S. T. P. Boyd

New Paramagnetic Susceptibility Thermometers for Fundamental Physics Measurements

New paramagnetic susceptibility thermometers have been developed for use in fundamental physics missions in earth orbit. These devices use a SQUID magnetometer to measure the variation in the dc magnetization of a thermometric element that consists of a dilute concentration of manganese in a palladium matrix. Near 2.2 K these new PdMn thermometers have demonstrated a temperature resolution of better than 100 pK/√Hz and a time constant of 50 ms when operated with a 50 K/W thermal resistance to the liquid helium sample. These thermometers have been observed to be remarkably stable, with a drift of less than 10 fK/s. The observed power spectral density of the noise from these thermometers is consistent with separate measurements of the device’s time constant and thermal standoff from the bath. Recently these PdMn materials have been made into thin films and microstructures for use in future studies of quantum liquids, and for possible use in a new class of bolometers and radiometers. These thermometers have been integrated into an experimental cell and thermal isolation network that are adequate to keep stray heats stable to within a few picowatts, with no systematic temperature errors greater than 60 pK, over the course of a planned fundamental physics experiment on Earth orbit.

Decoherence Under a Heat Flux Near the Superfluid Transition in 4He

Measurements of heat transport at the transition from perfect thermal superconductivity to nonlinear heat diffusion in pure 4He provide a very sensitive probe of matter wave coherence. Superfluid heat transport is proportional to the product of the superfluid density and the superfluid velocity, which are both directly related to the superfluid order parameter. From dynamic scaling theory, the correlation length near the superfluid transition provides a measure of the length over which phase fluctuations of the order parameter persist. Our measurements suggest that both the hydrostatic pressure variation within the liquid helium column, together with the heat flux Q, limit the otherwise divergent correlation length near the superfluid transition. Future measurements planned for the microgravity laboratory will provide the fast extensive experimental test of a renormalized, field theoretic description of heat transport near the superfluid transition. It will also provide a conclusive experimental study of the influence of hydrostatic pressure effects and dynamical effects on the correlation length. A new class of microgravity experiments is proposed that will permit measurements to within 10 pK of the superfluid transition temperature, allowing an entirely new class of ultra-accurate scientific investigations to be performed.

Development of a Precision Scanning Optical Pulser for Low-Temperature Particle Detectors

Gamma-ray spectroscopy using low-temperature microcalorimeter arrays has demonstrated breakthrough energy resolution, establishing an important new technique for the precise characterization of radioactive samples. However, the extraordinary dynamic range of these pixels, spanning tens of electronvolts to hundreds of kiloelectronvolts, places stringent requirements on the calibration accuracy of each pixel. In this report, we describe the design, development, and initial reflectivity-map testing of a new optical pulser intended for the precise determination of energy calibrations of microcalorimeter pixels and pixel arrays. A high-resolution XYZ scanner combined with 3 K optics achieving a 3-μm focused spot permit in situ imaging and precise placement of the energy deposition on the detector. Absolute absorbed power can be calibrated via a precision measurement of detector thermal stand-off.

OpenSQUID: A Flexible Open-Source Software Framework for the Control of SQUID Electronics

Commercially available computer-controlled SQUID electronics are usually delivered with software providing a basic user interface for adjustment of superconducting quantum interference device (SQUID) tuning parameters, such as bias current, flux offset, and feedback loop settings. However, in a research context it would often be useful to be able to modify this code and/or to have full control over all these parameters from researcher-written software. In the case of the STAR Cryoelectronics PCI/PFL family of SQUID control electronics, the supplied software contains modules for automatic tuning and noise characterization, but does not provide an interface for user code. On the other hand, the Magnicon SQUIDViewer software package includes a public application programming interface, but lacks auto-tuning and noise characterization features. To overcome these and other limitations, we are developing an “open-source” framework for controlling SQUID electronics, which should provide maximal interoperability with user software, a unified user interface for electronics from different manufacturers, and a flexible platform for the rapid development of customized SQUID auto-tuning and other advanced features. We have completed a first implementation for the STAR Cryoelectronics hardware and have made the source code for this ongoing project available to the research community on SourceForge (http://opensquid.sourceforge.net) under the GNU public license.

