrogen temperatures. Water vapour flow rates between 0.06 mg s^?1 and 420 mg s^?1 were used. The volumetric pumping speed was constant over the pressure range 2 × 10^?3 torr to 2 × 10^?2 torr and was, as expected, higher than that obtained in the free molecular flow region. Above 2 × 10^?2 torr the pumping speed decreased and possible reasons for this were investigated and are discussed. These included the effects of inadequate heat transfer from the liquid nitrogen refrigerant to the cryopump, a poor thermal conductivity of the cryodeposit, and an impurity, nitrogen gas, in the water vapour.     

He4 film flow suppressor for a dilution still and He3 purification

The design and construction of a still for a dilution refrigerator is described. This device yielded an He³He⁴ ratio of almost 300. In addition to its use in a dilution refrigerator the still has been used for He³ purification, where it proved capable of extracting a high proportion of He³ from gas with a large He⁴ contamination.     

A combined 3He-4He dilution refrigerator

A refrigerator capable of operating both in evaporation and dilution modes is described. The refrigerator provides preliminary cooling of investigated samples down to a starting temperature of about 0.3 K and permits the location of samples directly in the mixing chamber as well as their replacement at any stage of operation.The replacement of samples lasts about 20 min. The cooling time for a sample from room temperature down to 15 mK is about 3 h. The minimal temperature is 10 mK, and the ³He circulation rate is about 5 × 10^?4mols^?1.     

Can He3 - He4 dilution refrigerators operate aboard spacecraft?

Cooling to temperatures in the millikelvin range could be beneficial as a means of reducing noise and increasing sensitivity of instruments in certain experiments and measurements that are being considered for future space missions. The possibility of using a He³ - He⁴ dilution refrigerator for that purpose is explored in this paper. Calculations reported here indicate that electrostriction can be substituted for gravity as a practical means of achieving phase separation with well-defined interfaces in mixing chambers of dilution refrigerators operating aboard spacecraft.     

Simultaneous measurements of heat capacity and pressure coefficient of liquid He4 and He3 - He4 mixtures

An apparatus for simultaneous measurements of heat capacities and pressure coefficients was designed and constructed for experimental studies of critical phenomena near the lambda points of liquid He⁴ and He³ - He⁴ mixtures.The pressure was measured with a capacitance pressure sensor involving a copper-beryllium thin plate (0.4 mm thickness). An oscillator circuit, including the capacitance pressure sensor, and driven by a tunnel diode was attached to the bottom of the calorimeter. The resolutions of temperature and pressure in this apparatus are ～ 2 × 10^?7 K and ～2 × 10^?6 bar (～2 × 10^?1 N m^?2) respectively.     

An electronic vapour pressure controller

A system is described in which the vapour pressure of a liquid He⁴ bath is monitored by a capacitance gauge, the output of which is used to control the pumping rate of the vapour pump. The pressure is regulated to an accuracy of 0.1% or better over the range 3–1000 mm Hg and the system is usable down to at least 1 mm Hg. The controlled pressure is completely insensitive to magnetic field.     

Compressional cooling of He3 to millidegree temperatures

A study of adiabatic compressional cooling of a two-phase mixture of solid and liquid He ³ has been made. Details of the experimental arrangement are given. Using a flexible elastic low temperature He ³ cell that is compressed by liquid He ⁴ and precooled by a continuously operating dilution refrigerator, cooling to below 2 m°K was obtained from starting temperatures near 20 m°K. Frictional heating is remarkably low. Calculations are presented which investigate the fraction of solid formed and which suggest rather long thermal equilibrium times in the solid He ³ nuclear spin system. Application of the method to the production of bulk polarized He ³ nuclei is discussed.This research was supported by the U.S. Atomic Energy Commission under contract AT(11-1)-34, P.A. 143.This work is based on the thesis of R. T. Johnson, submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Illinois.     

