Cryostat Wall Research Articles

Thermal analyses of the W7-X plasma vessel for operation phase 2

The W7-X plasma vessel (PV) is part of the cryostat wall and forms a vacuum and thermal barrier between the hot vessel interior and the superconducting coils. The vessel is thermally loaded from the inside and is actively cooled by numerous cooling water pipes in order to control its temperature.However, due to the complex geometry and large number of different types of ports, the cooling pipes are distributed irregularly with varying distances in between them. So the temperature distribution of the PV becomes quite inhomogeneous even with homogeneous load density, and impermissible temperature hot spots >130 °C may arise. Particularly for the upcoming long pulse operation phase 2 (OP2), it is necessary to know the locally allowed thermal loads on the PV. Therefore, temperature distribution maps for different heat loads from 1 kW/m2 to 12 kW/m2 were calculated in order to determine the corresponding hot spots.The available CAD geometry was not convenient for this extensive and complex analysis; therefore, a simplified shell-beam model was employed to get an approximate temperature distribution, and a correction formula was derived to calculate more accurate values at the positions of interest.In some areas with large distances between cooling pipes, additional copper stripes are welded onto the PV to improve the heat transfer. For these regions local models were built to calculate the temperature distributions, and, in the course of this analysis, to judge the effect of the copper stripes.The calculation results indicate positions to be additionally protected and will be used to determine safe operation limits.

A Contactless Self-Regulating HTS Flux Pump

A series of high temperature superconducting (HTS) flux pumps have been developed to achieve the persistent current mode (PCM) operation for HTS magnets. However, most of the HTS flux pumps work with a small operating air gap, or with an iron core, the extra heat load brought by the conventional components deployed in the cryogenic environment is inevitable. To address this problem, in this study, in conjunction with the wireless power transfer (WPT) approach and transformer-rectifier type HTS flux pump, we have proposed a contactless self-regulating HTS flux pump. A circuit model and a 2D H-formulation finite-element model have been built for better understanding this system. Furthermore, a basic prototype was proposed to experimentally validate this concept. The measured results indicate that this method has the potential to charge the HTS magnet with all the conventional constituents outside the cryostat, and the operating frequency could be very low to evade inducing large eddy current as well as losses on the cryostat wall.

4 kV class splicing device to connect high-voltage instrumentation wires and cable

In ITER machine, the splicing devices (SDs) are designed to connect the high voltage (HV) individual wires bringing the signal from the sensors installed in the superconducting magnet system to the HV cables connected with the feedthroughs installed in the cryostat wall. SDs shall be designed and manufactured to ensure its safety operation at 4.5 K and under the environment of ionizing radiation and high magnetic field. As a kind of insulating structural part, the structural design of prototype SD aimed at achieving enough safe margin of insulating performance under helium (He) gas condition. To evaluate the electrical properties, a prototype SD was fabricated and the HV-leakage current tests were performed, the testing results indicate that the leakage currents are less than 2 μA under 4 kV DC and 20 μA under 4 kV AC respectively, which are less than the allowable values 5 μA and 30 μA. In addition, the testing results also indicate that there was no breakdown or current impulse occurred during Paschen discharge tests. However, when the bonded interface is not He gas tightness, SD can be damaged by Paschen discharge.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) Spectrographs

We describe the design and performance of the near-infrared (1.51–1.70 μm), fiber-fed, multi-object (300 fibers), high resolution (R = λ/Δλ ∼ 22,500) spectrograph built for the Apache Point Observatory Galactic Evolution Experiment (APOGEE). APOGEE is a survey of ∼105 red giant stars that systematically sampled all Milky Way populations (bulge, disk, and halo) to study the Galaxy’s chemical and kinematical history. It was part of the Sloan Digital Sky Survey III (SDSS-III) from 2011 to 2014 using the 2.5 m Sloan Foundation Telescope at Apache Point Observatory, New Mexico. The APOGEE-2 survey is now using the spectrograph as part of SDSS-IV, as well as a second spectrograph, a close copy of the first, operating at the 2.5 m du Pont Telescope at Las Campanas Observatory in Chile. Although several fiber-fed, multi-object, high resolution spectrographs have been built for visual wavelength spectroscopy, the APOGEE spectrograph is one of the first such instruments built for observations in the near-infrared. The instrument’s successful development was enabled by several key innovations, including a “gang connector” to allow simultaneous connections of 300 fibers; hermetically sealed feedthroughs to allow fibers to pass through the cryostat wall continuously; the first cryogenically deployed mosaic volume phase holographic grating; and a large refractive camera that includes mono-crystalline silicon and fused silica elements with diameters as large as ∼400 mm. This paper contains a comprehensive description of all aspects of the instrument including the fiber system, optics and opto-mechanics, detector arrays, mechanics and cryogenics, instrument control, calibration system, optical performance and stability, lessons learned, and design changes for the second instrument.

