Helium Bath Temperature Research Articles

Achievement of 26.5 T at 1.8 K and 24.0 T at 4.4 K in a Free Bore of 68-mm Diameter: Successful Commissioning of the HOMER II LTS/HTS High-Field Facility Upgrade

As an upgrade for higher fields, a REBCO high-temperature superconductor (HTS) insert coil with a remarkable cold bore of 68-mm diameter has been constructed with the goal to be routinely operated to attain total fields of 25 T and beyond in HOMER II. HOMER II is the most advanced superconducting high-field facility of the Institute for Technical Physics at the Karlsruhe Institute of Technology. With its basic magnet configuration, consisting of NbTi and (NbX) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn low-temperature superconductor solenoid sections, HOMER II can generate a magnetic field of 20 T in a large bore of 185-mm diameter at a Helium bath temperature of 1.8 K. The REBCO HTS insert coil was manufactured using the rarely implemented concept of layer winding technology and consists of five nested solenoids. In this article, we report on the design of the insert, its construction, and commissioning as well as related aspects. In the initial operation period, magnetic fields of up to 26.5 T were obtained at 1.8 K and 24.0 T at 4.4 K, respectively, without training steps and quenching or other problems.

Advanced Nb3Sn Conductors Tested in Racetrack Coil Configuration for the 11T Dipole Project

Within the 11T dipole magnet conductor development for the high-luminosity LHC upgrade, two types of Nb3 Sn cable based on the powder-in-tube (PIT) and the Rod and Restack processes (RRP) are tested in racetrack configuration in the short model coil (SMC) magnets. In 2016 and 2017, the performances of three different coils wound with 40-strand cables, one made of OST RRP 132/169 and two of PIT 120 strands have been measured using the SMC structure. This paper reports on the main design parameters of the SMC-11T-3 (RRP), and the SMC-11T-4 and SMC-11T-5 (PIT) magnets including details on the instrumentation, applied preload, maximum expected performances, residual resistivity ratio, splice resistances, and inductances. For SMC-11T-3, eight consecutive runs have been performed varying the azimuthal prestress from 150 to 200 MPa with the aim to study the impact of the transverse pressure on the magnet stability and degradation. Only two runs were done for SMC-11T-4 and SMC-11T-5. For the three magnets, the training behavior at both 4.2 and 1.9 K is analyzed in terms of training rate, quench location, maximum quench current and its dependence to the helium bath temperature, the level of transverse prestress, and current ramp rate. The main outcome of the tests on the coil performance is given as conclusion.

BOLUX: A cryogenic electrical-substitution radiometer as high accuracy primary detector in the 150–11,000 eV range

We have measured the electrical and radiometric properties of a cryogenic absolute radiometer BOLUX (Bolomètre pour l’Utilisation dans le domaine de rayons X). BOLUX is intended for use as a primary detector standard for radiant power measurement of a synchrotron beam from EUV to X-ray spectral ranges. The absolute radiometer uses a composite bolometer with a built-in electrical heating element. The bolometer's absorber, optimized for beam diameter and for X-ray absorption, is coupled both to a Ge doped thermometer and to the electrical resistance calibrator with similar thermal paths so that electrical and radiant heating are equivalent. This device operates between 4 K the liquid helium temperature, and 1.25 K obtained by pumping helium. The main advantage of BOLUX over other radiometers is its portability (weight 12 kg) which means it can be used with any synchrotron beamline. The second advantage is its very small time constant (20 ms) compared with other apparatus (typically a few min). We can use it in DC mode or in AC mode and Alpha sources calibration experiments have already been realized. We have measured synchrotron radiant power at the laboratory of the Physikalisch- Technische Bundesanstalt (PTB) at the Berlin electron storage ring BESSY II. We reported experiments in DC mode and showed the possibility to study the drift and the temporal stability at the exit of a synchrotron beamline. For a helium bath temperature of 1.33 K and in the spectral range of 150 eV–11 keV, we have measured a high limit of detection (in 1 s integrated) to be 10 −10 W, with a DC responsivity of 1.3×10 5 V/W, spatial nonuniformity over 4 mm diameter below 1% and possibility to use over five decades between a few nW and 100 μW with a good linearity (<1%). Typically available radiant power of 50 nW can be measured with a standard uncertainty as low as 1%. BOLUX response has been directly compared with qualified photodiodes [1]; we have observed a standard deviation between the two detectors lower than the total uncertainty of the detectors.

