Liquid Helium Bath Research Articles

Partial-Insulation Winding Technique for NbTi Coils

In 2010, we reported, for the first time, the no-insulation (NI) winding technique for high-temperature superconducting (HTS) pancake coils. Based on our test results of small NbTi NI coils, reported in 2011, the key benefits, i.e., enhanced mechanical integrity, compactness, and thermal stability, of the NI winding technique, appear intact for low-temperature superconducting (LTS) windings. However, the intrinsic charging delay observed in NI coils, caused by the lack of insulation, is more pronounced in LTS coils of a bare round wire than in HTS pancakes of thin, wide tape. Thus, to significantly reduce the charging delay in LTS coils of a bare round wire, we proposed a partial-insulation (PI) winding technique, a variation of the NI technique. In the PI winding of a bare round wire, a thin insulation sheet is introduced every few layers-note that in the PI winding, there are no turn-to-turn insulations. This paper reports results, experimental and analytical, of the PI winding technique in which bare-round-wire NI and PI coils were prepared to quantify the effects of PI winding technique. Three LTS coils of the identical dimension and magnet constant were wound with 0.4 mm diameter NbTi mono-filament wire and tested in a bath of liquid helium at 4.2 K, respectively, with three winding techniques: insulated (INS); NI; and PI. We analyzed the experimental results by applying an equivalent circuit model that had earlier been successfully applied to another set of experimental results. A graph model of resistance matrix was applied to estimate characteristic resistance of both NI and PI coils.

Optimization of gaseous helium heater for 2 K cryogenic system for VECC’s superconducting electron linac

Niobium superconducting radiofrequency cavities are generally operated at around 2K temperature to achieve a high quality factor by reducing residual surface losses. 2K temperature is produce by lowering down the pressure of the helium by employing a sub-atmospheric vacuum pumping system. The cavities are immersed in liquid helium bath, maintained in the helium chamber. A special heater is optimized for warming up the helium gas coming out from the helium chamber to 300K before it enters the pumping system. Keeping in view the uninterrupted and reliable operation of the superconducting electron linac and safe running of the liquid helium plant, a tubular heat exchanger type of heater is designed. Current is passed through the tubes of the heater so as to let the tube banks themselves act as heating element. He gas, passing through the tubes, absorbs the heat and warms up to the desired temperature. Unlike common notion, it has been observed that heater with longer length could reduce the requirement of the heater power but at the cost of extra pumping power, required to counter balance the excess pressure drop caused by the additional length of the heater. Pressure drop is kept within 50Pa for 2g/s helium flow rate. The whole lot of tubes, divided into 4 bundles, are electrically connected in series so that current rating of the feed-through could be kept within 750A. This paper discusses the methodology used for optimizing the design of the heater.

Current leads for superconducting magnets of ADS injector I

In an ADS injector I, there are five superconducting magnets in each cryomodule. Each superconducting magnet contains a solenoid magnet, a horizontal dipole corrector (HDC), and a vertical dipole corrector (VDC). Six current leads will be required to power the electrical circuits, from room temperature to the 2.1 K liquid helium bath: two leads carry 100 A current for the solenoid magnet while the other four carry 12 A for the HDC and the VDC. This paper presents the principle of current lead optimization, which includes the cooling methods, the choice of material and structure, and the issues for current lead integration.

Mechanical Design and Construction of Superconducting e-Lens Solenoid Magnet System for RHIC Head-on Beam-Beam Compensation

Each 2.6-m long superconducting e-Lens magnet assembly consists of a main solenoid coil and corrector coils mounted concentric to the axis of the solenoid. Fringe field and “antifringe field” solenoid coils are also mounted coaxially at each end of the main solenoid. Due to the high magnetic field of 6 T large interactive forces are generated in the assembly between and within the various magnetic elements. The central field uniformity requirement of ±0.50% and the strict field straightness requirement of ±50 microns over 2.1 m of length provide additional challenges. The coil construction details to meet the design requirements are presented and discussed. The e-Lens coil assemblies are installed in a pressure vessel cooled to 4.5 K in a liquid helium bath. The design of the magnet adequately cools the superconducting coils and the power leads using the available cryogens supplied in the RHIC tunnel. The mechanical design of the magnet structure including thermal considerations is also presented.