Impact of Magnet Geometry on Output of a Dynamo-Type HTS Flux Pump

A superconducting flux pump based on a high-temperature superconducting (HTS) dynamo has been demonstrated which employs rotating permanent magnets and a ReBCO conductor stator wire. This device is capable of injecting large dc currents (>100 A) into industrial superconducting magnet coils. Dynamic resistance, due to the interaction of the dc current with the ac magnetic field at the stator, provides a performance limit for this type of HTS dynamo exciter. It has been shown for a given device, the output of the HTS dynamo can be described by a simple circuit model linking short-circuit current (I sc ), dynamic resistance (R d ), and open-circuit voltage (V oc ). I sc represents the maximum dc current that can be delivered by the dynamo to a series-connected superconducting circuit and is found to be independent of rotational frequency for a fixed device geometry. We have recently demonstrated that this dynamo effect can be applied to excite a closed superconducting circuit through the cryostat wall, such that all active components of the dynamo are located outside of the cryogenic envelope. Here, we report experimental results which explore the relationship between rotor magnet geometry and ReBCO stator wire width on the output performance of a simple HTS dynamo. We characterize the effect of magnet orientation, aspect ratio and shape, at fixed flux gap, on I sc , R d , and V oc .

Impact of <bold>S</bold>tator Wire Width on Output of a Dynamo-Type HTS Flux Pump

Superconducting flux pumps enable dc supercurrents to be injected into a superconducting coil without direct connection to an external current supply. Here, we report experimental data from a dynamo-type flux pump employing a high-T c superconducting stator. This device employs a mechanical rotor that rotates outside the cryogenic envelope, and excites the superconducting circuit through the cryostat wall via a magnetic circuit formed between the rotor and stator. We show that the width of the stator wire employed in this device has a significant effect on its output performance. At low frequencies, both the short circuit current, I sc , and the open-circuit voltage, V oc increase with stator width, and we obtain a maximum value of I sc > 340 A using a stator wire width of 46 mm. However at higher operating frequencies, we observe that I sc reaches a maximum value before dropping with any further increase in stator width. We attribute this to thermally dissipative effects due to circulating eddy currents in these wide superconducting stators. At frequencies above ~400 Hz, we observe a sharp rise in the internal resistance of the stator wire that we attribute to a local quench due to the eddy currents driven by the local electromotive force (EMF) beneath the rotor magnet. This paper emphasizes the importance of frequency dependence when defining optimum operating parameters of a high-T c superconductor (HTS) flux pump. Our results also offer the promise that very high current flux pumps may be developed through maximizing the ratio of the stator wire width to the peak width of the rotor field profile.

Through-Wall Excitation of a Magnet Coil by an External-Rotor HTS Flux Pump

High-temperature superconducting (HTS) magnet systems conventionally require normal-conducting current leads, which connect between the HTS circuit and an external power supply located at room temperature. These current leads form a thermal bridge across the cryostat wall, and they represent the dominant heat load for many magnet applications. The use of a superconducting flux pump device is an alternative approach to exciting a magnet coil, which can eradicate this parasitic heat load, as such devices do not require direct physical connection to the HTS circuit. However, earlier proposed flux pump designs have required power-dissipating active components to be located within the cryogenic envelope, thus imposing their own parasitic heat load. Here, we report the successful demonstration of a mechanically rotating HTS flux pump, which operates entirely outside of the cryogenic envelope. This prototype device projects flux across a cryostat wall, leading to the injection of a direct current into a thermally isolated closed HTS circuit. This is achieved through the implementation of a flux-concentrating magnetic circuit employing ferromagnetic yoke pieces, which enables flux penetration of the HTS circuit at large flux gaps. We have demonstrated the injection of direct currents of > 30 A into a closed HTS circuit while operating this device across a cryostat wall.