The space infrared telescope facility (SIRTF) cryogenic telescope assembly (CTA) cryogenic and thermal system

SIRTF is the last of NASA's great observatories. The thermal function of the CTA is critical to the success of the SIRTF mission. The CTA cryogenic and thermal system will provide the low temperature environment that the telescope and science instruments require to conduct infrared observations. A key issue with the early flight data is the peak helium bath temperature, which depends upon cryostat isolation, the helium vent path, and pre-launch cryostat operations. This paper includes a general description of the CTA thermal/cryogenic system, followed by an update on the cryostat status and a discussion of the analysis and performance testing of the porous plug and helium vent path. The pre-launch cryostat preparations have been demonstrated, and those results are presented. There is also summary of performance from the observatory thermal balance test, which completes the verification of the CTA external thermal system. All of the pieces are in place for a successful SIRTF launch and mission.

The HAWC and SAFIRE adiabatic demagnetization refrigerators

High-resolution airborne wide-band camera (HAWC) and sub-millimeter and far-infrared experiment (SAFIRE) are far-infrared experiments which will fly on the stratospheric observatory for infrared astronomy (SOFIA) aircraft. HAWC's detectors will operate at 0.2 K, while those of SAFIRE will be at 0.1 K. Each instrument will include an adiabatic demagnetization refrigerator (ADR) to cool its detector stage from the liquid helium bath temperature (HAWC's at 4.2 K and SAFIRE's pumped to about 1.3 K) to its operating temperature. Except for the magnets used to achieve the cooling, the magnetic shielding, and a slight difference in the heat switch design, the two ADRs are identical. We describe the ADR design and present the results of performance testing.

Detector system with cryogenic semiconductor and superconductor bolometers

The design and characteristics are reported of a two-channel receiving device based on a semiconductor (Ge:Ga) and a superconductor (Pb+Sn) bolometers operating at deep cooling. The device is intended to detect radiation in the spectral region 50 to 500 μm at the aperture ratio 1:6 and the input hole of the field system 7 mm in diameter. The semiconductor composite bolometer has a noise equivalent power (NEP) of 3·10 −13 WHz − 1 2 at the response time 2·10 −3 s and the helium-bath temperature 1.34 K, and the superconductor one 2.1·10 −12 WHz − 1 2 at 3·10 −6 s and 4.23 K, respectively.

Longitudinal propagation of a normal phase along a superconducting wire and its transient heat evacuation to a normal helium bath

The longitudinal destruction of the superconducting state along a wire by an electrical subcritical current can occur if a strong enough thermal disturbance appears somewhere along the wire. The longitudinal expansion velocity of the normal phase along the wire is then strongly dependent on the different heat production and evacuation processes. As for a wire in a superfluid helium bath, a steady boundary thermal conductance across the wire wall and the normal helium bath can be deduced by adjusting the theoretical data with the experimental data. For the temperature range considered in this paper, the steady boundary thermal conductance seems to be independent of the helium bath temperature but not of the wire surface morphology. It is also shown that the experimental and theoretical data are in strong disagreement unless a transient term in the heat transferred across the wire wall and the liquid He I separating interface is introduced in the theoretical model. A transient heat transfer coefficient is defined, and its dependence on the different wire characteristics is investigated. An empirical relationship between the transient heat transfer coefficient and the wire characteristics is deduced for a given bath temperature. The transient heat transfer coefficient is shown to depend on the maximum thermal flux released into the bath across the wire wall and not on the wire surface morphology.