Heat Transfer Through the Electrical Insulation of <formula formulatype="inline"><tex Notation="TeX"> $Nb_{3}Sn$</tex></formula> Cables

For the LHC upgrade projects, CERN will rely on the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn technology to build high field dipole and quadrupole superconducting magnets. In the frame of this R&D program, cooling studies are carried out to determine the heat extraction from this new type of accelerator magnets and the relevant quench limits. In this paper, we present and discuss experimental results of heat transfer through the electrical insulation of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cables. A cable-stack was prepared using fiberglass sleeves and CTD-101K impregnation resin. Two different measurement methods were compared, consisting of instrumenting the sample before or after the vacuum impregnation. The tests were performed both in 1.9 K superfluid and 4.2 K liquid helium baths, using different heating configurations. We also present results of the numerical model developed to reproduce the experimental results.

Helium gas bubble trapped in liquid helium in high magnetic field

High magnetic field magnets are used widely in the area of the condensed matter physics, material science, chemistry, geochemistry, and biology at the National High Magnetic Field Laboratory. New high field magnets of state-of-the-art are being pursued and developed at the lab, such as the current developing 32 T, 32 mm bore fully superconducting magnet. Liquid Helium (LHe) is used as the coolant for superconducting magnets or samples tested in a high magnetic field. When the magnetic field reaches a relatively high value the boil-off helium gas bubble generated by heat losses in the cryostat can be trapped in the LHe bath in the region where BzdBz/dz is less than negative 2100 T2/m, instead of floating up to the top of LHe. Then the magnet or sample in the trapped bubble region may lose efficient cooling. In the development of the 32 T magnet, a prototype Yttrium Barium Copper Oxide coil of 6 double pancakes with an inner diameter of 40 mm and an outer diameter of 140 mm was fabricated and tested in a resistive magnet providing a background field of 15 T. The trapped gas bubble was observed in the tests when the prototype coil was ramped up to 7.5 T at a current of 200 A. This letter reports the test results on the trapped gas bubble and the comparison with the analytical results which shows they are in a good agreement.

Homogeneous performance and strain tolerance of long Bi-2223 HTS conductors under hoop stress

Two types of high-strength industrial Bi-2223 conductor, one laminated by copper alloy and the other laminated by stainless steel, have been tested to examine the effect of hoop stress on the transport property. The specimens (∼2 m long) were prepared by winding one layer around a GFRP mandrel and the measurements were made in a liquid helium bath with the hoop stress calculated from the BJR product applied by external magnetic field. A careful measurement wire configuration was necessary to cancel the noise pick-up from the environment for more accurate determination of Ic and n-value. We show for the first time that both conductors showed homogeneous voltage–current characteristics over a long length and degradations with hoop stress occurred uniformly, which is crucial information for the development of HTS magnet technology. The onset of degradation occurred at 200 MPa and 220 MPa, with additional bending stress present from the winding diameter of 108 mm, for copper alloy laminated and stainless steel laminated conductors, respectively. After considering the effect of bending strain, our result agrees well with the previously measured data.

Communication: H-atom reactivity as a function of temperature in solid parahydrogen: The H + N2O reaction

We present low temperature kinetic measurements for the H + N2O association reaction in solid parahydrogen (pH2) at liquid helium temperatures (1-5 K). We synthesize (15)N2(18)O doped pH2 solids via rapid vapor deposition onto an optical substrate attached to the cold tip of a liquid helium bath cryostat. We then subject the solids to short duration 193 nm irradiations to generate H-atoms produced as byproducts of the in situ N2O photodissociation, and monitor the subsequent reaction kinetics using rapid scan FTIR. For reactions initiated in solid pH2 at 4.3 K we observe little to no reaction; however, if we then slowly reduce the temperature of the solid we observe an abrupt onset to the H + N2O → cis-HNNO reaction at temperatures below 2.4 K. This abrupt change in the reaction kinetics is fully reversible as the temperature of the solid pH2 is repeatedly cycled. We speculate that the observed non-Arrhenius behavior (negative activation energy) is related to the stability of the pre-reactive complex between the H-atom and (15)N2(18)O reagents.

