Superfluid Helium Bath Research Articles

An experimental and numerical framework to assess the temperature distribution in complex He II-cooled magnet geometries

In the context of the High Luminosity upgrade of the Large Hadron Collider at CERN, a framework implementing experimental techniques and numerical analysis has been developed to systematically assess the temperature distribution in complex He II-cooled composite magnet geometries. The experiments are designed to measure the heat transfer coefficients in the magnet coil layers using coil samples in a stagnant superfluid helium bath. A numerical tool-kit has been developed to facilitate intensive parametric studies, in addition to estimation of helium content via a phenomenological model. The workflow of the tool-kit is built to handle complex geometries composed of different materials each with their temperature-dependent properties, at low computational cost. This framework has been validated with experimental data obtained from laboratory-scale experiments on impregnated coil samples, reported and discussed here. Three use cases for the developed numerical tool, with increasing levels of complexity, are presented and its results discussed.

Experimental study on a single-stage adiabatic demagnetization refrigerator with Chromium Potassium Alum

As an excellent paramagnetic material with low Curie temperature, Chromium Potassium Alum (CPA) is commonly used in the adiabatic demagnetization refrigerator (ADR) to achieve extremely low temperatures. A typical ADR consists of many components, such as a salt pill, a superconducting magnet, a heat switch, etc. An experimental system was built to test the performance of a single-stage ADR we designed. A two-stage GM refrigerator was used to provide ADR with a low-temperature environment of less than 4 K. A superfluid helium bath was connected to the ADR, via a mechanical heat switch. In the experiment, the CPA salt pill can be pre-cooled to 1.35 K by the superfluid helium bath. A superconductor power supply is used to magnetize and demagnetize the crystal. After applying a 1 T magnetic field, the single-stage ADR can obtain a minimum temperature of 318.5 mK.

Status of the MINERVA cryomodules and associated cryogenic system (MYRRHA phase 1)

MYRRHA at SCK CEN in Mol/Belgium will be a pre-industrial large-scale Accelerator Driven System for unparalleled research opportunities in spent nuclear fuel, nuclear medicine, and fundamental and applied physics. In 2018 the Belgian government released funding for MYRRHA’s first phase, MINERVA, for a staged implementation and operation. It covers the design, construction, and commissioning of the continuous-wave superconducting RF proton linac up to 100 MeV, as well as dedicated user target stations. The MINERVA proton linac will accommodate 30 identical cryomodules to boost the beam energy delivered by the normal-conducting frontend from 16.6 MeV to 100 MeV. Each cryomodule will contain two superconducting RF single-spoke cavities immersed in a superfluid Helium bath at 2 K. The design and architecture of the associated cryogenic system is derived from the stringent linac reliability requirements imposed by the future subcritical nuclear reactor. We present the architecture, design, and development status of the MINERVA cryomodules and associated cryogenic system towards the implementation phase of the project as part of a collaboration between ACS, CEA/DSBT, IJCLab, and SCK CEN. We also provide an overview of the initial outcomes of cryogenic and RF tests for the prototype MINERVA cryomodule, which are still ongoing at IJCLab.

Design Evolution of MADMAX Conductor to a Nb–Ti Cable in Copper Conduit

The MADMAX (MAgnetized Disc and Mirror AXion) project aims at detecting axion dark matter in the mass range around 100 μeV. Prerequisite for success is a very large dipole (1.35 m warm bore) with a high magnetic field up to 10 T. A first conductor design made of Nb-Ti Rutherford Cable On Conduit Conductor with a hole in the copper conduit (RCOCC type) was already presented. The coolant is static bath of superfluid helium at 1.8 K. Unfortunately, no conductor manufacturer with an available production line was found to take in charge the cable insertion/soldering process. Actually, such a production line should be compatible with large conductor section (∼400 mm²) including a solder wave device. The type of conductor was changed to use a line developed for ITER Cable In Conduit Conductor (CICC). Nevertheless, the jacket part was kept in copper. The main evolution of the MADMAX conductor design is presently reviewed. The final outer dimensions of the conductor, the void fraction and the required cold work of the copper induces a reverse engineering challenge on the initial shape of the copper profile. Then, the cable geometry modification involves a lots of differences in terms of self-field, strand diameters, transposition paths, joints technique and conductance between the strands and the stabilizer. The mechanical aspects are subject to evolution for the project due to the magnetic design optimization and conductor type change. A sub-modeling approach is used to detail the stress level in the stabilizer jacket. The possibility of wire motion and wire deformation during the conductor compaction in the production line is also discussed. Concerning the thermal analysis, the major difference is the thermo-hydraulic quench back that induces a much higher pressure drop of the helium in the conduit. So, higher pressure induced higher helium velocity and a fast detection after a quench event in superfluid environment becomes possible.