Normal phase propagation along a current carrying superconducting wire and its heat transfer to a cooling He II bath

A theoretical and experimental study of the longitudinal propagation velocity of a normal phase along a superconducting cylindrical wire suspended in a superfluid helium bath is presented. The theoretical model supposes a moving nonplanar separating boundary between the normal and superconducting phases and takes into account the latent heat absorbed during the destruction of the superconducting state. The comparison of the measured and calculated normal zone propagating velocities versus the circulating electrical currents shows an excellent agreement for all the bath temperatures considered. A boundary thermal conductance across the wire wall and superfluid helium bath interface is inferred by adjusting the calculated velocities with the experimental data. The deduced boundary thermal conductance is similar to the Kapitza conductance in the sense that both are proportional to the superfluid helium bath temperature raised to a constant power. Furthermore the deduced boundary conductance seems unaffected by the dynamical aspect of the longitudinal destruction process of the superconducting state and by the heat flux range across the solid and He II separating boundary covered by the normal zone propagation velocity versus the circulating electrical current plots. To the authors knowledge this is the first study of heat transfer using the data obtained from longitudinal normal zone propagation experiments.

Calculation of normal zone propagating velocities and transient thermal effects dependence on the superconducting wire parameters

Normal phase propagation velocities along superconducting wires immersed in a cooling normal helium bath were investigated experimentally and theoretically. The measurements have been done for bare tin wires with different radii and resistivities. The theoretical calculations of the normal zone propagation velocity versus the circulating current assume a transient mechanism for heat transfer between the wire wall and the normal helium bath. The transient-heat-transfer term is of the form alpha theta /sup 2/ delta theta / delta t, where theta is the temperature relative to the normal helium bath temperature. The transient-heat-transfer coefficient, alpha , is then defined, and good agreement between the theoretical and experimental data is obtained. The coefficient depends on the bath temperature and, nonrandomly, on the maximum power per unit surface released to the normal helium bath, which in turn depends on the wire radius, critical current, and resistivity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The 5 cm aperture dipole studies

The results obtained during the evolution of the design, construction, and testing program of the design B dipole, are presented here. Design B is one of the original three competing designs for the Superconducting Super Collider SSC arc dipoles. The latest design B cross-section The final design parameters were as follows: air cored (less than a few percent of the magnetic field derived from any iron present), aluminum collared, two layered 5.5T maximum operating field, and a 5cm cold aperture. There have been fourteen 64cm long 5cm aperture model dipoles cold tested (at 4.3K and less) in this program so far. There was a half length full size (6m) mechanical analog (M-10) built and tested to check the cryostat's mechanical design under ramping and quench conditions. Several deviations from the 'Tevatron' dipole fabrication technique were incorporated, for example the use of aluminum collars instead of stainless steel. The technique variations explored were dry winding, a technique with the cable covered with Kapton insulation only and wet winding where the Kapton was covered with a light coat of B stage epoxy. Test data include quench currents, field quality (Fourier multipole co-efficients), coil magnetization, conductor current performance, and coil loading. Quench current, loss per cycle, and harmonics were measured as a function of the magnitude and rate of change of the magnetic field, and helium bath temperature.

Heat transfer from metallic filaments (whiskers) into helium under overcritical pressure for temperatures between 3.7 and 7.2 K

The heat transfer coefficient α, near the critical temperature, T cO, was determined for several whiskers from the In-Pb alloy system. For this purpose the hysteresis of the voltage-temperature ( V- T B) transition curves at fixed currents, I, and of the V- I characteristics at fixed helium bath temperature, T B, was determined. The advantage of using measurements made with whiskers is that there is no heat transfer to a substrate and negligible heat transfer to the contracts. The only heat transfer is that to the surrounding helium.

Influence of self-heating on the step structures in the current-voltage characteristic of superconducting films

The current-induced transition between the normal and the superconducting state of Sn, In, and Pb films is investigated by varying the helium bath temperature, film thickness, mean free path, and an external magnetic field oriented parallel to the film surface. The experiments show that, farther fromTc, heating effects play an important role. The heating of the films relative to the helium bath by the liberation of Joule heat in the resistive state and the influence of hotspot formation on the step structures in the current-voltage characteristics of the samples are established with a graphite thermometer. the temperature and the size of the hotspots are determined by the power dissipation, the surface heat transfer, and the heat conduction within the film.