CW Measurement of the Upward‐Going Temperature Wave in the Helium‐4 Self‐Organized Critical State

We describe the first continuous‐wave (CW) measurements of the upward‐going temperature wave in the self‐organized‐critical (SOC) state which forms in 4He under conditions of downward heat flow near Tλ under gravity. The CW technique permits measurements with extremely low (<1 nK) excitation amplitudes, allows continuous measurement of the wave velocity as the SOC state grows, and has yielded the first quantitative measurements of the attenuation. The CW measurements appear to support predictions for the velocity but disagree with predictions for the attenuation. This new technique may help us understand the underlying mechanism of the SOC state.

Effect of Inhomogeneous Heat Flow on the Enhancement of Heat Capacity in Helium‐II by Counterflow near Tλ

In 2000 Harter et al. reported the first measurements of the enhancement of the heat capacity ΔCQ[equivalent]C(Q)-C(Q=0) of helium-II transporting a heat flux density Q near Tλ. Surprisingly, their measured ΔCQ was ~7–12 times larger than predicted, depending on which theory was assumed. In this report we present a candidate explanation for this discrepancy: unintended heat flux inhomogeneity. Because C(Q) should diverge at a critical heat flux density Qc, homogeneous heat flow is required for an accurate measurement. We present results from numerical analysis of the heat flow in the Harter et al. cell indicating that substantial inhomogeneity occurred. We determine the effect of the inhomogeneity on ΔCQ and find rough agreement with the observed disparity between prediction and measurement.

New propagating mode near the superfluid transition in 4He

Abstract We have observed a new temperature-entropy wave that propagates opposite to the direction of a steady heat flux Q when the helium column is heated from above. Counter-intuitively this new mode, which resembles second sound, propagates on the normal fluid side of the transition, but it exists only when the column of helium is heated from above. Such a new mode had been predicted to exist on the self-organized heat transport state for Q less than about 100 nW / cm 2 . We confirm that this mode exists in this regime, however we also observe that it propagates even when the helium is held away from the self-organized heat transport state.

Onset of superfluidity far from equilibrium: dynamical effects on the correlation length

Abstract Nonlinear heat conduction has recently been measured near the superfluid transition in pure 4He at very low heat flux Q. Since both dynamic effects and gravity limit the divergence of the superfluid correlation length near the transition at low-Q, these measurements must be repeated in the microgravity environment in order to observe the dynamic effects in isolation. Comparison of the microgravity data to similar data obtained on Earth will provide experimental insight into the effect of gravity on this nonlinear conduction region at low heat flux where theoretical predictions are lacking. While some measurement advantages exist in the microgravity laboratory, it is the study of the direct effect of gravity on the nonlinear conduction measurements that motivate the microgravity need.

A new experiment to measure static and dynamic heat flow in helium-II near the superfluid transition

Abstract A brief description is given of the physical context and technical features of a new experiment, presently in development, to study static and dynamic heat flow in helium-II near Tλ. The experiment uses immersed magnetocaloric thermometers with very high resolution ( −11 K / Hz ) and relatively high bandwidth. Heat flow is concentrated in a narrow channel to maintain a volume of thermally resistive helium with novel boundary conditions. This combination of features should be useful for a variety of studies.

Dynamic and Gravitational Effects on the Correlation Volume: Experimental Methods

We report on the design of a new prototype flight instrument that will be used to repeat previous Earth-based measurements of nonlinear heat transport near the superfluid transition in the microgravity laboratory. Since this nonlinear conductivity is associated with dynamic limitations to the divergent correlation length, and since gravitational acceleration also limits the correlation lengths divergence, we anticipate that the nonlinear conductivity will depend strongly on gravitational acceleration. The apparatus, data taking procedure, systematic corrections, and error sources are discussed here.