Initial Cooldown Research Articles

Cryogenic aspects of a 20 MW class low-temperature superconducting generator for the renewables industry

In this paper we give a progress update on the cryogenic design for cooling a 20 MW class, partially superconducting generator with stationary field coils for offshore wind renewables industry [1-2]. This is a continuation of an earlier program on a 10 MW system dating back ten years. Whereas this new power rating increase leads to a radial diameter expansion of the superconducting field coils from 4 to > 9.5 m, it maintains the axial length. We show the design based on this scaled up with enlarged diameter. The field coil size increase asks for higher cooling power that leads to a bigger cold box size to accommodate the cryocoolers. In the design, as many as 8 cryocoolers can be accessed and serviced from the nacelle that houses the main cryogenic components. A typical thermal load balance sheet with all components is given and compared against the available cryocooler cooling power at different operating conditions. Due to the cryocoolers’ local point of contact cooling within the nacelle, the temperature gradient of the thermal shield that fully encloses the cold mass with its embedded field coils needs to be balanced out to minimize the heat burden on the field coils. This requires additional analytical efforts and implementation of further design features. The extended nacelle houses the cold box with its cryogenic infrastructure. The interface for the envisaged cryogenic pushbutton closed-loop circulating system remains invisible, requires no handling of cryogenic liquids and is hermetically closed. The field coil diameter increase also leads to a greater initial helium gas storage volume. In addition, it requires higher initial room temperature fill pressure within the toroidal helium vapor storage tanks and in cooling tubes connecting to the cryocoolers. The toroidal helium gas tanks are thermally coupled with the thermal shield so that helium convection inside the storage tank can improve heat transfer and reduce the temperature gradient of the thermal shield. Besides those heat transfer challenges, additional mechanical strain within this large structure is exerted on the torque tubes during initial cooldown and when energizing field coils. Some of those design challenges are quite unexpected, leading to novel workarounds in order to maintain the chosen cooling strategy. Finally, we assess those design limitations in view of further cryogenic scalability with emphasis on manufacturability and assembly.

Optimization of thermal link for a conduction-cooled superconducting magnet system

The thermal link between the second stage of cryocooler and the superconducting magnet has a significant influence on the initial cooldown time of conduction-cooled superconducting magnet. In this paper, a numerical investigation is carried out on the initial cooldown characteristics of a superconducting magnet system used flexible copper strap as the thermal link. The optimal shape ratio (cross-sectional area to length) of the copper strap is obtained to minimize the cooldown time. The results show that optimal shape ratio decreases with increasing the length. In addition, the effect of copper purity on the cooldown time is little at the optimal shape ratio or at RRR (Residual Resistive Ratio) > 100.

Improving the cooldown times for next-generation cryocooled gravitational-wave interferometers

We propose and test an exchange gas technique for improving the cooldown times of cryocooled gravitational-wave interferometers. The technique works by utilizing low-pressure dry nitrogen gas to create a path for heat conduction to test masses while protecting the rest of the in-vacuum equipment from unwanted heat leakage. We show that the technique is capable of shortening the total wait time to reach the operating temperature by a factor of 3.5. Additionally, our tests show that the improvement in the heat transfer rate can be predicted to be within 10% error by using the Sherman-Lees interpolation equation. The technique is compatible with vibration isolation requirements of the cryogenic shielding of 124 K silicon interferometers and has the potential to improve the iteration time for research and development. The scalability of the prototype, the ability to predict the heat conduction, and the simplicity of the engineering make the strategy a good candidate to be included in the cryogenic design of future cryocooled gravitational-wave interferometers. The findings mark a first step in the investigation for a strategy to mitigate ice formation on the interferometer optics during initial cooldown.

