A review of current leads for cryogenic devices

In this review the present state of problems connected with the arrangement of current leads in cryostats is described. The methods of determining the heat flux along leads and also for finding their optimal geometry are presented. Various operating regimes of the leads are considered. The experimental results obtained are put in systematic order. Also existing designs of current leads are described.     

Numerical optimization of conduction cooled current leads for low current applications

The basic design principles of current leads for superconducting magnets are well established but HTS materials and conduction cooled systems call for new numerical methods. In this paper the design of current leads was formulated as an optimization problem. Both time integration and finite differencing were examined as possible ways to compute the temperature distribution inside the leads. Three examples about optimization of conduction cooled as well as gas cooled systems are presented. First, the design of tubular normal conducting gas cooled current leads was studied. Second, normal conducting leads cooled with a two-stage cryocooler were examined. Third, the optimization was applied to current leads consisting of HTS tapes at the low temperature end of a normal conducting bar. The study took into account the magnetic field and temperature dependent voltage-current characteristics of the anisotropic Bi-2223 material. The results are compared with traditional analytical ones and the numerical optimization is shown to be an efficient design tool for both normal conducting and HTS current leads.     

Disconnectable current leads for superconducting magnets

A pair of 100 A disconnectable current leads for superconducting magnets has been tested. This research note describes the device and gives the results of the tests. The liquid helium consumption due to the Joule dissipation of the gold contacts is small compared to the saving when the leads are disconnected. During an average duty cycle, the overall efficiency is, therefore, increased by a factor of two over that for permanent current leads.     

Evaluation of loss of current leads for HTS power apparatuses

This paper evaluates the losses of current leads made of various metals used in high-T_c superconducting power apparatuses operating at liquid nitrogen temperatures. The heat flow into liquid nitrogen conducting adiabatic current leads was measured by the nitrogen boil-off method. The results were used to derive average Lorenz numbers for metals between room temperature and liquid nitrogen temperatures. The average Lorenz number is a good index of performance of current leads. A theory of loss evaluation of current leads using the average Lorenz number and a method for the optimum current lead design were described as well. In addition, the loss of current leads made of Bi2223/Ag tape conductors was evaluated above the liquid nitrogen temperature, and a higher efficiency was achieved than with pure metal current leads.     

Thermomagnetic residual offset in integrated Hall plates

Integrated Hall plates show self-induced offset due to the current flow in their current leads. The magnetic field surrounding these current leads generates a magnetic offset signal, while their resistive heating causes thermoelectric offset voltages. This paper reports experimental results on these two effects. They are obtained using a novel test structure enabling to separate the two contributions. In addition, the experimental results concerning the thermal behavior of the test structure are compared with finite element (FE) simulations.     

Heat transfer and thermal exchange processes in low temperature current leads

This paper deals with the theory of heat transfer and thermal exchange processes in combined and normal current leads during cooling with cryogenic gases. The lead metal's thermal and electrical conductivity in the general case may be functions of temperature. The cross-section may also depend upon arbitrary coordinates; the optimum solutions in the analytical form being found. The optimum ratios between length, current, and cross-section are given, as well as the method of lead calculation at arbitrary values of the parameters. Methods of providing for safe operation of leads are considered, and the results are compared with available experimental data.     

Simple technique for increasing the conductivity of copper for current lead conductors

Current leads are used mainly as conductors for electrical current to be supplied to superconducting magnets. Low liquid helium losses in the current leads are of great importance and therefore the geometry of the current lead must correspond to a specific optimized current. Here it is shown that, by altering the residual resistivity of the conductor material, one can modify the optimized current of the current lead without changing its original geometry. A relatively easy technique for decreasing the resistivity of commercial electrolytic tough-pitch copper is described. Experimental results of copper annealing in a vacuum of 2.7 × 10⁻³ Pa (2 × 10⁻⁵ torr) in the temperature range of 295–703 K are discussed.     

