Liquid Helium Cryostat Research Articles

Safety studies on vacuum insulated liquid helium cryostats

The loss of insulating vacuum is often considered as a reasonable foreseeable accident for the dimensioning of cryogenic safety relief devices (SRD). The cryogenic safety test facility PICARD was designed at KIT to investigate such events. In the course of first experiments, discharge instabilities of the spring loaded safety relief valve (SRV) occurred, the so-called chattering and pumping effects. These instabilities reduce the relief flow capacity, which leads to impermissible over-pressures in the system. The analysis of the process dynamics showed first indications for a smaller heat flux than the commonly assumed 4W/cm2. This results in an oversized discharge area for the reduced relief flow rate, which corresponds to the lower heat flux.This paper presents further experimental investigations on the venting of the insulating vacuum with atmospheric air under variation of the set pressure (pset) of the SRV. Based on dynamic process analysis, the results are discussed with focus on effective heat fluxes and operating characteristics of the spring-loaded SRV.

Design and optimisation of low heat load liquid helium cryostat to house cryogenic current comparator in antiproton decelerator at CERN

The Cryogenic Current Comparator (CCC) is installed in the low-energy Antiproton Decelerator (AD) at CERN to make an absolute measurement of the beam intensity. Operating below 4.2 K, it is based on a superconducting quantum interference device (SQUID) and employs a superconducting niobium shield to supress magnetic field components not linked to the beam current. The AD contains no permanent cryogenic infrastructure so the local continuous liquefaction of helium using a pulse-tube is required; limiting the available cooling power to 0.69 W at 4.2K. Due to the sensitivity of the SQUID to variations in magnetic fields, the CCC is highly sensitive to mechanical vibration which is limited to a minimum by the support systems of the cryostat. This article presents the cooling system of the cryostat and discusses the design challenges overcome to minimise the transmission of vibration to the CCC while operating within the cryogenic limits imposed by the cooling system.

Low-temperature high magnetic field powder x-ray diffraction setup for field-induced structural phase transition studies from 2 to 300 K and at 0 to 8-T field.

A low-temperature and high magnetic field powder x-ray diffractometer (XRD) has been developed at UGC-DAE CSR (UGC: University Grant Commission, DAE: Department of Atomic Energy, and CSR: Consortium for scientific research), Indore, India. The setup has been developed around an 18 kW rotating anode x-ray source delivering Cu-Kα x-rays coming from a vertical line source. It works in a symmetric θ-2θ parallel beam geometry. It consists of a liquid helium cryostat with an 8 T split-pair Nb-Ti superconducting magnet comprising two x-ray windows each covering an angular range of 65°. This is mounted on a non-magnetic type heavy duty goniometer equipped with all necessary motions along with data collection accessories. The incident x-ray beam has been made parallel using a parabolic multilayer mirror. The scattered x-ray is detected using a NaI detector through a 0.1° acceptance solar collimator. To control the motions of the goniometer, a computer programme has been developed. The wide-angle scattering data can be collected in a range of 2°-115° of 2θ with a resolution of ∼0.1°. The whole setup is tightly shielded for the scattered x-rays using a lead hutch. The functioning of the goniometer and the artifacts arising possibly due to the effect of stray magnetic field on the goniometer motions, on the x-ray source, and on the detector have been characterized by collecting powder XRD data of a National Institute of Standards and Technology certified standard reference material LaB6 (SRM-660b) and Si powder in zero-field and in-field conditions. Occurrence of field induced structural-phase transitions has been demonstrated on various samples like Pr0.5Sr0.5MnO3, Nd0.49Sr0.51MnO3-δ and La0.175Pr0.45Ca0.375MnO3 by collecting data in zero field cool and field cool conditions.