Overview of the Iseult 11.7 T MRI Cryoplant Operation

The Iseult whole-body MRI delivered its first images in October 2021. The masterpiece of this MRI is an actively shielded NbTi magnet providing a homogeneous magnetic field of 11.7 T within a 90 cm warm bore. A dedicated cryoplant was constructed to cool the magnet at 1.8 K using a superfluid helium bath and it is in nominal operation since March 2019. This paper will present the cryoplant design, as well as the connection of the cryogenic ancillary equipment with the magnet. Estimated thermal losses will be compared with experimental data collected since the beginning of the cooling phase. Then, we will describe the system maintenance and the periodic controls of the various pressurized components performed keeping the continuous nominal operation of the MRI. Finally, we will present the first lessons learned on this unique cryogenic system operation and possible options to improve its reliability.

Progress Toward Medical Use of the Iseult Whole Body 11.7 T MRI: First Images

Neurospin is a neuroscience research center located in France at the Atomic Energy Commission (CEA Saclay). The facility is hosting an innovative whole-body 11.7 T MRI system that has delivered its first images in October 2021. The core part of the Iseult MRI is an actively shielded NbTi magnet cooled with a pressurized superfluid helium bath at 0.125 MPa and 1.8 K, providing a homogeneous magnetic field of 11.7 T within a 90 cm warm bore. After nearly twenty years of work and efforts, the magnet successfully reached its nominal field for the first time in July 2019. The field homogeneity has been adjusted and the control system tested against internal and external faults that could affect the future MRI operation. MRI peripheral equipment has been integrated and interactions between the gradient coils and the magnet and their impact on cryogenics and on the magnet safety system have also been studied. The MR scanner is now kept permanently at nominal field and the final calibration is on-going to prepare the first acquisition on a human volunteer. The paper will present the Iseult MRI commissioning status and the first images obtained, as well as a first feedback on the cryogenic plant operation after three years and a half at 1.8 K.

Superconductor thermohydraulical and resistive electrical analytical model (STREAM) applied to WEST TF coil quench analysis

The Toroidal Field (TF) system of the WEST tokamak comprises 18 NbTi superconducting coils, cooled in a static superfluid helium bath at 1.8 K and carrying a nominal current of 1255 A. The 19th December 2017, at the end of plasma run #52205, a quench of TFC09 was detected, first on a thermohydraulical signal (helium liquid level), and triggered the current Fast Safety Discharge (FSD).The quench was found to be induced by a neutron and gamma flux caused by highly energetic runaway electrons (30 MeV) colliding the outboard plasma facing components. Some previous analyses with SuperMagnet (CryoSoft) code confirmed a so called “smooth” quench with small initial heat deposition length (0.4 m) at low field region (external leg) and with a number of initially quenched conductors nearly equal to 100.A Superconductor Thermohydraulical and Resistive Electrical Analytical Model (STREAM) has been developed. This model comprises the electrical equations (magnetic field, current sharing temperature, quenched length evolution, Joule energy, resistive voltage, resistance and current) and a thermodynamical and an hydraulical evolutions in two following sequences. The first one comprises the helium pressure evolution with an isentropic compression (closed volume) of a cold volume by a hot (and heated) volume; the second one comprises the determination of expulsed helium mass flow in the exhaust circuit (from a given pressure threshold) with a limit at atmospheric pressure or Mach number equal to 1. The whole TFC09 circular coil is modelled by a single bath with one conductor of length equal to coil average perimeter.For WEST TFC09 quench STREAM calculations, the inputs are the Minimum Quench Energy (MQE) and the propagation law of the quenched (normal) length. Calculated STREAM results showed good agreement with the measurements as well as the calculated SuperMagnet results. The maximal resistive voltage, Joule energy and resistance are respectively at 1400 V, 18 MJ and 5 Ω. The maximal values of helium pressure, conductor temperature and expulsed helium mass flow are respectively 0.9 MPa, 80 to 90 K and 4 to 5 kg/s. This STREAM analytical model can be useful (with adaptation to CICC configuration) for other tokamak magnets safe operation and protection, notably considering that the execution time is rather small due to the set of solved equations.