A superconductor free of quasiparticles for seconds

Superconducting devices, based on the Cooper pairing of electrons, play an important role in existing and emergent technologies, ranging from radiation detectors to quantum computers. Their performance is limited by spurious quasiparticle excitations formed from broken Cooper pairs. Efforts to achieve ultra-low quasiparticle densities have reached time-averaged numbers of excitations on the order of one in state-of-the-art devices. However, the dynamics of the quasiparticle population as well as the time scales for adding and removing individual excitations remain largely unexplored. Here, we experimentally demonstrate a superconductor completely free of quasiparticles for periods lasting up to seconds. We monitor the quasiparticle number on a mesoscopic superconductor in real time by measuring the charge tunneling to a normal metal contact. Quiet, excitation-free periods are interrupted by random-in-time Cooper pair breaking events, followed by a burst of charge tunneling within a millisecond. Our results demonstrate the possibility of operating devices without quasiparticles with potentially improved performance. In addition, our experiment probes the origins of nonequilibrium quasiparticles in our device; the decay of the Cooper pair breaking rate over several weeks following the initial cooldown rules out processes arising from cosmic or long-lived radioactive sources.

High-Qoperation of superconducting rf cavities: Potential impact of thermocurrents on the rf surface resistance

For many new accelerator applications, superconducting radio frequency (SRF) systems are the enabling technology. In particular for CW applications, much effort is being expended to minimize the power dissipation (surface resistance) of niobium cavities. Starting in 2009, we suggested a means of reducing the residual resistance by performing a thermal cycle [1], a procedure of warming up a cavity after initial cooldown to about 20K and cooling it down again. In subsequent studies [2], this technique was used to manipulate the residual resistance by more than a factor of 2. It was postulated that thermocurrents during cooldown generate additional trapped magnetic flux that impacts the cavity quality factor. Here, we present a more extensive study that includes measurements of two additional passband modes and that confirms the effect. In this paper, we also discuss simulations that support the claim. While the layout of the cavity LHe tank system is cylindrically symmetric, we show that the temperature dependence of the material parameters results in a non-symmetric current distribution. Hence a significant amount of magnetic flux can be generated at the RF surface.

Development of a 1 L/h Scale Liquid Hydrogen System

A small-scale hydrogen liquefaction plant consists of cryocooler, cryostat, liquid nitrogen (LN2) precooling bath, two ortho–para (o–p) hydrogen converters, a heat pipe, multilayer insulations (MLI), sensors and auxiliary components. The liquefaction plant was designed as both a cryocooler-cooled liquefier and a long-term zero boil-off liquid storage reservoir. A Gifford–McMahon (GM) cryocooler was selected as a cold sink and purchased from Cryomech Inc. A vacuum-jacketed 200 L capacity cryostat was designed to provide highly efficient thermal protection to LH2 storage space. A heat pipe was uniquely designed and installed in the cryostat to enhance heat transfer between the cryocooler and hydrogen gas and liquid. A LN2-cooled precooling bath was designed to act as a precooler and an o–p hydrogen converter. MLT blankets with hydrogen getter under high vacuum in the cryostat minimize heat loss into LH2 storage. All the components were carefully assembled in dust, oil and solvent-free environment with extra caution of vacuum tightness followed by dry nitrogen and helium purge. After initial cool-down of the cryocooler under vacuum, 99.999% of 300 K hydrogen gas was introduced to the liquefaction plant. From a series of liquefaction tests with or without LN2 precooling, the hydrogen liquefaction plant successfully demonstrated 1.4 L/h of hydrogen liquefaction with LN2 precooling and two o–p hydrogen converters, and 0.7 L/h without LN2 precooling.

Fabrication and Test of an 8-T Superconducting Split Magnet System With Large Crossing Warm Bore

A conduction-cooled superconducting split magnet system with large crossing warm bore has been successfully constructed in our laboratory for material processing applications. The magnet system design was described in a previous paper. The magnet is composed of six NbTi low temperature superconducting coils, which generate 5.5-T central magnetic field and two Bi2223/Ag high temperature superconducting (HTS) insert coils, which generate 2.5-T central magnetic field and assembled in the form of split coil groups. The magnet has a 136-mm split gap to accommodate the crossing warm bore of 100 mm in diameter. The magnet system is cooled by two GM cryocoolers. The initial cooldown takes 9.2 days and the final temperature of the magnet is about 4.0 K. The HTS coils and NbTi coils are to be operated in the driven mode with two independent power supplies, under the operating currents of 200 A (HTS) and 139 A (NbTi), respectively. The magnet is successfully powered up to 8 T with a the ramp time of 290 min. In this paper, the fabrication and test of the superconducting split magnet system are presented.