Completion of the Commissioning of the EDIPO Test Facility

The main coils of the new European DIPOle (EDIPO) test facility, which is able to provide a background magnetic field up to 12.35 T, were commissioned at the Center for Research in Plasma Physics in July 2013. A field homogeneity of +/-1% was achieved over a 900-mm length, as compared to SULTAN's 10.9 T over 425 mm. To complete the commissioning of the test facility, the new superconducting transformer has been operated in 2015, and a high-current NbTi sample, which was earlier tested in the SULTAN test facility, has been tested in EDIPO under the same conditions as in SULTAN to benchmark the results. During the initial commissioning of the 100-kA transformer in 2013, the primary coil was found to be insufficiently cooled due to the debonding of the primary winding from its former that also served as the liquid helium cryostat. The NbTi primary coil was therefore modified to be in direct contact with liquid helium. At the next attempt of commissioning, in March 2014, a mechanical failure due to the imperfect alignment of the primary and secondary windings caused major damage. A replacement primary coil, which was wound in Summer 2014, was used in the third commissioning campaign in November 2014, but it failed due to the poor impregnation of the NbTi winding. Eventually, the original primary winding was repaired and commissioned in April 2015. The commissioning included a calibration of the current meter for the secondary winding and the high-current test, i.e., up to 95 kA at various background fields, of the “Trasek” conductor sample, which is a rectangular cable-in-conduit conductor made of 324 SnAg-coated NbTi strands. An excellent agreement was found between the results obtained from the two facilities. The sample holder unit is also equipped with a high-temperature superconducting (HTS) adapter and a counterflow heat exchanger to allow testing in EDIPO of HTS high-current samples over a broad range of operating temperatures, from 4.5 up to 50 K.

Simple insertible high performance variable temperature regulator for measurement of physical properties at low temperature.

An impedance capillary based Variable Temperature Regulator (VTR) for regulation of temperature in the range of 4.2 K-300 K, which can be detached and inserted into any experimental setup with a 50 mm diameter top access, has been designed, fabricated, and tested. The VTR may be used as a highly compact probe, which can be readily inserted in any liquid helium dewar or cryostat to realize uniform rates of cooling/heating and to achieve excellent temperature stability of ±1 mK at any temperature between 4.2 K and 300 K. VTR has been subjected to extensive experimental testing to arrive at optimum values of control parameters that are expected to influence its performance. The VTR may be integrated into any experimental setup for measurement of physical properties at low temperatures.

Commissioning of the cryogenic safety test facility PICARD

The sizing of cryogenic safety relief devices requires detailed knowledge on the evolution of the pressure increase in cryostats following hazardous incidents such as the venting of the insulating vacuum with atmospheric air. Based on typical design and operating conditions in liquid helium cryostats, the new test facility PICARD, which stands for Pressure Increase in Cryostats and Analysis of Relief Devices, has been constructed. The vacuum-insulated test stand has a cryogenic liquid volume of 100 liters and a nominal design pressure of 16 bar(g). This allows a broad range of experimental conditions with cryogenic fluids. In case of helium, mass flow rates through safety valves and rupture disks up to about 4kg/s can be measured. Beside flow rate measurements under various conditions (venting diameter, insulation, working fluid, liquid level, set pressure), the test stand will be used for studies on the impact of two-phase flow and for the measurement of flow coefficients of safety devices at low temperature. This paper describes the operating range, layout and instrumentation of the test stand and presents the status of the commissioning phase.

Cryogenics on the stratospheric terahertz observatory (STO)

The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon experiment designed to address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium. STO surveys a section of the Galactic plane in the dominant interstellar cooling line at 1.9 THz and the important star formation tracer at 1.46 THz, at ∼1 arc minute angular resolution, sufficient to spatially resolve atomic, ionic, and molecular clouds at 10 kpc. The STO instrument package uses a liquid helium cryostat to maintain the THz receiver at < 9 K and to cool the low noise amplifiers to < 20 K. The first STO mission (STO-1) flew in January of 2012 and the second mission (STO-2) is planned for December 2015. For the STO-2 flight a cryocooler will be added to extend the mission lifetime. This paper discusses the integration of the STO instrument into an existing cryostat and the cryogenic aspects of the launch and operation of the STO balloon mission in the challenging Antarctic environment.

Micron-scale magnetic resonance imaging of both liquids and solids

We describe and demonstrate a novel apparatus for magnetic resonance imaging (MRI), suitable for imaging of both liquid and solid samples with micron-scale isotropic resolution. The apparatus includes a solenoidal radio-frequency microcoil with 170μm inner diameter and a set of planar gradient coils, all wound by hand and supported on a series of stacked sapphire plates. The design ensures efficient heat dissipation during gradient pulses and also facilitates disassembly, sample changes, and reassembly. To demonstrate liquid state 1H MRI, we present an image of polystyrene beads within CuSO4-doped water, contained within a capillary tube with 100μm inner diameter, with 5.0μm isotropic resolution. To demonstrate solid state 1H MRI, we present an image of NH4Cl particles within the capillary tube, with 8.0μm isotropic resolution. High-resolution solid state MRI is enabled by frequency-switched Lee–Goldburg decoupling, with an effective rotating frame field amplitude of 289kHz. At room temperature, pulsed gradients of 4T/m (i.e., 170Hz/μm for 1H MRI) are achievable in all three directions with currents of 10A or less. The apparatus is contained within a variable-temperature liquid helium cryostat, which will allow future efforts to obtain MRI images at low temperatures with signal enhancement by dynamic nuclear polarization.