Thermohydraulic analysis of Tore Supra/WEST TF coil quench: Associated smooth quench occurrence in tokamak

The Toroidal Field (TF) system of the Tore Supra/WEST tokamak comprises 18 NbTi superconducting coils, cooled by a static superfluid helium bath at 1.8 K and carrying a nominal current of 1255 A. The 19th December 2017, at the end of plasma run #52205, a quench of TFC-09 was detected first on a secondary thermohydraulical signal (helium liquid level) and triggered the current Fast Safety Discharge (FSD). A numerical model of this quench has been developed with SuperMagnet (CryoSoft). The whole TFC-09 circular coil is modelled by THEA as a single large Cable-In-Conduit Conductor (CICC) with 2028 large rectangular strands (monolithic conductors of length equal to coil average perimeter). The external quench helium relief circuit (cold and warm safety valves, rupture disk and magnetic valve with corresponding pressure set) is modelled by FLOWER.The helium pressure in the coil, upstream of the cold safety valve (maximum value of 9 bar), and the expulsed helium temperature have been used as comparison between measurements and calculations which depend on the Minimum Quench Energy (MQE) used in THEA with small initial heat deposition length (few tens of centimeters) at low field region (external leg). This energy, in the order of few kJ, is compared to the real shape and energy of neutron and gamma flux caused by highly energetic runaway electrons colliding the outboard plasma facing components and which induced the quench. The expulsed helium mass flow rate of nearly 6 kg/s during 5 s has also been calculated. This event confirms on one hand the criticality and the possible occurrence of a so called “smooth quench” caused by small initial heat deposition length and at low field region and on the other hand the important interest of secondary quench detection which can be useful for other tokamak magnets safe operation and protection.

Tore Supra/WEST Toroidal Field Coil Quench Following a Plasma Disruption With Runaway Electrons

Tore Supra/WEST is a Tokamak operating at CEA/IRFM since 1988. Its toroidal field system is composed of 18 NbTi toroidal field coils (TFC) cooled by superfluid helium bath at 1.8 K. The system is used to operate at full current during every day of a plasma campaign, and is protected by a quench detection system (QDS) which can trigger a fast safety discharge (FSD). In December 2017, an FSD was triggered. After analysis, we could show that it was caused by the quench of TFC9, and that the quench had been triggered by a high flux of runaway electrons at the end of a plasma shot. The detection and protection systems were effective to protect the TF magnet, which is presently back in operation at nominal current without any fault. This paper presents the analyses conducted to determine the origin of the quench, the details of the QDS operation, and the impact of the quench on our cryomagnetic system.

Modeling of Beam Loss Induced Quenches in the LHC Main Dipole Magnets

The full energy exploitation of the Large Hadron Collider (LHC), a planned increase of the beam energy beyond the present 6.5 TeV, will result in more demanding working conditions for the superconducting dipoles and quadrupoles operating in the machine. It is hence crucial to analyze, understand, and predict the quench levels of these magnets for the required values of current and generated magnetic fields. A one-dimensional multi-strand electro-thermal model has been developed to analyze the effect of beam-losses heat deposition. Critical elements of the model are the ability to capture heat and current distribution among strands, and heat transfer to the superfluid helium bath. The computational model has been benchmarked against experimental values of LHC quench limits measured at 6.5 TeV for the Main Bending dipole magnets.

