Cryogenic Load Research Articles

Planck-LFI: design and performance of the 4 Kelvin Reference Load Unit

The LFI radiometers use a pseudo-correlation design wherethe signal from the sky is continuously compared with a stablereference signal, provided by a cryogenic reference load system. Thereference unit is composed by small pyramidal horns, one for eachradiometer, 22 in total, facing small absorbing targets, made of acommercial resin ECCOSORB CRTM, cooled to ∼ 4.5K. Horns and targets are separated by a small gap to allow thermaldecoupling. Target and horn design is optimized for each of the LFIbands, centered at 70, 44 and 30 GHz. Pyramidal horns are eithermachined inside the radiometer 20K module or connected via externalelectro-formed bended waveguides. The requirement of high stability ofthe reference signal imposed a careful design for the radiometric andthermal properties of the loads. Materials used for the manufacturinghave been characterized for thermal, RF and mechanical properties. Wedescribe in this paper the design and the performance of the referencesystem.

Focus on magnetics and electromagnetism

Focus on magnetics and electromagnetism

Sources of Error in AC Losses Measurement Using V-I Method

A significant cryogenic load is generated in rapidly-cycling superconducting magnets due to ac losses. This load should not be excessive, otherwise cryogenic system will not be able to evacuate heat and magnet will quench. To verify magnet design and to know real cryogenic load the test of ac losses has to be performed. Electrical and calorimetric methods can be used for this purpose. The calorimetric method is slower because it requires stabilization of cryogenic system. The electrical method called V-I is faster and it can be applied to any magnet independent of its cooling schemes. With this method ac losses are measured directly and results are provided on-line. However, the electrical method is considered more error prone, mainly due to the fact that power factor of superconducting magnets is close to zero.

Status of HTS current leads for WENDELSTEIN 7-X and JT-60SA

The stellarator WENDELSTEIN 7-X (W7-X) and the satellite tokamak JT-60SA will use High Temperature Superconductor (HTS) current leads to reduce cryogenic load and operational cost. Forschungszentrum Karlsruhe is in charge of design, construction and test of these HTS current leads. For W7-X fourteen current leads with a nominal current of 14 kA and a maximum current of 18.2 kA are required that are oriented upside down with the room temperature end at the bottom. JT-60SA is constructed in the frame of the Broader Approach Agreement and will use 26 current leads mounted in vertical, normal position with room temperature end at the top. All current leads will consist of an HTS part using Bi-2223 conductor and a classical Cu heat exchanger. The Cu heat exchanger covers the range between 60 K and room temperature and will be cooled with 50 K He while the HTS part is conduction cooled, only. For simplicity a common basic design of the current leads for W7-X and JT-60SA is planned which requests that the current leads work independently of the orientation, and from the experimental boundaries given by W7-X and JT-60SA the current leads have to withstand a loss of He coolant for more than 10 min and allow a discharge of the magnet system although a quench of the HTS part has happened. JT-60SA gives the more critical HV requirement with a Paschen tight insulation of 21 kV. All current leads shall be designed for low thermal loads, under full current and in standby mode. The paper will give details of the actual design status of these current lead developments for W7-X and JT-60SA based on a common basic design.

Flux Pump Excited Double-Helix Rotor for Use in Synchronous Machines

High temperature superconducting (HTS) machines have been demonstrated to benefit applications requiring high power density and torque. However, power density of existing superconducting machinery is limited by achievable flux density in the air gap and could be significantly increased with larger rotor currents. We propose a rotor based on MgB2 conductor running in persistent current mode. A novel MgB2 HTS flux pump transformer enables currents of several thousand amps in the rotor in a persistent mode. Large cryogenic heat loads associated with current leads are diminished by using a flux pump transformer for rotor excitation. All coils in the rotor and flux pump transformer are based on the double-helix winding configuration, which offers unmatched rotor robustness and reliability. This winding configuration also facilitates large bending radii, leading to an outstanding stress management and thus improved current carrying capacity of stress sensitive HTS conductors. The research presented in this paper analyses the performance of such a system through the design of a synchronous rotor for motors, generators and synchronous condensers.

