Superfluid Helium Bath Research Articles

Review of Quench Performance of LHC Main Superconducting Magnets

The regular lattice of the large Hadron collider (LHC) will make use of more than 1600 main magnets and about 7600 corrector magnets, all superconducting and working in pressurized superfluid helium bath. This complex magnet system will fill more than 20 km of the LHC underground tunnel. In this paper an overview of the cold test program and quality assurance plan to qualify all LHC superconducting magnets will be presented. The quench training performance of more than 1100 LHC main dipoles and about 300 main quadrupoles, cold tested to date, will be reviewed. From these results an estimate of the number of quenches that will be required to start operation of the whole machine at nominal energy will be discussed. The energy level at which the machine could be operated at the early phase of the commissioning without being disturbed by training quenches will be addressed. The LHC magnet program required the development of many new tools and techniques for the testing of superconducting magnet coils, magnet protection systems, cryogenics, and instrumentation. This paper will also present a summary of this development work and the results achieved.

Study of the transient thermal wave heat transfer in a channel immersed in a bath of superfluid helium

Thermal wave is a very interesting phenomenon in which heat is transported in the wave mode. It is different from the ordinary Fourier heat transfer discipline in which heat is transported in the diffusive mode. In the present study, transient thermal wave (second sound wave) heat transfer in He II (superfluid helium) is numerically studied. Quantized vortices in He II, a phenomenon related to the superfluid nature, which is an important factor affecting the behavior of the thermal wave has been taken into account. The present results show that the shape of the thermal wave does not deform seriously and the amount of the heat contained in and transported by the thermal wave does not decrease as the thermal wave transmits along the channel when it is free from the quantized vortices; while the shape of the thermal wave starts to deform at the moment of the emission of the thermal wave and the amount of the heat transported by the thermal wave decreases when it is subject to the quantized vortices. The deformation is in stronger magnitude in the case of the larger heat flux. The surplus amount of the heat which cannot be carried away by the thermal wave accumulates in the thermal boundary layer formed by the dense quantized vortices and then is transferred in a diffusion-like mode. It is found that Gorter–Mellink equation is not suitable to describe the transient heat transfer process in He II.

Design and calibration of a cryogenic blackbody calibrator at centimeter wavelengths

We describe the design and calibration of an external cryogenic blackbody calibrator used for the first two flights of the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) instrument. The calibrator consists of a microwave absorber weakly coupled to a superfluid liquid helium bath. Half-wave corrugations viewed 30° off axis reduce the return loss below −35 dB within a compact footprint. Ruthenium oxide resistive thermometers embedded within the absorber monitor the temperature across the face of the calibrator. The thermal calibration transfers the calibration of a reference thermometer to the flight thermometers using the flight thermometer readout system. The calibrator thermometry is stable in time over four years, with statistical uncertainty in the temperature calibration of order 2 mK near 2.7 K, limited primarily by thermal fluctuations in the liquid helium bath. Observations of the superfluid transition demonstrate that the absolute temperature scale is accurate within 0.3 mK.

Superconducting Magnets for the LHC Main Lattice

The main lattice of the Large Hadron Collider (LHC) employs about 1600 main magnets and more than 4000 corrector magnets. All superconducting and working in pressurized superfluid helium bath, these impressive line of magnets fills more than 20 km of the underground tunnel. With almost 70 main dipoles already delivered and 10 main quadrupoles almost completed, we passed the 5% of the production and now all manufacturers have fully entered into series production. In this paper the most critical issues encountered in the ramping up in such a real large scale fabrication is addressed; uniformity of the coil size and of prestress, special welding technique, tolerances on curvature (dipoles) or straightness (quadrupoles) and of the cold mass extremities, harmonic content and, most important, the integrated field uniformity among magnets. The actual limits and the solution for improvements are discussed. Finally a realistic schedule based on actual achievements is presented.