Possible energy saving by compensating heat inputs to liquid helium through current leads and thermal bridges

The possibility of energy saving by compensating heat inputs to liquid helium through current leads and thermal bridges is discussed. This saving is due to a suitable distribution of evaporated helium between current leads (or thermal bridges) and a regrigerator. In the analytical model considered energy savings for current leads compared to the theoretical minimum are of order 10–15%, for thermal bridges savings are possible only if the temperature of the hot end is equal to or lower than that of liquid nitrogen.     

Test results of 12/18 kA ReBCO coated conductor current leads

A pair of hybrid current leads (brass + stacked & soldered ReBCO tapes) rated for 12 kA in steady state and for up to 18 kA at pulsed over current conditions was designed, developed and tested at NRC “Kurchatov Institute” (NRC “KI”). During the experiment at LN2 temperature, the current leads (CLs) were successfully charged with 18 kA at 100 A/s ramp rate. To date, as far as we know, this is the highest current capacity achieved for 2G HTS current leads. The feasibility of “stack-and-soldering technique” for 10 kA+ class coated conductor CLs for accelerators and fusion was demonstrated. This paper gives an overview of the leads design and presents the preliminary test results. Detailed studies of magnetic properties and current sharing process for the stacked and staggered HTS joints are also reported.     

On minimizing the heat leak of current leads in cryogenic vacuum systems

Electronic devices such as infrared detectors in remote sensing instruments are frequently placed in cryogenic vacuum systems, such that the ratio of signal to noise can be enhanced. The heat leak of leads supplying usually small electric current to these electronic devices is a critical issue in system designs. Cu is a typical material for current leads. In this work, however, we suggest employing the Peltier current lead to lower the heat leak. It comprises a thermoelectric element and a Cu lead at the hot and cold ends, respectively. In considering the heat radiation between Cu leads and the vacuum vessel, both all-Cu and Peltier current leads are optimised so as to minimise their resulting heat leaking into cryogenic vacuum systems. Moreover, temperature–entropy (T–s) diagrams are constructed to track the actual thermodynamic processes in the two types of current leads. The heat leak of a current lead in the presence of heat radiation can be easily identified from the simple 2-D T–s plane. It is also proved that the effect of heat radiation on the heat leak of current leads is usually negligible.     

Investigation on vapor-cooled current leads operating in a pulse mode

This paper describes transient numerical modeling for thermal characteristic of vapor-cooled current leads under pulse operation. The transient thermal analysis considers the temperature difference between a helium gas flow and a copper lead with temperature dependent properties of helium gas, copper and stainless steel. The numerical analysis was validated by an experiment with commercially available 100 A vapor-cooled current leads. A proper overloading factor calculation and the effect of intermediate cooling are presented for the current leads under pulse operation, which can significantly reduce heat input to a cryostat.     

ITER 68kA高温超导特大电流引线的冷端漏热

ITER高温超导电流引线载流能力最大要达到68 kA,其试验件于2008年底加工完成并进行了低温试验,在载流能力、接头电阻、漏热和安全性等关键参数上取得了重要进展.讨论了ITER高温超导电流引线超导段冷端(5 K)漏热的理论值计算,并用热导率积分法进行了低温漏热测试,结果表明理论值与试验值基本吻合,试验件冷端漏热满足ITER国际组要求.     

Determination of the refrigerating capacity of cryogenic gas machines for cooling current leads containing high-temperature superconductors

A method of calculation of current leads containing high-temperature semiconductors and cooled by cryogenic gas machines (cryocoolers) has been proposed. The method is based on subdivision of current leads into individual portions (sections) differing in the character of traversal of the current and cooling conditions. The results of the change in the required refrigerating capacity of the coolers operating at different temperature levels as a function of the length and arrangement of the sections have been shown. The distinctive features of the variation in the temperature and the heat flux along cryocooler-cooled current leads have been analyzed.     

Propagation of normal zone in superconducting tapes due to heating in near-electrode area

Local disturbances of thermal stability in the superconducting materials may initiate the formation of the normal zones. One of the reasons for these disturbances is thermal processes in metal leads connected to the superconductor. Here, superconducting tapes with metal leads are analyzed thermally and from the point of view of current distribution. The results show that the Joule's heat, generated in the vicinity of the metal-superconductor junction may initiate propagation of a normal zone in the superconductor and may cause its thermal runaway.     

