Large Bolometer Arrays with Superconducting NbSi Sensors for Future Space Experiments

New techniques in microelectronics allow to build large arrays of bolometers filling the focal plane of submillimeter and millimeter telescopes. The expected sensitivity increase is the key for the next generation of space experiments in this wavelength range. Superconducting bolometers offer currently the best prospects in terms of sensitivity and multiplexed readout. We present here the developments led in France based on NbSi alloy thermometers. The manufacturing process of a 23 pixel array and the test setup are described.      

Operation of a Wideband Terahertz Superconducting Bolometer Responding to Quantum Cascade Laser Pulses

We make use of a niobium film to produce a micrometric vacuum-bridge superconducting bolometer responding to THz frequency. The bolometer works anywhere in the temperature range 2–7?K, which can be easily reached in helium bath cryostats or closed-cycle cryocoolers. In this work the bolometer is mounted on a pulse tube refrigerator and operated to measure the equivalent noise power (NEP) and the response to fast (μs) terahertz pulses. The NEP above 100?Hz equals that measured in a liquid helium cryostat showing that potential drawbacks related to the use of a pulse tube refrigerator (like mechanical and thermal oscillations, electromagnetic interference, noise) are irrelevant. At low frequency, instead, the pulse tube expansion-compression cycles originate lines at 1?Hz and harmonics in the noise spectrum. The bolometer was illuminated with THz single pulses coming either from a Quantum Cascade Laser operating at liquid nitrogen temperature or from a frequency-multiplied electronic oscillator. The response of the bolometer to the single pulses show that the device can track signals with a rise time as fast as about 450?ns.     

The Development of 5.5 T High Gradient Superconducting Magnetic Separator

A new High Gradient Superconducting Magnetic Separator (HGMS) system was developed to process kaolin. This paper describes the development of a 5.5 T central field, 300 mm room temperature bore, HGMS system. A zero boil-off helium cryostat, a double canisters system, and a PLC (Process Logic Controller) fully automatic control system based on SCADA (Supervisory Control And Data Acquisition) system have been designed for the HGMS. Every part works very well. Some experiments on processing kaolin clay used HGMS have been carried out. This paper will show the detailed development of every part.     

Smoothing of Magnetic Fields for Superconducting Bearings by Multilayers

Losses in magnetic bearings, whether conventional or superconducting, are caused by any magnetic inhomogeneities in the direction of motion. These can be smoothed by inserting a permeable sheet between the magnet and superconductor. However, this will also have the effect of reducing the field of the main magnet, which supplies the levitating force. Drawing on the parallel with shielding, where shields with air-gaps are more effective than solid shields of the same thickness (Rucker in Philos. Mag. 37:95, 1894), the same principle can be applied to smoothing layers. Analytic solutions are easy to obtain by using an anisotropic permeability in the smoothing layer. An air gap makes the normal permeability about unity while maintaining the parallel smoothing permeability which shorts out small wavelength inhomogeneities. For inhomogeneities which are mainly in the direction of motion we can striate the layer so that the in plane permeability is also inhomogeneous. The results are applied to a magnet track made of permanent magnets joined together with an effective gap of 0.1 mm. A suitable ferromagnetic layer can reduce the dip in field by a factor of 53 while only reducing the main field by 3 %.     

Vortex States of a Three-Dimensional Mesoscopic Superconducting Torus in an External Magnetic Field

The time-dependent Ginzburg–Landau equations have been solved numerically by the finite-element method for a three-dimensional mesoscopic superconducting torus. We obtain the different vortex patterns as a function of the applied field perpendicular to its surface. And we find that multivortex states are ground state in a three-dimensional mesoscopic torus. These results show that our approach is effective and useful to interpret experimental data on vortex states in mesoscopic superconductors.     