Vacuum system for superconducting LINAC at TIFR

The superconducting heavy ion LINAC booster at the TIFR-BARC facility has been operational. Seven super conducting cryostats containing 4 quarter wave resonators each along with beam lines, bending magnets, switching magnet, diagnostics and vacuum system have been commissioned. The heart of the cryogenic system for the heavy ion superconducting LINAC booster is a custom-built liquid helium refrigerator made by Linde Kryotechnik, Switzerland. The Refrigerator is rated for 300 Watts at 4.5 K with a dual JT (Joule-Thomson valve) at the final cooling stage, which allows simultaneous connections to the cryogenic loads (the LINAC module cryostats) and to a liquid helium storage dewar (1000 litres). The two-phase helium at 4.5 K produced at the JT stage in the refrigerator is delivered to the cryostats through a cryogenic distribution system. The cryogenic distribution system for the LINAC is designed to deliver both liquid helium and liquid nitrogen to the cryostats. The details of UHV system, indigenously developed beam line components, pumps and module cryostats will be presented.

Towards steady state operation in large tokamaks: The experience of TORE SUPRA superconducting magnet system

The TORE SUPRA (TS) tokamak is the first important application of superconductivity in Plasma Physics on a large scale. Overall, despite the differences in design and size, the accumulated reliable experience over 16 years of operation is a useful tool to prepare the manufacture and the operation of the ITER magnets. An overview is given about the main design choices of the superconducting toroidal field (TF) system. The acceptance tests of the systems are described with details concerning the problems encountered and solved during this phase. The actual status of the system is described through a presentation of the present operation and of the solutions to mitigate the effects of fast safety discharges. The behaviour of the TF system during long plasma discharges in TS is a good introduction to the situation in a steady state tokamak reactor. It can be observed that the long plasma discharges do not bring a significant cryogenic load.

Characterization of ferrites at low temperature and high frequency

Complex values of dielectric permittivity and magnetic permeability of 10 different materials, ferrites and ceramics, are measured in a frequency range from 1 to 40 GHz at room temperature and at 80 K. Recommendations are done for material usage in a cryogenic load for higher-order modes in the superconducting RF cavities of the Energy Recovery Linac.

R&D Progress on the KSTAR Current Leads

The Korea Superconducting Tokamak Advanced Research (KSTAR) device requires R&D of 20-30 kA overloaded current leads (OCL) for the superconducting poloidal field (PF) magnet. The PF magnet is under zero current state for a very long time, as it is often the case for superconducting magnets used in nuclear fusion. In such case, a simple approach to reduce the cryogenic load would be to design the current leads to be overloaded. Two main problems in this R&D area are the reduction of the cryogenic heat load and the temperature rise due to the current overloading. In this paper, the current overloading tests and results of the numerical analysis for brass leads are presented and discussed in detail.

A New Series-Connected Hybrid Magnet System for the National High Magnetic Field Laboratory

The National High Magnetic Field Laboratory (NHMFL) proposes to build a new hybrid magnet system with three specific goals: 1) address the needs of users requiring high homogeneity, high temporal stability, and long residence times at moderately high fields; 2) enhance user service through use of a single 10 MW power supply (allowing simultaneous operation of multiple systems); and 3) make the system attractive in terms of combined capital and operating costs. The basic configuration are series-connected hybrid, wherein resistive insert and superconducting outsert are connected in series and powered by a single 10-MW unit (20 kA and 500 V) of the NHMFL dc power system, a configuration with advantages for handling faults and off-normal operating conditions and with improved temporal stability. Specific design goals are 35 T, 10-ppm uniformity over a 10-mm DSV, and access for 40-mm diameter probes. By powering the superconducting outsert with 20-kA high-temperature-superconductor (HTS) current leads, cryogenic loads are kept small, substantially smaller even than the present 45-T Hybrid. The net result is a magnet design: 1) with capability for both higher field and higher field quality than present resistive-only systems, 2) that permits simultaneous service of two or more users, 3) that is sufficiently compact to fit within the standard resistive-magnet cell, 4) whose ease of operation is comparable to resistive-only systems, and 5) whose lifetime cost (including construction and operation) can be significantly lower than comparable resistive-only systems. We discuss details of the existing conceptual design, the methodology for its creation, the perceived development needs, and the projected program to realize this system.