Reduction of helium loss from a superconducting accelerating cavity during initial cool-down and cryostat exchange by pre-cooling the re-condensing cryostat

A Zero-Boil-Off (ZBO) cryostat is designed to realize a compact, stand-alone cryogenic system for the AIST superconducting accelerator (SCA). Under normal operation there is no evaporative helium loss from the cryomodule and therefore operating costs associated with the supply of liquid helium can be eliminated. The only significant loss of helium from the module occurs during the initial cavity cool-down procedure or when the re-condensing cryostat is replaced. It takes about 3h to cool down the cryostat head from room temperature (300K) to 4K. During this time around 100L of liquid helium is lost due to evaporation. By pre-cooling the cryostat inside a low heat load vacuum tube before transfer to the cryomodule, this evaporative loss could be essentially eliminated, significantly reducing the volume of liquid helium required for the initial cryomodule cool-down. The pre-cooling system also provides an efficient method to test the cryostat prior to use.

Performance Test of Cooling System for 500 m HTS Cable in KEPCO Power Grid

The high temperature superconducting (HTS) power cable has been successfully installed and operated for feasibility confirmation since 2006 at the power testing center of Korea Electric Power Corporation (KEPCO). The three phase 100 m long HTS cable with 22.9 kV/1.25 kA was manufactured and cooled by the open-loop of sub-cooled liquid nitrogen generated by a decompression cooling unit. No major fault has been observed during long-term operating test. As a next step, the design study of a 500 m long HTS cable system has initiated in 2008. Now, the HTS power cable was manufactured and installed at Icheon substation located near Seoul. In this paper, the cooling performance test of the 500 m long HTS cable in the real power grid is reported. Pre-cooling process using nitrogen gas is presented and the result of initial cool-down of the HTS cable is reported. Also, temperature as well as pressure drop in the system is discussed with respect to the loading current level.

Nanofriction of adsorbed monolayers on superconducting lead

With a quartz crystal microbalance (QCM) we have studied the nanofriction of neon and other simple gases deposited on lead at low temperature. Special care has been paid to the investigation of some artifacts in the QCM response encountered after the initial cooldown. Extensive results obtained with QCMs that have been thermally annealed confirm that Ne films slide on Pb below 10 K. However, no extra dissipation is observed when the Pb electrode is driven either magnetically or thermally into the metallic state. This means that the electronic contribution to friction is negligible for this system, probably, because of the small polarizability of Ne atoms. More polarizable adsorbates like N${}_{2}$, Kr, and Xe are instead found to be completely pinned to Pb below 10 K.

Initial Cooldown Characteristic of Conduction-Cooled High Field Magnet System

Cooldown time is one of the major factors in the conduction-cooled superconducting magnet system. Cooldown time means a time cooling a superconducting magnet from room temperature to cryogenic temperature within a stipulated amount of time. The estimation of cooldown time seeks the elapsed time to cool the superconducting magnet by cryocooler during initial cooldown process. This procedure includes the modeling of properties of low temperature superconductor, the dimensions of superconducting magnet, the cryogenic loads, the thermal link between magnet and cryocooler, and the available refrigerating capacity of cryocooler. This method is applied to the conduction-cooled superconducting magnet system developed by ourselves for accuracy confirmation. The effects of magnetic field, size of magnet and thermal link on the initial cooldown are also discussed.