Radiation-stimulated superconductivity

Eliashberg has predicted that absorption of microwaves in a superconducting film leads to an increase of the energy gap by creating a nonequilibrium quasiparticle distribution. The frequency has to exceed the inverse relaxation time for inelastic scattering. In the present paper measurements are reported of the critical current of long, narrow, superconducting thin-film strips of aluminum subjected to high-frequency radiation (10 MHz–10 GHz). Above a critical frequency of about 200 MHz considerable enhancement of critical current and critical temperature is observed. Analysis of the results is performed by taking the critical current for a measure of the energy gap. The results are in reasonable agreement with Eliashberg's theory. As predicted, the transition between the superconducting and the normal states becomes of first order. The experimental results on critical current enhancement of micro-bridges (Dayem-Wyatt effect) can be explained consistently with gap enhancement.     

Experimental study of gas-cooled current leads for superconducting magnets

Design details and experimental test results from several design variations of the gas-cooled, copper current leads used in conjunction with the superconducting dipole magnets for ESCAR (Experimental Superconducting Accelerator Ring) are reported. Thermal acoustic oscillations, which were experienced with an initial design, were eliminated in subsequent designs by a reduction of the hydraulic diameter. The occurrence of these oscillations is in general agreement with the stability analysis of Rott,¹ but the observed gas flow dependence is not in agreement with some other recently reported results for leads operated with supercritical phase coolant.² An empirically determined correlation was obtained by plotting lead resistance vs enthalpy gain of the coolant gas. The resulting family of curves can be reduced to a single line on a plot of effective resistivity vs, the product of current and cross-sectional area divided by the product of the square of the mass flow of the coolant and the lead length. This correlation, which should be applicable to other designs of copper current leads in which ideal heat transfer to the coolant gas is approached, predicts that the enthalpy gain of the coolant, and therefore the peak lead temperature, is proprtionalo to the cube of the ratio of current to coolant mass flow. The effective value of the strongly temperature-dependent kinematic viscosity of the coolant gas was found to vary linearly with the effective resistivity of the lead.     

高温超导交流磁体气冷电流引线的优化设计和实验

通过研究交流超导磁体气冷电流引线两段式模型的优化设计,使得其向低温系统的漏热达到最小,并自行设计热测法实验装置进行了实验研究.结果表明在引线顶端温度为300 K、引线下端温度为77 K、引线传输电流为460 A的情况下,得到最佳引线直径为7.9 mm,与实验值8 mm符合.对交流电流引线的趋肤效应进行了分析和讨论.     

Spin Effects on Heat Current Through a Quantum Dot Attached to Ferromagnetic Leads

A heat current originating from electron–phonon coupling in a quantum dot (QD) molecule connected to ferromagnetic leads is studied by the non-equilibrium Green’s function technique. The system is driven out of equilibrium by a temperature gradient (thermal bias) applied across the two terminals of the structure. We find that when the magnetic moments of the two leads are arranged in parallel configuration, the heat current is not sensitive to the leads’ ferromagnetism, whereas in the case of antiparallel configuration, the magnitude of the heat current increases with increasing spin polarization of the leads, with the reduction of the electric current’s intensity. We also find that the ferromagnetism on the leads can amplify the heat rectification effect occurring for some particular dot levels, i.e., the strength of the heat flowing between the QD and the phonon bath can be very small for one direction of the temperature gradient, while it becomes quite large when the corresponding direction of the temperature gradient is reversed.     

Thermally stimulated current study in polyvinyl chloride (PVC)-mica composites

Mica powder was dispersed in polyvinyl chloride (PVC) in different percentages by weight and the thermally stimulated discharge current of the composites was studied. The results are discussed in terms of dipolar and space charge polarization that leads to induced dipole formation. Activation energies have been calculated for all the samples by the initial rise method.