Low-Dimensional Confining Structures on the Surface of Helium Films Suspended on Designed Cavities

We investigate the formation of quantum confined structures on the surface of a liquid helium film suspended on a nanostructured substrate. We show theoretically that, by nanostructuring the substrate, it is possible to change the geometry of the liquid helium surface, opening the possibility of designing and controlling the formation of valleys with different shapes. By applying an external electric field perpendicular to the substrate plane, surface electrons can be trapped into these valleys, as in a quantum dot. We investigate how the external parameters, such as the electric field strength and the height of the liquid helium bath, can be tuned to control the energy spectrum of the trapped surface electrons.

Modeling of pulsed heat load in a cryogenic SHe loop using Artificial Neural Networks

The pulsed heat load to the cryoplant is an important issue in the design and operation of tokamaks adopting superconducting (SC) magnets for the magnetic confinement, as the International Thermonuclear Experimental Reactor (ITER). The smoothing of the heat load during plasma operation is being addressed by experiments, e.g. in the HELIOS facility at CEA Grenoble, and simulations. The assessment of the operation of the cryoplant mainly requires the knowledge of the evolution of the heat load to the liquid helium (LHe) baths that are used as interfaces/buffers between the magnets cooling loops and the cryoplant itself. In this paper, an innovative approach based on Artificial Neural Networks (ANNs) is presented, leading to a simplified but fast model of the transient heat load from the magnets to the LHe baths. An ANN model is developed for the HELIOS loop and the resulting network is trained using detailed transient simulations performed with the 4C code, which was previously extensively validated against experimental data from HELIOS. The predictive capability of the (simplified) ANN model is then demonstrated by considering another, independent dataset, not used during the ANN training, and comparing the evolution of the heat load to the LHe bath computed by the ANNs with that obtained from the (detailed) 4C model.

Superconducting Dipole Magnet for SAMURAI Spectrometer

A superconducting dipole magnet for a large-acceptance spectrometer named SAMURAI has been constructed and installed at the RIKEN RI Beam Factory. The important features of the SAMURAI superconducting dipole magnet are a large pole gap, a wide horizontal opening, and a large momentum bite. The magnet is an H-type dipole, having circular superconducting coils and cylindrical pole pieces with a diameter of 2 m and a pole gap of 880 mm. The coils are orderly wound by the wet winding method developed by Toshiba using a Nb/Ti superconducting wire. The upper and lower coils are installed in two separate cryostats and cooled by the liquid helium bath cooling method. Each cryostat has six cryocoolers: one for a coil vessel at 4 K, four for thermal shields, and one for high- TC superconducting power leads. The size of the iron yoke is 6.7 m wide, 3.5 m deep, 4.64 m tall, and the total weight of the magnet is about 650 tons. The maximum magnetic field is 3.08 T at 563 A (1.922 MA turns/coil), which gives a bending power (field integral) of 7.05 Tm. The maximum stored energy amounts to 27.4 MJ and the inductance varies from 396 H to 150 H as the magnetic field increases. The fringe fields are smaller than 5 mT at 0.5 m from the magnet. The construction of the SAMURAI magnet started in 2008 and was completed in June 2011. The commissioning of the SAMURAI spectrometer was successfully performed using RI beams in March 2012.

Long-Term Operation of the Superconducting Triplet Quadrupoles With Small Cryocoolers for BigRIPS In-Flight Separator and RI-Beam Delivery Line at RIKEN

We report long-term operation of the superconducting triplet quadrupoles (STQs) with small cryocoolers for the BigRIPS in-flight separator and RI-beam delivery line. Each STQ is cooled by liquid helium bath cooling method with a small stand-alone cryogenic system in which a Gifford-McMahon (GM)/Joule-Thomson cryocooler and a GM cryocooler are mounted on the cryostat. While the GM/Joule-Thomson cryocooler recondenses evaporating helium gas in the helium vessel, the GM cryocooler cools the 70 K shield and current leads by conduction. We operate 21 STQ systems forming beam lines for more than five years. We report on how we maintain cryocoolers without warming up the magnets. Operational experiences including troubles in the long-term operation are also reported.