Development of a Bolometer Detector System for the NIST High Accuracy Infrared Spectrophotometer

A bolometer detector system was developed for the high accuracy infrared spectrophotometer at the National Institute of Standards and Technology to provide maximum sensitivity, spatial uniformity, and linearity of response covering the entire infrared spectral range. The spatial response variation was measured to be within 0.1 %. The linearity of the detector output was measured over three decades of input power. After applying a simple correction procedure, the detector output was found to deviate less than 0.2 % from linear behavior over this range. The noise equivalent power (NEP) of the bolometer system was 6 × 10⁻¹² W/Hz at the frequency of 80 Hz. The detector output 3 dB roll-off frequency was 200 Hz. The detector output was stable to within ± 0.05 % over a 15 min period. These results demonstrate that the bolometer detector system will serve as an excellent detector for the high accuracy infrared spectrophotometer.     

Automatic Bolometer Bridge Using an Adaptive Control Technique for RE Power Measurement

When RF power is measured using a bolometer bridge, generally its accuracy depends on the power level to be measured and the sensitivity of the bolometer mount in use. The former is principally due to the accuracy of bridge current measurement and the latter is due to an imperfection of bridge control. These are the dominant errors of the bridge measurement system. The objective of this paper is to design a bolometer bridge which offers an effective reduction of these errors and simplify its automation. As a result, a system introducing a digital control technique is proposed and tested.     

Driving Mechanism of a Superconducting Magnetic Bearing System

Driving the rotor of a superconducting magnetic bearing without mechanical contact in the optimum conditions is an important task for high operational speeds. For this reason, an alternative eddy current mechanism is proposed to drive the rotor by means of magnetism with a speed higher than that of the driver. The designed driving system provides strong and stable magnetic coupling between the relatively rotating parts. The drag and lift forces between the rotor and driver disc are discussed by considering various conditions, such as the rotor configurations and the saturation of the magnetic field within the conducting material. The overall results indicate that the designed electromechanical driving system has potential solutions for the various applications for magnetic bearings from the effective driving mechanism point of view.     

The Magnetic Moment of a Superconducting Ball in a Axisymmetric Magnetic Field

Journal of Superconductivity and Novel Magnetism - The magnetic moment of a spherical superconductor that is located on the axis of the external axisymmetric magnetic field is found. It is proved...     

Local Study of Magnetic Structures in High-Temperature Superconducting Composites

The results of magneto-optical investigations of the magnetization processes in high-temperature superconducting coated conductor composites are presented. The studies were performed in the temperature range of 4–80 K. It is shown that the re-magnetization of composites in external magnetic field is accompanied by formation and propagation of the magnetic flux annihilation waves (areas with zero magnetic induction). The relationship between penetration depth of annihilation front and the value of external magnetic field at different temperatures as well as temperature dependence of the rate of magnetic flux annihilation front motion are found. The experimental data agree qualitatively with the results of numerical calculations performed by using the Monte Carlo method.     

Interplay of Magnetic and Superconducting Subsystems in Ho-Doped YBCO

Journal of Superconductivity and Novel Magnetism - Superconducting and paramagnetic contributions to the magnetization of polycrystalline Y1?xHoxBa2Cu3O7?δ samples were...     

Testing of a 5.5 T High-Gradient Superconducting Magnetic Separator

A 5.5 T central field high-gradient superconducting magnetic separator (HGMS) has been designed, fabricated, and tested at the Institute of High Energy Physics, Chinese Academy of Sciences. It has been developed for processing kaolin, to increase the brightness or whiteness whether it is for paper or ceramic applications. The HGMS system mainly consists of an NbTi superconducting magnet, a double-canister system, and a PLC (Process Logic Controller) control system. The solenoid magnet, with operating current of 148 A, has a clear bore of 300 mm. By using a G-M cryocooler (35 W/50 K, 1.5 W/4.2 K), the results of the experiment show that the HGMS system can be run in zero boil-off regimes without any additional cooling. A pressure control system for the helium vessel can accurately respond to variations of pressure above the pressure outside the vessel. The system will process kaolin slurry at typically 0.6～2.5 cm/s, resulting in a production rate of approximately 80～120 tones per day (dry basis). The operation is fully automatic, with a simple system for adjusting the process parameters. The HGMS magnet suffered no quench and operated stably during many experiments since January 2012. The details of the design, fabrication, and performance results of the HGMS are presented in this paper.     