Anode-coupled readout for light collection in Liquid Argon TPCs

This paper will discuss a new method of signal read-out from photon detectors in ultra-large, underground liquid argon time projection chambers. In this design, the signal from the light collection system is coupled via capacitive plates to the TPC wire-planes. This signal is then read out using the same cabling and electronics as the charge information. This greatly benefits light collection: it eliminates the need for an independent readout, substantially reducing cost; it reduces the number of cables in the vapor region of the TPC that can produce impurities; and it cuts down on the number of feed-throughs in the cryostat wall that can cause heat-leaks and potential points of failure. We present experimental results that demonstrate the sensitivity of a LArTPC wire plane to photon detector signals. We also simulate the effect of a 1 μs shaping time and a 2 MHz sampling rate on these signals in the presence of noise, and find that a single photoelectron timing resolution of ∼30 ns can be achieved.

A Novel Rotating HTS Flux Pump Incorporating a Ferromagnetic Circuit

High-temperature superconductor (HTS) flux pumps enable large currents to be injected into a superconducting coil without requiring normal-conducting current leads. We present results from an experimental axial-type HTS rotating flux pump that employs a ferromagnetic circuit to focus incident flux upon a coated-conductor stator wire. We show that this device can inject currents of > 50 A into an HTS coil at 77 K and is capable of operating at flux gaps greater than 18 mm. Accommodating a cryostat wall within this flux gap will enable future flux pump designs, in which all moving parts are located outside the cryostat.

Busbar arcs at large fusion magnets: Model experiments on busbar arcing in a double-walled feeder tube with the LONGARC device

Electric arcs moving along the power cables (the so-called busbars) of the toroidal field (TF) coils of large fusion devices like DEMO and ITER may reach and penetrate the cryostat wall. The present work concludes a series of model experiments studying the propagation and destruction mechanisms of such busbar arcs in small scale. While it went clear from previous findings that the inner feeder tube of a busbar would not withstand a powerful arc, the present work investigated the impact of the arc on the second, coaxial outer feeder tube. The key finding is that the outer feeder tube will stay intact in any case, proving the robustness of the ITER TF busbar confinement concept. As the basic DEMO magnet concept will be “ITER-like”, the knowledge of weaknesses and strengths of the ITER TF busbar feeder design may be helpful in the preparation phase of a detailed DEMO magnet feeder design.

Development of a brushless HTS exciter for a 10 kW HTS synchronous generator

HTS synchronous generators, in which the rotor coils are wound from high-Tc superconducting wire, are exciting attention due to their potential to deliver very high torque and power densities. However, injection of the large DC currents required by the HTS rotor coils presents a technical challenge. In this paper we discuss the development of a brushless HTS exciter which operates across the cryostat wall to inject a superconducting DC current into the rotor coil circuit. This approach fundamentally alters the thermal load upon the cryogenic system by removing the need for thermally inefficient normal-conducting current leads. We report results from an experimental laboratory device and show that it operates as a constant voltage source with an effective internal resistance. We then discuss the design of a prototype HTS-PM exciter based on our experimental device, and describe its integration with a demonstration HTS generator. This 200 RPM, 10 kW synchronous generator comprises eight double pancake HTS rotor coils which are operated at 30 K, and are energised to 1.5 T field through the injection of 85 A per pole. We show how this excitation can be achieved using an HTS-PM exciter consisting of 12 stator poles of 12 mm YBCO coated-conductor wire and an external permanent magnet rotor. We demonstrate that such an exciter can excite the rotor windings of this generator without forming a thermal-bridge across the cryostat wall. Finally, we provide estimates of the thermal load imposed by our prototype HTS-PM exciter on the rotor cryostat. We show that duty cycle operation of the device ensures that this heat load can be minimised, and that it is substantially lower than that of equivalently-rated conventional current leads.