Stability modeling of the LHC Nb-Ti Rutherford cables subjected to beam losses

The Large Hadron Collider (LHC) at CERN is being prepared for its full energy exploitation during run III, i.e., an increase of the beam energy beyond the present 6.5 TeV, targeting the maximum discovery potential attainable. This requires an increase of the operating field of the superconducting dipole and quadrupole magnets, which in turn will result in more demanding working conditions due to a reduction of the operating margin while the energy deposited by particle loss will increase. Beam-induced magnet quenches, i.e., the transition to normal conducting state, will become an increasing concern, because they could affect the availability of the LHC. It is hence very important to understand and be able to predict the quench levels of the main LHC magnets for the required values of current and generated magnetic fields. This information will be used to set accurate operating limits of beam loss, with sufficient but not excessive margin, so to achieve maximal beam delivery to the experiments. In this study we used a one-dimensional, multistrand thermal-electric model to analyze the maximum beam losses that can be sustained by the LHC magnets, still remaining superconducting. The heat deposition distribution due to the beam losses is given as an input for the stability analysis. Critical elements of the model are the ability to capture heat and current distribution among strands, and heat transfer to the superfluid helium bath. The computational model has been benchmarked against energy densities reconstructed from beam-induced main dipole quenches during LHC operation at 6.5 TeV. The model was then used to evaluate the stability margin of both main dipole and main quadrupole magnets at different beam energies, up to the expected ultimate operating energy of the LHC, 7.5 TeV. The comparison between the quench levels underlines how the increase of beam energy implies a substantial reduction of magnets stability and will require much stricter setting on the allowable beam losses to avoid resistive transitions during operation.

Design and thermal calculations of FRESCA2 heat exchangers for pressurised superfluid helium baths temperature controlling

In order to characterise and qualify superconducting cables at currents up to 70 kA, at background magnetic fields up to 13 T and at 1.9 K or 4.3 K temperature levels the Facility for Reception of Superconducting Cable (FRESCA2) is currently under design in collaboration between CERN and WUST. The FRESCA2 design using the double cryostat concept, with superconducting magnet external cryostat and a cable sample insert internal cryostat. The magnet and the cable sample are held in the pressurised superfluid helium bath of the particular cryostat, whose temperature is controlled by a heat exchanger (HX), where the saturated superfluid helium (Hells) is used as the cooling medium. The paper presents the design and the thermal model of the HXs as well as the calculation of their minimum area necessary to cool - down the particular bath from 4.3 K to 1.9 K in the required time in respect to an available pumping speed of warm compressors.

The safety helium collectors for the ESS superconducting linac: functional specification and detailed design

The ESS proton linac includes 43 separate cryomodules which are cooled down by means of a saturated superfluid helium bath. For safety reasons, access to the machine is limited and strictly controlled. However, this does not exclude some oxygen deficiency hazards. In order to prevent exposure of personnel to helium releases during access to the tunnel (Beam OFF mode), it was decided to equip the machine with two safety helium collectors, which are intended to collect the helium released by the burst disks of the cryomodule 2 K helium vessels, in case of an accidental event and vent it outside the tunnel. However, in Beam ON mode, when more than one cryomodule can discharge cold helium at the same time, the collectors must not cause any significant back pressure to the burst disks or cavities.The paper presents the functional specification, feasibility study and design of the safety helium collectors.

In-House Industrial Production of the Superconducting Conductor for the 43 T Hybrid Magnet of LNCMI-Grenoble

The hybrid magnet being built at LNCMI-Grenoble is based on the combination of resistive inserts, made up of Bitter and polyhelix coils, with a large bore superconducting outsert. It will produce, in the first step, an overall continuous magnetic field of 43 T in a 34 mm warm bore aperture. The superconducting coil will provide a nominal magnetic field of 8.5 T in a 1.1 m cold bore diameter. It relies on the novel development of an Nb-Ti/Cu Rutherford Cable On Conduit Conductor (RCOCC) cooled down to 1.8 K by a bath of superfluid helium pressurized at 1200 mbar. After a brief reminder of the specificity of hybrid magnets, namely the strong electromechanical coupling between resistive and superconducting coils, the main requirements of the RCOCC for efficient and reliable magnet operation are given. The test and validation phases prior to the industrial production of the RCOCC are reported, including the trial production of the ½ hard-drawn (H02 cold-worked) Cu-Ag 0.05% hollow stabilizer in continuous lengths of 325 m. The novelty of the RCOCC development concerns the assembly and the induction soft-soldering of the Rutherford cable on the Cu-Ag hollow stabilizer allowing a strict control of the interstrand contact resistance. Studies, developments, and the integration of the industrial production line at LNCMI-Grenoble are presented together with the result of the production of all RCOCC unit lengths.