Magnetization steps inZn1−xMnxO:Four largest exchange constants and single-ion anisotropy

Magnetization steps (MST's) from Mn pairs in several single crystals of Zn_(1-x)Mn_xO (0.0056<=x<=0.030, and in one powder (x=0.029), were observed. The largest two exchange constants, J1/kB=-18.2+/-0.5K and J1'/kB=-24.3+/-0.6K, were obtained from large peaks in the differential susceptibility, dM/dH, measured in pulsed magnetic fields, H, up to 500 kOe. These two largest J's are associated with the two inequivalent classes of nearest neighbors (NN's) in the wurtzite structure. The 29% difference between J1 and J1' is substantially larger than 13% in CdS:Mn, and 15% in CdSe:Mn. The pulsed-field data also indicate that, despite the direct contact between the samples and a superfluid-helium bath, substantial departures from thermal equilibrium occurred during the 7.4 ms pulse. The third- and fourth-largest J's were determined from the magnetization M at 20 mK, measured in dc magnetic fields H up to 90 kOe. Both field orientations H||c and H||[10-10] were studied. (The [10-10] direction is perpendicular to the c-axis, [0001].) By definition, neighbors which are not NN's are distant neighbors (DN's). The largest DN exchange constant (third-largest overall), has the value J/kB=-0.543+/-0.005K, and is associated with the DN at r=c. Because this is not the closest DN, this result implies that the J's do not decrease monotonically with the distance r. The second-largest DN exchange constant (fourth-largest overall), has the value J/kB=-0.080 K. It is associated with one of the two classes of neighbors that have a coordination number z=12, but the evidence is insufficient for a definite unique choice. The dependence of M on the direction of H gives D/kB=-0.039+/-0.008K, in fair agreement with -0.031 K from earlier EPR work.

A compact, high-performance continuous magnetic refrigerator for space missions

We present test results of the first adiabatic demagnetization refrigerator (ADR) that produce true continuous cooling at sub-kelvin temperatures. This system uses multiple stages that operate in sequence to cascade heat from a “continuous” stage up to a heat sink. Continuous operation avoids the usual constraints of long hold times and short recycle times that lead to the generally large mass of single-shot ADRs, and allows us to achieve much higher cooling power per unit mass. Our design goal is 10 μW of cooling at 50 mK while rejecting heat to a 6–10 K heat sink. The total cold mass is estimated to be less than 10 kg, including magnetic shielding of each stage. These parameters envelop the requirements for currently planned astronomy missions. The relatively high temperature heat rejection capability allows it to operate with a mechanical cryocooler as part of a cryogen-free, low temperature cooling system. This has the advantages of long mission life and reduced complexity and cost. At present, we have assembled a three-stage ADR that operates with a superfluid helium bath. Additional work is underway to develop magnetocaloric materials that can extend its heat rejection capability up to 10 K. Design, operation and performance of the ADR are discussed.

Thermal wave measurements in superfluid helium II

AbstractIn this paper, a thermal wave in the bath of superfluid helium II is measured by a new type of superconductor temperature sensor under different heat fluxes and bath temperatures, and at the same time, a thermal shock wave is also studied experimentally and theoretically. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 419–425, 2001

Modelling of helium-mediated quench propagation in the LHC prototype test string-1