Beam Heat Loads to Superconducting Quadrupoles for BigRIPS In-Flight Separator at RIKEN

The BigRIPS in-flight fragment separator in the RIKEN Radioactive Isotope Beam Factory (RIBF) project was commissioned in 2007 and has been operated for six years. The RIBF accelerator complex has provided a wide range of heavy ion beams from deuteron to uranium with the energy of 250-345 MeV/nucleon. More than 70 experiments of unstable nuclei have been performed using the RI beams produced by the BigRIPS fragment separator. The production target of the RI beams is in close proximity to the first superconducting triplet quadrupoles (STQs) in the BigRIPS fragment separator, so that the STQ is exposed to the high radiation, which causes “beam heat loads” to the cryogenic system. The STQ consists of three air-core NbTi coils with the maximum field gradients of 24 T/m, assembled in the liquid-helium bath cryostat with the warm bore radius of 90 mm. This paper presents measurements of the radiation heat loads to the STQ cryostat for a number of experienced heavy ion beams from oxygen to uranium with the beam power of 1-7 kW. Corresponding radiation transport calculations are presented, and radiation doses and damages to the STQ system are also discussed.

In situ electrical characterization of palladium-based single electron transistors made by electromigration technique

We report the fabrication of single electron transistors (SETs) by feedback-controlled electromigration of palladium and palladium-nickel alloy nanowires. We have optimized a gradual electromigration process for obtaining devices consisting of three terminals (source, drain and gate electrodes), which are capacitively coupled to a metallic cluster of nanometric dimensions. This metal nanocluster forms into the inter-electrode channel during the electromigration process and constitutes the active element of each device, acting as a quantum dot that rules the electron flow between source and drain electrodes. The charge transport of the as-fabricated devices shows Coulomb blockade characteristics and the source to drain conductance can be modulated by electrostatic gating. We have thus achieved the fabrication and in situ measurement of palladium-based SETs inside a liquid helium cryostat chamber.

Development and performance of a 129-GHz dynamic nuclear polarizer in an ultra-wide bore superconducting magnet.

We sought to build a dynamic nuclear polarization system for operation at 4.6 T (129 GHz) and evaluate its efficiency in terms of (13)C polarization levels using free radicals that span a range of ESR linewidths. A liquid helium cryostat was placed in a 4.6 T superconducting magnet with a 150-mm warm bore diameter. A 129-GHz microwave source was used to irradiate (13)C enriched samples. Temperatures close to 1 K were achieved using a vacuum pump with a 453-m(3)/h roots blower. A hyperpolarized (13)C nuclear magnetic resonance (NMR) signal was detected using a saddle coil and a Varian VNMRS console operating at 49.208 MHz. Samples doped with free radicals BDPA (1,3-bisdiphenylene-2-phenylallyl), trityl OX063 (tris{8-carboxyl-2,2,6,6-benzo(1,2-d:4,5-d)-bis(1,3)dithiole-4-yl}methyl sodium salt), galvinoxyl ((2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy), 2,2-diphenylpicrylhydrazyl (DPPH) and 4-oxo-TEMPO (4-Oxo-2,2,6,6-tetramethyl-1-piperidinyloxy) were assayed. Microwave dynamic nuclear polarization (DNP) spectra and solid-state (13)C polarization levels for these samples were determined. (13)C polarization levels close to 50 % were achieved for [1-(13)C]pyruvic acid at 1.15 K using the narrow electron spin resonance (ESR) linewidth free radicals trityl OX063 and BDPA, while 10-20 % (13)C polarizations were achieved using galvinoxyl, DPPH and 4-oxo-TEMPO. At this field strength free radicals with smaller ESR linewidths are still superior for DNP of (13)C as opposed to those with linewidths that exceed that of the (1)H Larmor frequency.