Design, Assembly, and Use of a Device to Eliminate Earth Faults Caused by Metallic Debris in the LHC Main Dipole Circuits

The superconducting dipole magnets of the Large Hadron Collider operate in a superfluid helium bath at 1.9 K. As a part of the magnet quench protection system, each dipole magnet is equipped with a bypass diode located in the helium bath. The connection between the superconducting magnet and the cold by-pass diode is made through a clamping system called “half-moon,” located at the lowest point of the cold-masses. This area is prone to receiving metallic debris residual from the assembly technological processes. The metallic debris might move and create an earth fault during helium flows that occur not only during the cool-down, warm-up, and flushing of the cryogenic installation but also during magnet quenches at high currents. In case of appearance, the earth fault is detected by the protection system of the circuit and as a consequence, the current is ramped down to zero. Subsequently, with the circuit current already at zero, the fault can be eliminated using a device denominated as the Earth Fault Burner. The fault elimination must follow a strict procedure as it is not fully risk-free. This paper describes the details of such earth fault elimination, including preliminary diagnostics and necessary hardware. Two examples from the LHC operation are described and discussed.

Progress in the Construction of the 43 T Hybrid Magnet at LNCMI-Grenoble

By combining resistive polyhelix and Bitter insert coils with a large bore superconducting outsert one, the new hybrid magnet in construction at LNCMI-Grenoble will produce an overall continuous magnetic field of 43 T in a 34-mm warm bore aperture. After a brief reminder of the specificity of hybrid magnets, namely the strong electromagnetic and mechanical coupling between resistive and superconducting coils, the main specificities of the proposed design are presented. The superconducting coil of 1.1-m cold bore diameter will provide a nominal magnetic field of at least 8.5 T. It relies on the specific development of the Nb–Ti/Cu Rutherford cable on a conduit conductor (RCOCC) cooled down to 1.8 K by a bath of superfluid helium pressurized at 1200 mbar. The novelty of the RCOCC concerns the in-laboratory assembly with induction soft-soldering of the Rutherford cable on a Cu–Ag hollow stabilizer, allowing a strict control of the quantity of the solder alloy used and therefore of the interstrand contact resistance. A stainless steel reinforced copper shield inserted between the superconducting and resistive coils will reduce the coupling currents induced within the RCOCC, as well as the mechanical force exerted on the superconducting coil. After successful thorough reviews of the Grenoble hybrid magnet design anticipating possible upgrades of the maximum magnetic field that can be produced, this project is now well engaged in its construction phase. The status of this project is presented in details together with the next milestones.

Design and on-orbit operation of the soft x-ray spectrometer adiabatic demagnetization refrigerator on the Hitomi observatory

The Soft X-ray Spectrometer (SXS) instrument that flew on the Astro-H observatory was designed to perform imaging and spectroscopy of x-rays in the energy range of 0.2 to 13 keV with a resolution requirement of 7 eV or better. This was accomplished using a 6x6 array of x-ray microcalorimeters cooled to an operating temperature of 50 mK by an adiabatic demagnetization refrigerator (ADR). The ADR consisted of three stages in order to operate using either a 1.2 K superfluid helium bath or a 4.5 K Joule-Thomson (JT) cryocooler as its heat sink. The design was based on the following operating strategy. After launch, while liquid helium was present (cryogen mode), two of the ADR's stages would be used to single-shot cool the detectors, using the helium as a heat sink. When the helium was eventually depleted (cryogen-free mode), all three ADR stages would be used to continuously cool the helium tank to about 1.5 K, and to single-shot cool the detectors (to 50 mK), using the JT cryocooler as a heat sink. The Astro-H observatory, renamed Hitomi after its successful launch in February 2016, carried approximately 36 liters of helium into orbit. Based on measurements during ground testing, the average heat load on the helium was projected to be 0.66 mW, giving a lifetime of more than 4 years. On day 5, the helium had cooled to <1.4 K and ADR operation began, successfully cooling the detector array to 50 mK. The ADR's hold time steadily increased to 48 hours as the helium cooled to a temperature of 1.12 K. As the commissioning phase progressed, the ADR was recycled (requiring approximately 45 minutes) periodically, either in preparation for science observations or whenever the 50 mK stage approached the end of its hold time. In total, 18 cycles were completed by the time an attitude control anomaly led to an unrecoverable failure of the satellite on day 38. This paper presents the design, operation and on-orbit performance of the ADR in cryogen mode as the foreshortened mission did not provide an opportunity to test cryogen-free mode.