Conduction Cooling Test of a Splittable Quadrupole for ILC Cryomodules

A superconducting splittable quadrupole magnet was designed at Fermilab for use in ILC-style cryomodules, in which the magnet is to be assembled around the beam tube to avoid contaminating the ultraclean superconducting RF beam volume. This quadrupole was built and first tested in a liquid helium bath environment at Fermilab, where its quench and magnetic performance were characterized. The device is intended to be cooled by conduction when installed in cryomodules, so a separate test was made at KEK where an appropriate conduction cooling test facility exists. We present results of the thermal performance of the magnet in the conduction cooling mode, and discuss its excitation characteristics in this operating mode.

Low attenuation of GHz Rayleigh-like surface acoustic waves in ZnO/GaAs systems immersed in liquid helium

Low attenuation of Sezawa modes operating at GHz frequencies in ZnO/GaAs systems immersed in liquid helium has been observed. This unexpected behaviour for Rayleigh-like surface acoustic waves (SAWs) is explained in terms of the calculated depth profiles of their acoustic Poynting vectors. This analysis allows reproduction of the experimental dispersion of the attenuation coefficient. In addition, the high attenuation of the Rayleigh mode is compensated by the strengthening provided by the ZnO layer. The introduction of the ZnO film will enable the operation of SAW-driven single-photon sources in GaAs-based systems with the best thermal stability provided by the liquid helium bath.

Field performance of a prototype compact YBCO “annulus” magnet for micro-NMR spectroscopy

A prototype compact annulus YBCO magnet (YP1070) for micro-NMR spectroscopy was constructed and tested at 77K and 4.2K. This paper, for the first time, presents comparison of the 77-K and 4.2-K test results of our annulus magnet. With a 26-mm cold bore, YP1070 was comprised of a stack of 1070 thin YBCO plates, 80-μm thick and either 40-mm or 46-mm square. After 1070 YBCO plates were stacked “optimally” in 214 groups of 5-plate modules, YP1070 was “field-cooled” at 77K after being immersed in a bath of liquid nitrogen (LN2) with background fields of 0.3 and 1T and also at 4.2K in a bath of liquid helium (LHe) with background fields of 2.8 and 5T. In each test, three key NMR magnet field-performance parameters—trapped field strength, spatial field homogeneity, and temporal stability—were measured. At 4.2K, a maximum peak trapped field of 4.0T, equivalent to 170MHz 1H NMR frequency, was achieved with a field homogeneity, within a |z|<2.5mm axial space, of ∼3000ppm. YP1070 achieved its best field homogeneity of 182ppm, though at a reduced trapped field of 2.75T (117MHz). The peak trapped fields at 4.2K were generally ∼10times larger than those at 77K, in direct proportion to ∼10-fold enhancement in superconducting current-carrying capacity of YBCO from 77 to 4.2K. Temporal stabilities of ∼110 and ∼17,500ppm/h measured at 77K, with trapped fields respectively of 0.3 and 1T, show that temporal stability deteriorates with trapped field strength. Also, temporal enhancement of trapped fields at 4.2K was observed and reported here for the first time.

Thermal Conductivity Test of YBCO Coated Conductor Tape Stacks Interleaved With Insulated Stainless Steel Tapes

A 32 Tesla, all-superconducting user magnet, which consists of two high temperature superconductor YBCO inner coils producing a field of 17 T in an low temperature superconductor Nb3Sn and NbTi outer magnet producing a background field of 15 T, is being developed at the National High Magnetic Field Laboratory. The YBCO inner coils are pancake-wound with YBCO coated conductor tapes with an interleaved insulation of sol-gel coated stainless steel tapes. The coils are to be cooled directly in liquid helium bath. Heat losses in the windings, such as ac losses during ramping and heat loss in the internal joints, are supposed to be transferred to the coil external surfaces through heat conduction. Thus, thermal conductivity of the coil structure is critical for the internal cooling of the coil and also quench propagation if any. Thermal conductivity measurements were carried out in the radial direction on stacks of alternating YBCO tapes and stainless steel tapes. This paper presents the test results that showed a very low thermal conductivity in the radial direction. For comparison purposes, calculated thermal conductivities in the axial and azimuthal direction are also presented.