The Large Hadron Collider (LHC) prototype test string-1, hereafter referred to as the string, is composed of three 10-m long prototype dipole magnets and one 6-m long prototype quadrupole magnet. The magnets are immersed in a pressurized static bath of superfluid helium that is maintained at a pressure of about 1 bar and at a temperature of about 1.9 K. This helium bath constitutes one single hydraulic unit, extending along 42.5 m of the string length. We have measured the triggering of quenches of the string magnets due to the quenching of a single dipole magnet located at the string's extremity, i.e., “quench propagation”. Previously reported measurements enabled to establish that in this configuration the quench propagation is mediated by the helium and not by the inter-magnet bus bar connections [L. Coull, D. Hagedorn, G. Krainz, F. Rodriguez-Mateos, R. Schmidt, Quench propagation tests on the LHC superconducting magnet string, in: S. Myers, A. Pacheco, R. Pascual, C. Petit-Jean-Genaz, J. Poole (Eds.), Fifth European Particle Accelerator Conference – EPAC '96, Sitges, Barcelona, Spain, 10–14 June 1996, IOP, Bristol, 1996; F. Rodriguez-Mateos, R. Schmidt, L. Serio, Thermo-hydraulic quench propagation at the LHC superconducting magnet string, in: D. Dew-Hughes, R.G. Scurlock, J.H.P. Watson (Eds), 17th International Cryogenic Engineering Conference (ICEC-17), Bournemouth, UK, 14–17 July 1998, IOP, Bristol, 1998]. We present a model of helium-mediated quench propagation based on the qualitative conclusions of these two previous papers, and on additional information gained from a dedicated series of quench propagation measurements that were not previously reported. We will discuss the specific mechanisms and their main parameters involved at different timescales of the propagation process, and apply the model to make quantitative predictions.

The 8-T, 1.1 m bore superconducting solenoid for the 40 T hybrid magnet of the Grenoble High Magnetic Field Laboratory

The Grenoble High Magnetic Field Laboratory is developing a 40-T Hybrid Magnet System to support research in steady, high magnetic fields. Oxford Instruments has been selected to supply the superconducting outsert system for the hybrid magnet The scope of supply includes the superconducting magnet, the liquefier, a 1.8 K refrigerator, the cryostat, the power supply and the control-protection system. The superconducting magnet is a 8-T, 1.4 m long and 1.1 m bore NbTi solenoid. It will be operated in a 1.8 K superfluid helium bath at atmospheric pressure In order to develop the magnet, a specific R&D program has been carried out. This paper will describe the principal aspects of the project including the main results obtained to date.

Influences of hydrostatic bath pressure, van der Waals pressure, and fountain pressure on the heat transfer in a bath of superfluid helium

Considering the basic heat transfer rules of superfluid helium and the phase diagram of helium, pressure effects on heat transfer to HeII are especially studied. If the bath pressure is less than λ-pressure (Pλ), special peak heat flux density relations are shown to correlate bath pressure, hydrostatic head, and modified pressure items (van der Waals pressure and fountain pressure). If the bath pressure is greater than λ-pressure (P > Pλ), a generalized formula of peak flux density of HeIIp bath is shown. © 1999 Scripta Technica, Heat Trans Asian Res, 29(1): 15–21, 2000

Influences of hydrostatic bath pressure, van der Waals pressure, and fountain pressure on the heat transfer in a bath of superfluid helium

Influences of hydrostatic bath pressure, van der Waals pressure, and fountain pressure on the heat transfer in a bath of superfluid helium

Heat transfer in electrical insulation of LHC cables cooled with superfluid helium

The electrical insulation of the Large Hadron Collider (LHC) cables constitutes a thermal barrier between the conductor and the superfluid helium bath. This can prevent removal of the heat dissipated in the cable by the current rise in the dipoles or by the beam losses. The main experimental results, obtained with stacks of insulated conductors representing a piece of the actual coil, are given. The mock-ups vary only by the material composition and the structure of the electrical insulation. Analysis of the temperature distribution measured in the conductors as a function of the dissipated heat power makes it possible to determine the dominant heat transfer mode in each type of tested insulation and to classify these according to their permeability to superfluid helium. Thermal numerical modelling of the experimental mock-ups clarifies the heat transfer path in the complex structure of the insulation and enables calculating values of the thermal quantities characteristic of each insulation. The results of these studies have led to the choice of the cable insulation of the LHC magnets.