Enthalpy—pressure (H-p) diagram of He3 in the range 1.0 K ≤ T ≤ 4.17 K and 0 ≤ p ≤ 6.5 atm and inversion curve for T ≤ 4.17 K

The enthalpy-pressure (H-p) diagram of He³ was determined by experiment. All important data for the operation of a continuous He³ refrigerator working on the Joule-Thomson principle can be derived from this diagram. In particular, the achievable refrigerating capacity at preselected operating conditions may be immediately ascertained. The inversion curve of He³ for T ? 4.17 K was determined from the H-p diagram and is in fair agreement with the data for T ? 4 K already known.     

Vapour pressure thermometry in high magnetic fields using a Pirani gauge

Vapour pressure thermometry has the advantage of not being affected by magnetic fields. Therefore, a study was carried out of thermometry below 1 K using a Pirani pressure gauge in the vapour of He³. Its sensitivity and error in temperature measurements above 0.55 K are dV/dT ≥ K^?1 and 0.4% respectively.Additionally, a semiconductor pressure sensor in the vapour of He⁴ was examined for the vapour pressure thermometry.     

Study of a liquid helium jet pump for circulating refrigeration systems

A basic diagram of the refrigeration system, in which liquid helium is circulated by means of a jet pump, is described. The equations have been derived to design jet devices which operate with liquid helium.In the experimental setup with a jet pump the flow rate of liquid was 5 to 10 times larger than that of compressed gas in a direct stream of the refrigerator. After pumping the pressure was equal to (0.15–0.40) × 10⁵Nm^?2. A circulating stream of liquid helium at supercritical pressure, due to the heat load corresponding to the refrigerator capacity, was heated to 0.15–0.25 K.The results of our studies permit one to determine circulation loop parameters and main geometric dimensions of the jet pump.     

Comparison of two pumping speed measuring methods of oil diffusion pumps

This paper presents a comparative study of two pumping speed measurement methods recommended for oil diffusion pumps. The methods used are the test dome method and the conductance tube method with an auxiliary pumping system which is used at very low pressures when the throughput is below 10^?5 Nm s^?1 (～ 0.1 μl s^?1). It is found that, in order to obtain the comparative results in the overlapping pressure region on the speed measurement of oil diffusion pumps by these two methods, the conductance tube in the conductance tube method should be located at a height 1.2 D from the pump mouth when the position of the inlet gauge is at a distance D/2 from the pump flange, i.e. at the same position as recommended in the dome method to obtain the intrinsic speed of the pump.     

SOME MECHANICAL CHARACTERISTICS OF NITROGEN STRUCTURAL STEELS

In this paper, test conditions and microstructural factors have been investigated for newly developed nitrogen steels at load rates and test temperatures suitable for future operating conditions. Static tests have been carried out at ε～ 10^?4 s^-1 and dynamic tests at ε～ 10² s^?1. Different test temperatures enable fracture mechanisms and fracture toughness variations to be studied. Anisotropy of some characteristics in the initial (non-heat-treated) state, together with heat treatment effects and structural changes under two load rates have been analysed. Fracture appearance transition temperature (FATT) values have been obtained from absorbed energy and fracture surface crystallinity values. Using yield strength and fracture toughness data, an estimate has been made of critical crack lengths for unstable fracture.     

A continuous He cryostat with pulse-tube pre-cooling and optical access

We have designed and constructed a continuously operating ³He cryostat with windows for laser ablation and spectroscopy. A two-stage pulse-tube refrigerator cools two platforms to base temperatures of approximately 40 K and 4 K respectively. The platforms are equipped with heat exchangers that cool separate streams of ⁴He and ³He. The ⁴He stream is used to run a thermally isolated evaporative refrigerator with a base temperature of approximately 1.5 K and a cooling power of 20 mW. The ⁴He refrigerator is used to condense ³He, which is used to run a ³He evaporative refrigerator on the experimental cell. The cryostat runs continuously at temperatures from 10 K to 0.4 K with a cooling power of 1.5 mW at 0.5 K.