Geometrical Barriers and the Growth of Flux Domes in Thin Ideal Superconducting Disks

When an ideal (no bulk pinning) flat type-II superconducting disk is subjected to a perpendicular magnetic field H _a , the first vortex nucleates at the rim when H _a =H ₀, the threshold field, and moves quickly to the center of the disk. As H _a increases above H ₀, additional vortices join the others, and together they produce a domelike field distribution of radius b. In this paper I present analytic solutions for the resulting magnetic-field and sheet-current-density distributions. I show how these distributions vary as b increases with H _a , and I calculate the corresponding field-increasing magnetization.     

Heat Treatment of Two-Turn Nb3Sn Superconducting Coil

A heat treatment system was built at High Magnetic Field Laboratory, Chinese Academy of Sciences (CHMFL) for the heat treatment of large-scale Nb₃Sn superconducting coils and conductors. In order to evaluate the manufacturing technique of the joints for the Nb₃Sn CICC, a two-turn Nb₃Sn superconducting coil with two superconducting joints had been developed. A two-turn coil together with several Nb₃Sn witness samples was heat-treated. Experimental and finite elemental analysis was also discussed. The results of the measurements met the requirements of the magnet design.     

Magnetic Interaction Force and a Couple on a Superconducting Sphere in an Arbitrary Dipole Field

The calculation of the magnetostatic potential and levitation force due to a point magnetic dipole placed in front of a superconducting sphere in the Meissner state is readdressed. Closed-form analytical expression for the scalar potential function that yields the image system for an arbitrarily oriented magnetic dipole located in the vicinity of a superconducting sphere is given. Analytic expression for the lifting or levitation force acting on the sphere is extracted from the solution for a general dipole. A special case of our expression where the initial magnetic dipole makes an angle with the z-axis is derived. Our expression for the force in this particular case shows that a recently obtained result (J. Supercond. Nov. Magn. 21:93–96, 2008) for an arbitrary dipole is incorrect. A brief discussion of another erroneous result (J. Supercond. Nov. Magn. 15:257–262, 2002) for a transverse/tangential dipole–sphere configuration, corrected elsewhere recently, is reproduced. Correct expressions for the interaction energy with some limiting cases are also provided. The result derived here demonstrates that the value of the levitation force for a dipole that makes an angle with z-axis lies between the values for a radial dipole–sphere and transverse dipole–sphere configurations providing upper and lower bounds. It is found that for a magnetic dipole making an angle with z-axis, there exits a second force component along the negative y-direction, which influences a couple acting on the superconducting sphere. It is also shown that the couple is proportional to the second force component and that both the couple and second force components vanish for a radial dipole–sphere and transverse dipole–sphere configurations, respectively. These results appear to be new and have not had received due attention in the context of superconductivity.     

Development of Superconducting Microresonators for a Neutrino Mass Experiment

The determination of the neutrino mass is still an open issue in particle physics. The calorimetric measurement of the energy released in a nuclear beta decay allows to measure all the released energy, except the fraction carried away by the neutrino: a finite neutrino mass m \(_\upnu \) causes the energy spectrum to be truncated at Q  \(-\)  m \(_\upnu \) , where Q is the transition energy. The electron capture of \(^{163}\) Ho (Q \(\sim \) 2.5 keV) results to be an ideal decay. In order to achieve enough statistics, a large number of detectors ( \(\sim \) 10 \(^4\) ) is required. Superconducting microwave microresonators are detectors suitable for large-scale multiplexed frequency domain readout, with theoretical energy and time resolution of \(\sim \) eV and \(\sim \!\upmu \) s. Our aim is to develop arrays of microresonator detectors applicable to the calorimetric measurement of the energy spectra of \(^{163}\) Ho. Currently, a study aimed to the selection of the best design and material for the detectors is in progress. In this contribution, a comparison between the measurements (critical temperature, gap parameter, quasiparticle recombination time and X-ray energy spectra) made with stoichiometric, sub-stoichiometric TiN and Ti/TiN multilayer films are presented.