Aerogel insulation systems for space launch applications

New developments in materials science in the areas of solution gelation processes and nanotechnology have led to the recent commercial production of aerogels. Concurrent with these advancements has been the development of new approaches to cryogenic thermal insulation systems. For example, thermal and physical characterizations of aerogel beads under cryogenic-vacuum conditions have been performed at the Cryogenics Test Laboratory of the NASA Kennedy Space Center. Aerogel-based insulation system demonstrations have also been conducted to improve performance for space launch applications. Subscale cryopumping experiments show the thermal insulating ability of these fully breathable nanoporous materials. For a properly executed thermal insulation system, these breathable aerogel systems are shown to not cryopump beyond the initial cooldown and thermal stabilization phase. New applications are being developed to augment the thermal protection systems of space launch vehicles, including the Space Shuttle External Tank. These applications include a cold-boundary temperature of 90 K with an ambient air environment in which both weather and flight aerodynamics are important considerations. Another application is a nitrogen-purged environment with a cold-boundary temperature of 20 K where both initial cooldown and launch ascent profiles must be considered. Experimental results and considerations for these flight system applications are discussed.

Influence of Mobile Air-Conditioning on Vehicle Emissions and Fuel Consumption: A Model Approach for Modern Gasoline Cars Used in Europe

The influence of air-conditioning activity on the emissions and fuel consumption of passenger cars is an important issue, since fleet penetration and use of these systems have reached a high level. Apart from the MOBILE6 study in the United States, little data is available on the impact of air-conditioning devices (A/Cs). Since weather conditions and A/C technologies both differ from those in the U. S., a test series was designed for the European setting. A fleet of six modern gasoline passenger cars was tested in different weather conditions. Separate test series were carried out for the initial cooldown and for the stationary situation of keeping the interior of the vehicle cool. As assumed, CO2 emissions and fuel consumption rise with the thermal load. This also causes a notable rise in CO and hydrocarbons (HCs). Moreover, A/Cs do not stop automatically at low ambient temperatures; if necessary, they produce dry air to demist the windscreen. A model is proposed that shows a constant load for lower temperatures and a linear trend for higher temperatures. The initial cooldown tests highlight significant differences among cars but show that A/C operation for the initial cooling of an overheated passenger compartment does not result in any extra emissions for the fleet as a whole.

Cryocooler-cooled high field superconducting magnet

A high field NbTi/Nb3Sn superconducting magnet with no use of liquid helium was developed by adopting high-Tc Bi2Sr2Ca2Cu3O10 current leads and 4 K GM-cryocoolers. The initial cooldown from room temperature to 4.1 K for the coil was attained within 40 hours by conductively cooling in vacuum. The cryocooler-cooled superconducting magnet successfully achieved to generate a central field of 10.7 T in a 52 mm room temperature experimental bore, and to hold a field at 10.5 T for 24 hours.

Time-Dependent Behavior of Continuous-Fiber-Reinforced Metal Matrix Composites: Modeling and Applications

A time-dependent approach employing a four-phase concentric cylinder model has been developed to predict the response of metal matrix composites (MMCs) subjected to thermomechanical loadings in which both plastic and creep responses of the composites are considered. The progressive development of plasticity in the matrix phase is determined using the deformation theory of plasticity while the creep deformation of this phase is estimated using the Bailey-Norton equation with an Arrhenius-type expression for the time-dependent creep coefficient. The model is applied to SCS-6/Ti-β21S composite to study the evolution of the stress and strain states in the constituents of the composite during initial cool-down and subsequent thermal cycles. The model is then employed to examine the influence of several critical parameters on the composite internal stress and strain states. These parameters include the thickness of the equivalent composite media, the type of fiber coating material, the thickness of the reaction zone, cooling rate during initial cool-down, and the kinetics of creep process during thermal cyclic loading. Results of these applications indicated that the process-induced thermal stresses in the matrix phase can be relaxed due to creep following initial cool-down from fabrication. This stress reduction is enhanced at a slower cooling rate. Comparison of different fiber coating materials shows that the use of carbon coating induces compressive stress state in the brittle interfacial region. TiB2-coated fibers, however, are found to be less affected by the growing interphase thickness in preserving the compressive radial stress component in the matrix and the interphase zone. Furthermore, it is found that the matrix activation energy for creep, Q, is history-dependent and can be correlated with the level of creep strain accumulated in the matrix phase. In addition, the residual thermal stresses induced in the matrix phase during initial cool-down can be relaxed by the application of subsequent thermal cycles.