Two step phase transition model of heat transfer in bath of subcooled superfluid helium

Heat transfer in a bath of subcooled superfluid helium is different if compared with that in other coolants, which is mainly related to the superfluid characteristics and also the three phases of gas helium (GHe), helium I liquid (HeI) and subcooled helium II liquid (HeIIp). A two step phase transition model is developed, in which the interface phenomena of GHe/HeI and HeI/HeII are considered. The model explains experimental results well.

Pressure effect on the heat transfer in bath of superfluid helium

Considering the basic heat transfer rules of superfluid helium and the phase diagram of helium, pressure effects on heat transfer to HeII are especially studied. If the bath pressure is less than λ-Pressure ( P λ ), special peak heat flux density relations are shown to correlate bath pressure, hydrostatic head and modified pressure items (van der Waals pressure and fountain pressure). If the bath pressure is greater than λ-pressure( P>P λ ), a generalized formula of peak flux density of HeIIp bath is shown.

Evidence for intrinsic Kerr bistability of high-Q microsphere resonators in superfluid helium

Quality factors up to 109 have been obtained in the whispering gallery modes (WGMs) of fused silica microspheres immersed in a superfluid helium bath. We have observed a dispersive bistable behaviour of the WGM resonances with a threshold power of 10 μW, due to the intrinsic Kerr nonlinearity of silica. These results open the way to the realization of a thresholdless microlaser and other cavity QED projects with microspheres.

Transient heat transfer from thin wires to a bath of sub-cooled superfluid helium

The temperature rise due to a step-wise pulse current/heat through horizontally immersed thin wires in a bath of sub-cooled superfluid helium (Hellp) is studied. Simplifications are made to establish the mathematical model according to the transient heat transfer characteristics of Hellp. The computer-simulated theoretical results are found to fit the experimental results well.

Angular distribution of rotons generated by alpha particles in superfluid helium: A possible tool for low energy particle detection.

We report measurements of the distribution of rotons generated by \ensuremath{\alpha} particles interacting in a bath of superfluid helium. The roton flux is found to be anisotropic; it is about 4 times larger transverse to the track direction than along it. This asymmetry may provide a powerful tool in particle and astrophysics experiments where sensitivity to low energy recoil track direction is important.

Measurement of steady state heat transfer in a bath of sub-cooled superfluid helium

This paper reports a study of the peak and recovery heat flux densities (qP and qR) in a bath of subcooled superfluid helium (He lip). A RhFe wire (d=51 mu m, L=34.8 mm) was horizontally immersed in a He IIp cryostat, which served as sample, heater and temperature sensor. For steady state measurement a programmed voltage method with a triangle waveform of 20 s was applied. The measured qP and qR values cover the temperature range 1.81-2.1 K at a pressure of 0.1 MPa.

Some Remarks on the Steady State Heat Transfer Measurement in the Bath of Subcooled Superfluid Helium

Peak and recovery heat flux densities (qP, qR) in the bath of subcooled superfluid helium (HeIIp, from 1.81 to 2.1 K at 0.1 MPa). Have been measured, in which a programmed voltage method with a triangle waveform of 20 s is applied, and a RhFe wire (d = 51 μm, L = 34.8 mm) is horizontally immersed in a HeIIp cryostat, which served as sample, heater and temperature sensor.

Peak and recovery heat flux densities in bath of subcooled superfluid helium

This paper describes the measurement of the peak and recovery heat flux densities ( q P, q R) in a bath of subcooled superfluid helium. A RhFe wire ( d = 51 μm, L = 34.8 mm) was horizontally immersed in a He Ilp cryostat, which served as the sample, the heater and the temperature sensor. For the steady state measurement a programmed voltage method with a triangular waveform of 20 s was applied. The measured q P and q R values cover the temperature range 1.81–2.1K at a pressure of 0.1 MPa. On the basis of the heat and mass transfer through phase interfaces, theoretical relations (without geometrical parameters) of q P and q R are derived, which have very wide application range due to the relative critical heat flux density method used for the calculations. The theoretical results fit the experimental results well for both q P and q R.