Design of Post Metal Shields Through Electric Field Distribution Analysis for a 154-kV SFCL

Korea Electric Power Corporation has developed a 154-kV superconducting fault current limiter (SFCL). This report is a part of the design process of the SFCL, particularly for fixation of posts supporting the superconducting element on the cryostat wall side. For supporting the superconducting element, the use of a post insulator is inevitable; however, the post insulator and cryostat with liquid nitrogen (L-N2) during operation of the SFCL form three junction points where electric field is intensified. In this study, we aim to design the metal shield in order to relax the electric field intensity at triple points (TPs) through numerical analysis of electric field distribution. For the electric field distribution analysis, a commercial software based on the finite-element method was employed. Each design for the metal shield was checked whether it makes the electric field intensity at the TP sufficiently lower than dielectric strength in L-N 2 for 750-kV input and whether there is any electrically weak point on the metal. The designs of the metal shields were improved through four critical steps where thermal contraction, manufacture tolerance, and insulation distance in L-N 2 were considered. It was experimentally verified that there was no electric breakdown in L-N 2 between the metal shield and the fiber-reinforced plastic post insulator for the lightning impulse test and the ac breakdown voltage test according to the IEC 60137 standard.

Sliding weight supports for W7-X magnet system: structural aspects

The Wendelstein 7-X (W7-X) stellarator is presently under commissioning at the Max-Planck-Institut für Plasmaphysik in Greifswald. The coil system consisting of 70 superconducting coils of seven different types is supported by a massive central support structure (CSS), and thermally protected by the cryostat. The magnet system weight is borne by supports which are bolted to the cold CSS. These ten so-called cryo-legs penetrate through the cryostat wall to the warm machine base. The design of the cryo-legs incorporates glass-reinforced plastic tubes to guarantee relatively small thermal conductivity. In order to ensure free thermal shrinkage of the magnet system and to reduce stresses in the cryo-legs, sliding and rotating bearings are used as interfaces to the machine base. Tie-rods between the machine base and the warm ends of the cryo-legs prevent toroidal rotation of the magnet system, as well as any other horizontal shifts due to asymmetric loads. The assembly of the magnet system introduced some vertical imperfections in the cryo-leg positions causing considerable additional internal stresses which were not considered during the design stage. In addition, originally not planned trim coils induce unsymmetrical cyclic loads. Therefore, the previously used method to analyse one magnet system module with periodical boundary conditions is not applicable. Consequently, a model of the complete magnet system, including all five modules, was created and analysed. Fatigue analyses of the cryo-legs under the new cyclic loads, applied on top of the approximately 100 t static weight, have been performed in order to evaluate the lifetime. The paper presents the progress in structural analyses of the W7-X magnet system under the as-built conditions, loads due to the trim coil operation, and results of the weight support fatigue analysis.

Thermal Design of the Mu2e Detector Solenoid

The reference design for a superconducting detector solenoid (DS) for the Mu2e experiment has been completed. In this study, the main functions of the DS are to provide a graded field in the region of the stopping target, which ranges from 2 to 1 T and a uniform precision magnetic field of 1 T in a volume large enough to house a tracker downstream of the stopping target. The inner diameter of the magnet cryostat is 1.9 m and the length is 10.9 m. The gradient section of the magnet is about 4 m long and the spectrometer section with a uniform magnetic field is about 6 m long. The inner cryostat wall supports the stopping target, tracker, calorimeter and other equipment installed in the DS. This warm bore volume is under vacuum during operation. It is sealed on one end by the muon beam stop, while it is open on the other end where it interfaces with the Transport Solenoid. The operating temperature of the magnetic coil is 4.7 K and is indirectly cooled with helium flowing in a thermosiphon cooling scheme. This paper describes the thermal design of the solenoid, including the design aspects of the thermosiphon formore » the coil cooling, forced flow cooling of the thermal shields with 2 phase LN2 (Liquid Nitrogen) and the transient studies of the cool down of the cold mass as well.« less

Busbar arcs at large fusion magnets: Conductor to feeder tube arcing model experiments with the LONGARC device

Electric arcs moving along the power cables (the so-called busbars) of the toroidal field (TF) coils of ITER may reach and penetrate the cryostat wall. Model experiments with the new LONGARC device continue the VACARC (VACuum ARC) experiments that were initiated to investigate the propagation and destruction mechanisms of busbar arcs in small scale [1]. The experiments are intended to support the development and validation of a numerical model. LONGARC overcomes the space limitations inside VACARC and allows also for advanced 1:3 (vs. ITER full scale) model setups. The LONGARC device and first results are presented below.

