J. C. Wheatley

Principles and methods of low-frequency electric and magnetic measurements using an rf-biased point-contact superconducting device

A comprehensive study has been made of some low-frequency electric and magnetic measurements which can be made using an rf-biased point-contact superconducting device of the symmetric two-hole type introduced by Zimmerman. Section 1 begins with a qualitative description of the phenomena which are observed and utilized in making the measurements. In Section 2 a semiquantitative and semiempirical analysis is made of the weakly superconducting system as it interacts with the rf field. Reasonable agreement between the analysis and experiment is obtained, thereby establishing a basis for optimization of conditions for measurement. Empirical data are also given as needed on coupling-coil characteristics. In Section 3 noise in the measuring system is discussed and the characteristics of the flux-locked loop used in the measurements introduced. It is shown that the total observed noise may be divided into at least three parts: one intrinsic to the sensor, one which reflects voltage noise in the amplifier, and one coupled into the instrument by the measuring circuit. The noise power spectra of the first two are shown to be comparable in an actual system. Below 100 Hz the spectrum, relative to the square of the flux quantum, is white and estimated by 10−8 Hz−1. In Section 4 the characteristics are discussed of a dc voltmeter using a flux-locked loop and having a relatively high input impedance and good noise figure. An experimental study of noise is presented. A particular case with subpicovolt sensitivity limited by Johnson noise is discussed. In Section 5 low-frequency ac measurements of resistance, self-inductance, and mutual inductance are discussed using a novel bridge circuit and the superconducting sensor in a flux-locked loop as null detector. The noise characteristics are described and studied experimentally. Extreme sensitivity is achieved, both in resistance and inductance. A 1 µω resistor may be measured with a precision of 1 part in 105 with subpicowatt power dissipation. Measurements of the mutual inductance due to a 1.6-mg crystal of CMN are described in which the crystal temperature was reduced to the millidegree region. In Section 6 measurements of static magnetization are discussed and the ideas applied to measurement of temperature using milligram quantities of CMN. Measurements using a pair of thermometers, one inside and the other outside the mixing chamber of a dilution refrigerator, are presented. In Section 7 the concept of a device-noise temperature is introduced and shown to be in the microdegree region. A very simple Johnson noise thermometer is then described. Measurements with this thermometer in the mixing chamber of a dilution refrigerator were carried out to 12 mK, the effective device temperature being measured to be +0.8±1.3mK. A qualitative experimental estimate places the actual device temperature well below 1 mdeg. In appendices a technical discussion of the flux-locked loop and of noise is given.

Microchannel crossflow fluid heat exchanger and method for its fabrication

A microchannel crossflow fluid heat exchanger and a method for its fabrication are disclosed. The heat exchanger is formed from a stack of thin metal sheets which are bonded together. The stack consists of alternating slotted and unslotted sheets. Each of the slotted sheets includes multiple parallel slots which form fluid flow channels when sandwiched between the unslotted sheets. Successive slotted sheets in the stack are rotated ninety degrees with respect to one another so as to form two sets of orthogonally extending fluid flow channels which are arranged in a crossflow configuration. The heat exchanger has a high surface to volume ratio, a small dead volume, a high heat transfer coefficient, and is suitable for use with fluids under high pressures. The heat exchanger has particular application in a Stirling engine that utilizes a liquid as the working substance.

An intrinsically irreversible thermoacoustic heat engine

Certain thermoacoustic effects are described which form the basis for a heat engine that is intrinsically irreversible in the sense that it requires thermal lags for its operation. After discussing several acoustical heating and cooling effects, including the behavior of a new structure called a ‘‘thermoacoustic couple,’’ we discuss structures that can be placed in acoustically resonant tubes to produce both substantial heat pumping effects and, for restricted heat inputs, large temperature differences. The results are analyzed quantitatively using a second‐order thermoacoustic theory based on the work of Rott. The qualities of the acoustic engine are generalized to describe a class of intrinsically irreversible heat engines of which the present acoustic engine is a special case. Finally the results of analysis of several idealized intrinsically irreversible engines are presented. These suggest that the efficiency of such engines may be determined primarily by geometry or configuration rather than by temp...

Continuously Operating 4He Evaporation Refrigerator

A simple and compact device was developed to provide continuous, self‐regulating refrigeration at approximately 1.3 K. The temperature of the device remains nearly constant, independent of external power, up to a critical power. For a molar flow rate of 10−4 moles/sec, the refrigerator can absorb 4.5 mW. Such a refrigerator should be suitable for condensing 3He in a 3He–4He dilution refrigerator.