Recondensation performance of liquid helium cryostat for a 28 GHz electron cyclotron resonance ion source

Cryostat performance is essential for the stable operation of a superconducting magnet. A closed-cycle liquid helium cryostat was adopted for use for a superconducting electron cyclotron resonance (ECR) ion source by recondensing liquid helium vapor. The goal was to maintain the liquid helium filled reservoir at a constant level without transferring any liquid helium during the normal operation of the ECR ion source. To accomplish this, Gifford-McMahon (GM) refrigerators, which have two cold heads, were installed on the top of the cryostat. The cooling power of the GM cryocooler is 1.5 W at the second stage and 50 W at the first stage. Each stage was connected to the liquid helium reservoir, a radiation shield including high-Tc current lead, and related items. Before commissioning the ECR ion source, a preliminary evaluation of the recondensation performance was carried out with the magnet in partial operation. The design of the cryostat, its fabrication, and the experimental results are reported.

Development of a helium cryostat for laser spectroscopy of atoms with unstable nuclei in superfluid helium

We are developing a new nuclear laser spectroscopic technique for the study of nuclear structure that can be applied to short-lived low-yield atoms with unstable nuclei. The method utilizes superfluid helium (He II) as a trapping medium for high-energy ion beams. A liquid helium cryostat with optical windows is a key apparatus for this type of experiment. We describe the design and the performance of the cryostat which is developed for the present project.

Engineering Design of a Special Purpose Functional Magnetic Resonance Scanner Magnet

A 2 T open magnetic resonance imaging scanner for functional magnetic resonance imaging investigations of subjects maintaining a natural stance and free access to the environment is presented. The self-shielded multicoil magnet is composed of solenoids optimized in positions and cross-sectional shape. It provides a room temperature gap of 800 mm at the subject shoulders, tapering down to 600 mm at its narrowest point. The system consists of symmetrical magnetic poles, five NbTi superconducting coils each, maintained at 4.2 K in a liquid helium cryostat with a re-condenser. Windings, coil formers, mechanical structure, cryostat, vacuum chamber, thermal shield, mechanical supports, and cryogenics have been modeled in detail using Catia CAD software, accounting for the interactions among components and the constraints posed by winding and assembly methods. The cable characteristics have been chosen based on simulations of thermodynamic stability in operating conditions; self-protection against quench has been provided by inductance subdivision with bypass diodes.

Particle tracking at 4 K: The Fast Annihilation Cryogenic Tracking (FACT) detector for the AEgIS antimatter gravity experiment

The AEgIS experiment is an international collaboration with the main goal of performing the first direct measurement of the Earth's gravitational acceleration on antimatter. Critical to the success of AEgIS is the production of cold antihydrogen (H¯) atoms. The FACT detector is used to measure the production and temperature of the H¯ atoms and for establishing the formation of a H¯ beam. The operating requirements for this detector are very challenging: it must be able to identify each of the thousand or so annihilations in the 1ms period of pulsed H¯ production, operate at 4K inside a 1T solenoidal field and not produce more than 10W of heat. The FACT detector consists of two concentric cylindrical layers of 400 scintillator fibres with a 1mm diameter and a 0.6mm pitch. The scintillating fibres are coupled to clear fibres which transport the scintillation light to 800 silicon photomultipliers. Each silicon photomultiplier signal is connected to a linear amplifier and a fast discriminator, the outputs of which are sampled continuously by Field Programmable Gate Arrays (FPGAs).In the course of the developments for the FACT detector we have established the performance of scintillating fibres at 4K by means of a cosmic-ray tracker operating in a liquid helium cryostat. The FACT detector was installed in the AEgIS apparatus in December 2012 and will be used to study the H¯ formation when the low energy antiproton physics programs resume at CERN in the Summer of 2014. This paper presents the design requirements and construction methods of the FACT detector and provides the first results of the detector commissioning.