Design and Development of Superconducting DC Cable for Railway Applications

We report the recent development in the national project dealing with a prototype of the next generation dc superconducting cable for railways. The main goal of the project is to upgrade the feeder of the overhead contact line system connecting the electric train with the substation. The superconducting cables are usually considered to reduce the resistive losses generated in a conventional feeding system mainly in the catenary wires. In this paper, various cable structures existing in the railway network were studied with respect of their use in the next generation of railways. The superconducting cables need to be cooled below 77 K. They consist of coaxially configured conductors and sheaths. Liquid nitrogen is forced to flow through these coaxial cables. Various designs and configurations of the cooling systems possess always some advantages and some drawbacks. Our systematic analysis showed that a `go and return' system, where the liquid nitrogen flows through the hollow former and returns through the space between the outer layer of the HTS tapes and the cryostat wall is most effective for the railway applications; it saves both space and costs.

Safety of fusion magnets: Model experiments to high current arcs at ITER busbars and feeder lines

Abstract Electric arcs moving along the power cables (the so-called busbars) of the toroidal field (TF) coils of ITER may reach and penetrate the cryostat wall. Model experiments with the VACARC (VACuum ARC) device were initiated to investigate the propagation and destruction mechanisms of busbar arcs in small scale. These experiments are intended to support the development and validation of a numerical model. The present setup simulates the fact that the busbar is contained by an inner and an outer feeder tube which absorb radiated energy and will be in contact with hot metal melt from the busbar. The tubes may also direct the hot plasma jet flow of the arc as long as they are intact. Experiments at scaled values for tube diameter, wall thickness and expected power density suggest serious damage to the inner feeder tube by the arc. This asks for further investigations that include the outer tube, too. The extent of damage to the tube samples depends on arc power, as expected. At high currents an additional luminous effect takes place, which is attributed to a reaction of oxygen and hydrogen probably set free from the dissociation of the insulation resin by the heat of the arc.

Design and assembly technology for the thermal insulation of the W7-X cryostat

The Max-Planck-Institut für Plasmaphysik in Greifswald is building up the stellarator fusion experiment Wendelstein 7-X (W7-X). To operate the superconducting magnet system the vacuum and the cold structures are protected by a thermal insulated cryostat. The plasma vessel forms the inner cryostat wall, the outer wall is realised by a thermal insulated outer vessel. In addition 254 thermal insulated ports are fed through the cryogenic vacuum to allow the access to the plasma vessel for heating systems, supply lines or plasma diagnostics. The thermal insulation is being manufactured and assembled by MAN Diesel & Turbo SE (Germany). It consists of a multi-layer insulation (MLI) made of aluminized Kapton with a silk like fibreglass spacer and a thermal shield covering the inner cryostat surfaces. The shield on the plasma vessel is made of fibreglass reinforced epoxy resin with integrated copper meshes. The outer vessel insulation is made of brass panels with an average size of 3.3 × 2.0 m 2. Cooling loops made of stainless steel are connected via copper strips to the brass panels. Especially the complex 3 D shape of the plasma vessel, the restricted space inside the cryostat and the consideration of the operational component movements influenced the design work heavily. The manufacturing and the assembly has to fulfil stringent geometrical tolerances e.g. for the outer vessel panels +3/−2 mm.

Safety of fusion magnets: Pressure impact on arcing in model experiments to high-current arcs propagating along the busbars of large fusion magnet coils

The paper reports the results of the latest experiments to support the development and validation of the numerical model for high-current arcs at the power cables (busbars) to the toroidal field (TF) coils. The busbar arcs have significant potential of danger in case of an accident due to the huge inductive energy of about 40 GJ stored permanently inside the TF coils. In a worst case these arcs may propagate to and penetrate the cryostat wall. The experiments were set up to understand the propagation and damage potential of such arcs. The present work investigates the influence of residual pressure on arcing.

Safety of Fusion Magnets: Model Experiments to High Current ARCS Propagating along the Busbars of Large Fusion Magnet Coils

The inductive energy of about 40GJ stored permanently inside the toroidal field (TF) coils of ITER provides a considerable potential of hazard in case of an accident. While for most accidents it could be proved that the damage is limited to the coils themselves, possible high current arcs at the busbars of the TF coils may propagate to and penetrate the cryostat wall. Model arc experiments were setup to understand the propagation and damage potential of such arcs to provide a database for the development and validation of a numerical model as the next step. This work reviews the basic arc propagation and burning modes found so far and introduces new experimental setups and findings.