Evaluation of the scattering in critical current and Niobium-Titanium volume fraction along a length of conductor

The knowledge of critical current at any point of a length of a conductor has always been a wish for a coil designer. It is now possible to contemplate this matter without running the whole length at helium bath temperature. From the local resistance per metre of composite measured at room temperature it is possible to derive the cross sectional area of the Niobium-Titanium. We show from samples taken along lengths of different conductors that the cross sectional areas derived from weighing the Niobium-Titanium and from the resistance per metre measurements are in very good agreement and that the current density is constant along a given length. As a result the critical current can be estimated from three measurements: critical current, weight of the Niobium-Titanium in a sample taken at one end and the measurement on line of the resistance per metre.

Critical current anomaly in superconducting proximity-effect bridges

The current-voltage characteristics of long AgSn proximity-effect bridges obtained at helium bath temperature ( T) close to the λ-point of helium ( T λ ) show an abnormal jump in the critical current ( T c ) at T = T λ . It is observed that I c 2 3 varies linearly with T which is similar to what has been reported by previous workers, but the slope changes at T = T λ . The implications of the observed phenomena are examined.

Investigations on unsaturated optical gain spectra of GaAs-GaAlAs-DHS lasers at above-threshold conditions

Directly measured unsaturated gain spectra of GaAs-GaAlAs double heterostructure wafers at above-threshold conditions are reported for different temperatures. Maximum gain values are in the order of 100 cm−1 for room temperature and up to 1000 cm−1 for liquid helium bath temperature. Best fit of the gain spectra is obtained using lineshapes corresponding to band-to-band transitions without validity ofk-selection rule and including tail states. The temperature dependence of the maximum gain agrees with these assumptions.

Infrared Helium cooled bolometers in the presence of background radiation: Optimal parameters and ultimate performances

We discuss in this paper the optimization of the principal parameters for a helium cooled bolometer designed for a given experiment and particularly when the optical background from the warm optics is important. Optimal values of diameter, thermal conductivity between the detector element and the cryostat, time constant, and helium bath temperature are studied. Simple formula for the calculation of photon noise are given. The ultimate performances possible in the present state of art for the three-part bolometer are reviewed in the different cases and compared to the theoretical limits.

Modified heating theory of nonequilibrium superconductors

Most recent experiments on nonequilibrium superconductors have been interpreted in terms of a theoretical model that assumes the recombination time of quasiparticles acts as a bottleneck in the quasiparticle relaxation process. However, in all experiments to date, the actual bottleneck is the escape time of recombination phonons from the superconductor. We show that all previous experiments can be interpreted in terms of a modified heating theory where the quasiparticles remain in both thermal and chemical equilibrium at an effective temperature ${T}^{*}$ greater than the helium-bath temperature. The temperature ${T}^{*}$ is determined from a consideration of only those phonons with energy greater than $2\ensuremath{\Delta}$.

Packaged Tunable L-Band Maser System

A low-noise tunable L-band maser system is described. The maser uses a pink ruby crystal oriented at 90° and is tunable from 850 to 2000 mc. The voltage-gain bandwidth product is as high as 37.5 mc at a liquid helium bath temperature of 1.5°K. An L-band circulator has been developed for use with the maser. It has an insertion loss of 0.3 db, operates over a 200-mc frequency range at L-band, and determines the usable tuning range of the circulator-maser system. The maser and circulator have been packaged into an operational unit that includes all auxiliary components, and has a system noise factor of 0.5 db (35°K). Electrical and mechanical features of the system are described and performance data are given.

Two Types of Regulators and the Precision Control of Helium Bath Temperature

A pneumatic and an electronic regulator for helium bath temperature are described. The pneumatic regulator, an automatic pressure regulating valve with large throughput, gives a simple controller suitable for regulation to a millidegree for temperatures above 1.4°K. The electronic control uses a carbon resistor for the thermometer and a heater inside the bath for the control element. Its regulation is an order of magnitude better, and it is usable at all achievable temperatures. Its theoretical analysis, circuit diagram, construction and operating details, and performance are described. Included are some general remarks and results concerning temperature regulation in He-I and at the lambda point.