A model superconducting helical undulator fabricated using a small filament, tube-type, multifilamentary Nb3Sn wire

A model helical undulator 250 mm in length, designed and fabricated with a period of 14 mm, was tested in a liquid helium bath. The helical coil was wound with a multifilamentary, tube-type Nb3Sn strand with 217 sub-elements. The coil Ic and the magnetic field in the bore were measured at 4.2 K in a liquid helium bath using a Hall probe. Ic was found to be 842 A, and the bore field reached the design specification of 0.8 T. The transverse bore field was also measured as a function of position along the length of the undulator axis and was seen to exhibit the expected sinusoidal behavior. The measured Ic was compared with that obtained from a crossing of a load line (determined from FEM modeling) with the Ic(B) curve of a short sample Nb3Sn strand. The coil exhibited the same critical current as the short sample.

Stability and Normal Zone Propagation in a 50 Tesla Solenoid Wound of YBCO Coated Conductor Tape—FEM Modeling

We present an analysis of stability, heating and quench propagation in a 50 Tesla solenoid immersed in a liquid helium bath at 4.2 K, obtained by numerical Finite Element Method (FEM) modeling. The solenoid was assumed to be wound of a cable made of YBCO coated conductor tapes. Full heat transfer curve in the whole temperature range (from 4.2 K up to 300 K) for liquid helium, taken from experiment, was used. An anisotropic continuum model of the winding for thermal propagation, with input data taken from experiments, was developed and adopted in computations. Magnetic field and temperature dependent parameters, such as electrical and thermal conductivities, heat capacities etc., also taken from experiments, were considered. We modeled a single-coil 50 T solenoid as well as a segment approach, in which a 34 T winding, segmented radially, is the inner-most part of a system of and NbTi solenoids providing a background field of 16 Tesla. Stress-strain modeling showed that due to a strong anisotropy of in YBCO film, the critical current of the solenoid is not limited by mechanical forces, but by the radial magnetic field component in the winding, i.e. by the field component parallel to c-axis of YBCO film. We found quite a high degree of stability of the 50 Tesla solenoid with existing stagnant normal zones and significant heat conduction across the winding.

HTS Current Leads with Traveling Transition Point

The heat flux into liquid helium bath is evaluated for HTS current leads (CL) without intermediate heat removal intercepts in comparison with conventional CL for two extreme cases: no gas cooling and perfect cooling with evaporated gas. It is shown that in the first case the gain is very low (about 5%) while the gain reaches several tens of percents for perfect gas cooling both for hybrid CL made of copper or brass. Quite similar to usual CL the hybrid CL made of brass are much less sensitive to overloading compared to copper CL. In the optimum case, the considered hybrid CL are about 15% thicker than usual CL. The design simplicity of HTS CL without heat intercepts makes them attractive especially for high voltage applications and laboratory scale superconducting magnets.

Possible Application of NbTi Artificial Pinning Centers Wire for Insertion Devices

Abstract Superconductive insertion devices (IDs) allow higher fields for a given gap and period length compared to the classical permanent magnet IDs. This technological concept enables an increase in the brilliance and/or the photon energy. The workhorse for superconducting magnets are multifilament NbTi wires, which are nowadays also used for superconducting insertion devices. Even higher magnetic fields can be reached by using a conductor with enhanced critical current density.Here, we propose a possible application for superconducting undulators, wound with NbTi wire with artificial pinning centers, developed by SupraMagnetics, Inc. We report the critical current characteristic, Jc(B), of a short wire measured in a liquid helium bath, and the load-line of a racetrack coil, designed to simulate the field configuration on the conductor as in a superconducting undulator. Based on the measured load line, we report on the simulations of the magnetic field on axis and of the spectrum in a third generation light source of a possible undulator wound with a wire having similar properties to the measured one.