Thermal stability in precision cosmology experiments: the Planck LFI case

After the great success of NASA's satellite missions COBE and WMAP, the Planck mission represents the third generation of mm-wave instruments designed for space observations of CMB anisotropies. Two instruments, the Low-Frequency Instrument (LFI) and the High-Frequency Instrument (HFI) will produce CMB maps with unprecedented angular resolution, sensitivity and frequency coverage. This ambitious task will be achieved by using low noise HEMT detectors cryogenically cooled at ∼20K for the LFI and bolometric detectors cooled at 0.1K for the HFI; in particular, the LFI is based on pseudo-correlation receivers in which the sky signal is continuously compared to a cryogenic reference load in thermal contact with the HFI 4K stage. Such high sensitivity in Planck detectors calls for a strict control of systematic effects, which must be kept at μK level in the final maps; this in turn imposes tight requirements on the thermal and electrical stability of the different stages in the instrument. In this paper we discuss a study of the impact of thermal fluctuations at the level of the 20K cooler cold-end on the Planck-LFI measurements and present some viable solutions that have been adopted to keep the residual systematic error within the required values for Planck-LFI.

Accelerator physics issues in eRHIC

The accelerator physics issues associated with adding electron–gold and polarized electron–proton collisions to the RHIC repertoire are discussed. Two scenarios are considered: ring–ring and linac–ring collisions. In the first scenario a third electron ring, possibly made from permanent magnets, is added to the RHIC tunnel. In the second scenario the considerable beam power is recuperated by recycling the beam through the full energy linac injector. Straw man ring optics are presented, including an integrated spin rotator which provides longitudinally polarized electrons at the interaction points. Electron and ion intensity limitations are investigated, for operations with as many as 2520 bunches. Synchrotron radiation, beam–beam, electron cloud, and cryogenic heat load limits are examined. The implications for new and upgraded ion ring systems are discussed, including electron cooling, RF systems, the ion abort dumps, and injection and extraction kickers.

HTS current leads for the LHC

Leads for transferring heavy current from power converters, working at room temperature, into the liquid helium environment required to operate superconducting magnets, have been long recognized as being an immediate application of the emerging technology of high temperature superconductivity. By correct dimensioning of such leads it should be possible to reduce significantly the cryogenic load which these leads represent. The Large Hadron Collider (LHC) under construction at CERN is by far the largest user of superconducting magnets requiring the transfer of over 3 million amperes of current, and therefore has much to gain from the use of this technology. After a brief reminder of the motives and the outline of the project, a review is given of the program to provide helium gas cooled current leads incorporating HTS sections.

Radiation from the cryogenic calibration load for microwave radiometers

A styrofoam layer is introduced to the cryogenic calibration load for microwave radiometers in order to keep the antenna at ambient temperature while calibrating. Obviously the insulation layer has nonuniform temperature profile. A novel approach based on the transmission-line theory is presented to calculate the emission from the load. According to the sample calculation through this new approach, the contribution of the insulation layer to the load radiation can not be neglected.

Dual-channel sis receivers for the iram Plateau de Bure Interferometer

The five antennas of the Plateau de Bure Interferometer have been instrumented with dual-channel receivers in the λ 3 mm and λ 1 mm bands. Polarisation diplexing allows simultaneous observations in the two bands. Each receiver has ambient and cryogenic calibration loads, and one receiver is equipped with a beam switching chopper for total power flux measurements. Typically the receiver noise temperatures are<50 K in both the λ 3 mm band and the λ 1 mm band. Initial observations show that at 115 GHz the sensitivity is doubled compared to the previous receivers, and high quality fringes have been obtained at 230 GHz. Preliminary experiments show that the receiver stability is good enough to correct atmospheric phase variations by monitoring the fluctuations in atmospheric emission at 225 GHz. VLBI fringes have been detected between one 15-m antenna and the IRAM 30-m antenna in Spain.