Stress Induced Void Nucleation in Narrow Aluminum Alloy Lines

AbstractThe stress induced failure of passivated aluminum based metallizations is the combined result of the stress induced void nucleation and growth of voids. The present work investigates the dependence of the void nucleation on aging temperature and cooling rate. Narrow Al, Al(0.2%Cu) and Al(2%Si) lines of different widths were fabricated by lift-off and passivated with various thicknesses of Si3N4. After a 400°C anneal, the specimens were cooled directly to the selected aging temperature and aged for up to 1100 hours. Void nucleation was found to occur predominantly during the initial cool-down. The rate of nucleation varied in quite different ways with the combination of aging temperature and cooling rate for the different alloys, which is consistent with a grain boundary sliding controlled nucleation mechanism.

Stress Induced Void Nucleation in Narrow Aluminum Alloy Lines

ABSTRACTThe stress induced failure of passivated aluminum based metallizations is the combined result of the stress induced void nucleation and growth of voids. The present work investigates the dependence of the void nucleation on aging temperature and cooling rate. Narrow Al, Al(0.2%Cu) and Al(2%Si) lines of different widths were fabricated by lift-off and passivated with various thicknesses of Si3N4. After a 400°C anneal, the specimens were cooled directly to the selected aging temperature and aged for up to 1100 hours. Void nucleation was found to occur predominantly during the initial cool-down. The rate of nucleation varied in quite different ways with the combination of aging temperature and cooling rate for the different alloys, which is consistent with a grain boundary sliding controlled nucleation mechanism.

Anomalous electrical resistivity of the Kondo system Ce(Rh1-xCox)3B2.

Electrical resistivity measurements have been carried out on the high-Curie-temperature ferromagnetic compound Ce${\mathrm{Rh}}_{3}$${\mathrm{B}}_{2}$ (${T}_{C}=110\ensuremath{-}115$ K). The temperature dependence of the electrical resistivity $\ensuremath{\rho}(T)$ during the initial cooldown above ${T}_{C}$ gives the first solid indication of the Kondo-like behavior in this system. However, the resistivity is irreversible above ${T}_{C}$ when warming up from low temperature and $\ensuremath{\rho}(T)$ approaches the previous reported form. This irreversibility is closely related to microscopic cracks created by the strong internal magnetic field of the ferromagnetic state and was not observed in the nonmagnetic compound Ce${\mathrm{Co}}_{3}$${\mathrm{B}}_{2}$ with the same hexagonal structure. Further proof of this Kondo state can be obtained in the study of the pseudoternary system $\mathrm{Ce}{({\mathrm{Rh}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{3}{\mathrm{B}}_{2}$ where the resistivity increases with decreasing temperature during the initial cooldown and a local minimum Kondo anomaly was observed. The magnetic state is rapidly broken up with the replacement of Rh and Co and the resistivity anomaly disappears after the disappearance of ferromagnetic order.

Preliminary results of the partial-array LCT coil tests

The Large Coil Task (LCT) is a collaboration between the US, Euratom, Japan, and Switzerland for the production and testing of 2.5 × 3.5 m bore, superconducting 8-T magnets. The definitive tests in the design configuration, the six coils arrayed in a compact torus, will begin in 1985. Partial-array tests are being done in 1984. In January the initial cooldown of two coils was aborted because of helium-to-vacuum leaks that developed in certain seal welds when the coil temperatures were 170 to 180 K. In July three adjacent coils (designated JA, GD, CH) were cooled, and in August two were energized to the limits of the test facility. An overview of the results is presented, including facility, cooldown, energization, dump, recovery from intentional normal zones, strain, and displacement, for operation up to 100% of design current but below full field and stress. These initial results are highly encouraging.

A Microprocessor-Based Controller for a Liquid Hydrogen Target Refrigerator

A microprocessor-based cryogenic refrigerator controller developed at the Los Alamos Scientific Laboratory provides automatic cooldown and subsequent monitoring of liquid hydrogen targets. The controller performs the initial cooldown of the cryogenic system and provides continuous monitoring of the important system parameters. An alarm is sounded to summon an operator in the event of any parameter exceeding predetermined limits.