Cryocooled Facilities for Superconducting Coils Testing in Gaseous Helium

Abstract Two superconducting coil test facilities equipped by Sumitomo SRDK-415D cryocoolers were developed, manufactured and tested. The motivation for their constructing was to make cheaper the testing (and especially training of LTS magnets) by liquid helium (LHe) saving. It is well known that the helium price increases rapidly and this tendency most probably will continue for a long time, as the demand of helium grows faster than its production. The utilization of heat-exchange gas considerably reduces many problems, that arise in the design of completely dry LTS magnets. The goal was to decrease or even completely avoid the consumption of rather expensive liquid helium for testing the laboratory size Nb-Ti and Nb3Sn coils including their training process. Several superconducting magnets were tested by using these facilities. For example, the first facility was successfully used for testing of 13 T, 60 kg coil cooled by cryocooler in helium gas (several torr pressure) heat exchange atmosphere. The precooling time was about 45 hours. The quench current (240 A at 4.2 K) was equal to that reached in the pool boiling LHe cryostat. The second facility with 420 mm wide access bore can be used for testing of corresponding size superconducting coils with very modest consumption of liquid helium with its level well below the lower flange of the coil. Each test facility is equipped by 2 pairs of HTS current leads. Design and operational experience of one of them is described.

Magnetic levitation on a type-I superconductor as a practical demonstration experiment for students

We describe and discuss an experimental set-up which allows undergraduate and graduate students to view and study magnetic levitation on a type-I superconductor. The demonstration can be repeated many times using one readily available 25 l liquid helium dewar. We study the equilibrium position of a magnet that levitates over a lead bowl immersed in a liquid hand-held helium cryostat. We combine the measurement of the position of the magnet with simple analytical calculations. This provides a vivid visualization of magnetic levitation from the balance between pure flux expulsion and gravitation. The experiment contrasts and illustrates the case of magnetic levitation with high temperature type-II superconductors using liquid nitrogen, where levitation results from partial flux expulsion and vortex physics.

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.

Isolation and Characterization of Homodimeric Type-I Reaction Center Complex from Candidatus Chloracidobacterium thermophilum, an Aerobic Chlorophototroph

The recently discovered thermophilic acidobacterium Candidatus Chloracidobacterium thermophilum is the first aerobic chlorophototroph that has a type-I, homodimeric reaction center (RC). This organism and its type-I RCs were initially detected by the occurrence of pscA gene sequences, which encode the core subunit of the RC complex, in metagenomic sequence data derived from hot spring microbial mats. Here, we report the isolation and initial biochemical characterization of the type-I RC from Ca. C. thermophilum. After removal of chlorosomes, crude membranes were solubilized with 0.1% (w/v) n-dodecyl β-D-maltoside, and the RC complex was purified by ion-exchange chromatography. The RC complex comprised only two polypeptides: the reaction center core protein PscA and a 22-kDa carotenoid-binding protein denoted CbpC. The absorption spectrum showed a large, broad absorbance band centered at ∼483 nm from carotenoids as well as smaller Q(y) absorption bands at 672 and 812 nm from chlorophyll a and bacteriochlorophyll a, respectively. The light-induced difference spectra of whole cells, membranes, and the isolated RC showed maximal bleaching at 840 nm, which is attributed to the special pair and which we denote as P840. Making it unique among homodimeric type-I RCs, the isolated RC was photoactive in the presence of oxygen. Analyses by optical spectroscopy, chromatography, and mass spectrometry revealed that the RC complex contained 10.3 bacteriochlorophyll a(P), 6.4 chlorophyll a(PD), and 1.6 Zn-bacteriochlorophyll a(P)' molecules per P840 (12.8:8.0:2.0). The possible functions of the Zn-bacteriochlorophyll a(P)' molecules and the carotenoid-binding protein are discussed.

Polarization and temperature dependent spectra of poly(3-hydroxyalkanoate)s measured at terahertz frequencies

Temperature-dependent terahertz (THz) absorption spectra of poly(3-hydroxyalkanoate)s (PHAs) were measured by using a Fourier transform far-infrared (FT-FIR) spectrometer and a THz time-domain spectrometer over a temperature range of 10 K to 465 K with a liquid helium cryostat and a heating cell. Clear differences were observed between the spectra of crystalline and amorphous polyhydroxybutyrate (PHB), indicating that the absorption peaks observed in the THz spectra originated in the higher-order conformation of PHB. The polarization spectra of a stretched PHB sample were measured, and the direction of the vibrational transition moment was determined. The temperature dependences of the spectra reveal frequency shifts and broadening of the absorption peaks with temperature, suggesting large anharmonicity of the vibrational potential. The temperature shift behaviour is quite different in each transition. Some of the transitions show a blue shift, which cannot be explained by a simple anharmonic potential model. Frequency shifts of the peaks were mainly observed below 10 THz, which suggests a large anharmonicity of the vibrational potential at lower frequencies.