酸化物超伝導電流リード Ｙ系大容量直流電流リードの開発

(Y, Ho)-Ba-Cu-O bulk samples have been prepared by the unidirectional solidification method. Since these samples exhibit low thermal conductivity and high critical current density, it is desirable to apply Y-based bulk sample to current lead, especially for a large-scale superconducting apparatus. A high-Tc superconducting current lead model using (Y, Ho)-Ba-Cu-O bulk has successfully transported a high current of 7, 550A. Warm ends of bulk samples were 50.3K and 45.8K, respectively, at 7, 550A. The liquid helium boil-off rate was 4.39l/h at 5, 560A, which corresponds to 3.1W heat leakage. The cryogenic heat load was reduced to about half the value of a conventional copper current lead.

Cryogenic load calculation of high Tc current lead

High temperature superconducting current leads are composed of a normal metal part, conducting the current from room temperature to an intermediate temperature (often 77 K), and a high T c part, conducting the current down to liquid helium temperature. The cryogenic load and the corresponding power consumption of both parts are compared to the electrical power consumption of an equivalent all metal current lead. The temperature profiles and the 4.2 K heat load of the superconducting part are evaluated by a computer calculation. The calculation uses material parameters from sintered Y-123 and melt-cast processed Bi-2212 tubes, materials which already exist in usable dimensions for current leads. The 1 μV cm −1 criterion turns out to be much too conservative and some dissipation due to flux flow in the upper part of the lead does not affect the overall performance of the current lead if the thermal runaway current I TR is not exceeded. The maximum stable current is evaluated as a function of conductor length for operation in a self-field and applied fields up to 0.2 T. Long leads result in very low 4.2 K heat loads but shorter leads result in higher stable transport currents. The use of very short high T c conductor parts (50 mm) and the possibility of fabricating a short metallic lead for 77 K operation allows us to design very short (< 0.3 m) and high performance current leads for 4.2 K applications. This current lead can be designed to pass right through the insulating vacuum of a cryostat, presenting new opportunities for a more compact cryostat system. Stability considerations indicate that the current lead is not sensitivie to flux jumps. As currents I > I TR cannot be accepted, a safety margin must be considered in the design of the current lead.

AC losses in the SSC high energy booster dipole magnets

The baseline design for the SSC (Superconducting Super Collider) high energy booster (HEB) has dipole bending magnets with a 50-mm aperture. An analysis of the cryogenic heat load due to AC losses generated in the HEB ramp cycle is reported for this magnet. Included in this analysis are losses from superconductor hysteresis, yoke hysteresis, strand eddy currents, and cable eddy currents. The AC loss impact of 2.5 mu m vs. 6 mu m filament conductor is presented. A 60-mm aperture design is also investigated. >

The CLEO II detector magnet: design, tests, and performance

The design features of the CLEO II magnet, test results and operating results are described. The design is intended to keep the good features of the smaller CLEO I solenoid, such as low cryogenic load, while eliminating some of its weaknesses, such as its poor insulating vacuum, sensitivity to refrigerator performance, and susceptibility to utility failures. A persistent current mode, proposed in view of the frequent need to de-energize the magnet for maintenance and accelerator studies, was considered and rejected. The requirements were then increased stability, low cryogenic heat load, passive cooling, and passive protection. At this writing, the magnet has been operating at high fields without serious incident for about six months. The cryostat vacuum has remained at 10/sup -6/ torr with no external pumping, and the effective refrigeration load has been comfortably within the capabilities of the Koch model 1430 (Oxford estimated 8.3 W and 14 1/h). All the various styles of cryogenic operation have been attempted successfully. Operation is largely unattended except for routine refrigerator maintenance and field rampup and rampdown. >

A high-power magnetically switched superconducting rectifier operating at 5 Hz

Above a certain current level, the use of a superconducting rectifier as a cryogenic current source offers advantages compared to the use of a power supply at room temperature which requires large current feed-throughs into the cryostat. In some cases, the power of such a rectifier is immaterial, for example if it is to be used as a current supply for short test samples with low inductances. Usually, however, a rectifier is intended to energize large superconducting magnets, so the maximum power available becomes an important parameter since it determines the loading time. One method of increasing the power of a rectifier is to raise the operating frequency. In this respect, magnetically controlled switches with very fast switching times are preferable to thermally controlled ones. This paper reports on the design, as well as the experimental results of a magnetically switched full-wave superconducting rectifier. Once this rectifier is brought to its design frequency of 5 Hz, the average power delivered to the cryogenic load will be 500 W.