Acoustic cooling engine

An acoustic cooling engine with improved thermal performance and reduced internal losses comprises a compressible fluid contained in a resonant pressure vessel. The fluid has a substantial thermal expansion coefficient and is capable of supporting an acoustic standing wave. A thermodynamic element has first and second ends and is located in the resonant pressure vessel in thermal communication with the fluid. The thermal response of the thermodynamic element to the acoustic standing wave pumps heat from the second end to the first end. The thermodynamic element permits substantial flow of the fluid through the thermodynamic element. An acoustic driver cyclically drives the fluid with an acoustic standing wave. The driver is at a location of maximum acoustic impedance in the resonant pressure vessel and proximate the first end of the thermodynamic element. A hot heat exchanger is adjacent to and in thermal communication with the first end of the thermodynamic element. The hot heat exchanger conducts heat from the first end to portions of the resonant pressure vessel proximate the hot heat exchanger. The hot heat exchanger permits substantial flow of the fluid through the hot heat exchanger. The resonant pressure vessel can include a housing less than one quarter wavelength in length coupled to a reservoir. The housing can include a reduced diameter portion communicating with the reservoir. The frequency of the acoustic driver can be continuously controlled so as to maintain resonance.

Noise thermometry at ultralow temperatures

The mean square Johnson noise currents flowing in a low-temperature seriesL–R circuit are used to measure absolute temperatures in a new method using an rf-biased superconducting magnetometer. In one set of experiments a quantitative study was made of the noise currents in a beryllium copper resistor located in the mixing chamber of a dilution refrigerator in the temperature range 5.4 mK to 4.2 K as a function of the magnetic temperature determined from measurements of the static magnetization of powdered CMN. In this temperature range the measured mean square noise current is found to be linearly proportional toT* with a coefficient which agrees within the experimental accuracy of±3 % with the theoretically predicted value based on the N yquist relation and independent measurements of all necessary calibration factors. A model for a SQUID operated in the flux-locked loop configuration with a partly resistive input circuit is presented and is used in the determination of the “device noise” temperature, which for the above experiment is found to be 0.16±0.02 mK. A calculation is presented of the length of time necessary to average the output of our thermometer in order to achieve any desired precision in the estimation of the mean square Johnson noise currents, and a comparison with the observed precision is made. In a second set of experiments the relationship between the Johnson noise temperature of a copper resistor and the 16-Hz magnetic temperature of two powdered paramagnetic salts, CMN and CDP, each in the shape of a right-circular cylinder with diameter equal to height, located in an adiabatic demagnetization cell, was determined from 2 to 20 mK. The device noise temperature characterizing the measuring system for these experiments was measured to be ∼0.05 mK. A discussion of the possible sources of inaccuracy associated with Johnson noise thermometry is presented. Particular attention is paid to the possibility of a heat leak directly to the noise resistor.

Magnetic behavior of dilute Cu(Mn) alloys at very low temperatures

The static magnetization of dilute Cu(Mn) alloys with Mn concentrations between 4.2 and 106 ppm has been measured between 10 mK and 0.4 K in fields between 1 and 200 Oe. Magnetic ordering is observed and attributed to impurity-impurity interactions. However, a concentration-independent Weiss constant and a decreasing moment per impurity in the low-field, low-temperature limit are evidence for the increasing importance of single-impurity effects with characteristic temperatureTk∼ 2 mK.

PRINCIPLES AND METHODS OF DILUTION REFRIGERATION. II.

This paper is an extension of, and complementary to, an earlier paper by Wheatley, Vilches, and Abel. Some important fundamental questions which lead to degradation in the performance of a dilution refrigerator are considered: namely, convection on the dilute side and superfluid on the concentrated side. Experimental results are presented for a film suppressing still used in conjunction with a continuous-heat exchanger (several designs are considered) and a variable number (0–6) of step-heat exchangers of a copper-foil type with a high surface-area-to-viscous-impedance ratio. The first quantitative measurements of viscous heating on the dilute side are presented as well as measurements of thermal resistance on the dilute and concentrated side. Finally, the properties of the refrigerator under external heat load are considered.

Propagation of fourth sound in superfluid

Pressure waves propagate through a superleak in both

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