Parametric study on the thermal performance of beam screen samples of the High-Luminosity LHC upgrade

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will increase the accelerator’s luminosity by a factor 10 beyond its original design value, giving rise to more collisions and generating an intense flow of debris. A new beam screen has been designed for the inner triplets that incorporates tungsten alloy blocks to shield the superconducting magnets and the 1.9 K superfluid helium bath from incoming radiation. These screens will operate between 60 K and 80 K and are designed to sustain a nominal head load of 15 Wm−1, over 10 times the nominal heat load for the original LHC design. Their overall new and more complex design requires them and their constituent parts to be characterised from a thermal performance standpoint. In this paper we describe the experimental parametric study carried out on two principal thermal components: a representative sample of the beam screen with a tungsten-based alloy block and thermal link and the supporting structure composed of an assembly of ceramic spheres and titanium springs. Results from both studies are shown and discussed regarding their impact on the baseline considerations for the thermal design of the beam screens.

Conceptual design of the cryostat for the new high luminosity (HL-LHC) triplet magnets

The High Luminosity LHC (HL-LHC) is a project to upgrade the LHC collider after 2020-2025 to increase the integrated luminosity by about one order of magnitude and extend the physics production until 2035. An upgrade of the focusing triplets insertion system for the ATLAS and CMS experiments is foreseen using superconducting magnets operating in a pressurised superfluid helium bath at 1.9 K. This will require the design and construction of four continuous cryostats, each about sixty meters in length and one meter in diameter, for the final beam focusing quadrupoles, corrector magnets and beam separation dipoles. The design is constrained by the dimensions of the existing tunnel and accessibility restrictions imposing the integration of cryogenic piping inside the cryostat, thus resulting in a very compact integration. As the alignment and position stability of the magnets is crucial for the luminosity performance of the machine, the magnet support system must be carefully designed in order to cope with parasitic forces and thermo-mechanical load cycles. In this paper, we present the conceptual design of the cryostat and discuss the approach to address the stringent and often conflicting requirements of alignment, integration and thermal aspects.

Conceptual study of the cryostats for the cold powering system for the triplets of the High Luminosity LHC

The High Luminosity LHC (HL-LHC) is a project aiming to upgrade the Large Hadron Collider (LHC) after 2020-2025 in order to increase the integrated luminosity by about one order of magnitude and extend the operational capabilities until 2035. The upgrade of the focusing triplet insertions for the Atlas and CMS experiments foresees using superconducting magnets operating in a pressurised superfluid helium bath at 1.9 K. The increased radiation levels from the particle debris produced by particle collisions in the experiments require that the power converters are placed in radiation shielded zones located in a service gallery adjacent to the main tunnel. The powering of the magnets from the gallery is achieved by means of MgB2 superconducting cables in a 100-m long flexible cryostat transfer line, actively cooled by 4.5 K to 20 K gaseous helium generated close to the magnets. At the highest temperature end, the helium flow cools the High Temperature Superconducting (HTS) current leads before being recovered at room temperature. At the magnet connection side, a dedicated connection box allows connection to the magnets and a controlled boil-off production of helium for the cooling needs of the powering system. This paper presents the overall concept of the cryostat system from the magnet connection boxes, through the flexible cryostat transfer line, to the connection box of the current leads.