A superconducting coil indirectly cooled by forced helium flow

A small superconducting coil is indirectly cooled by a forced flow of helium. The coil, wound from Nb-Ti-Zr multifilament superconducting composite, is 15 mm id, 24 mm od and 30 mm long. The maximum central field is 30.3 kG at 4.2 K. Contact between the cooling tube and the coil is achieved using grease.The experimental results are expressed by a relation of critical current and energizing rate. Typical transient temperatures of the coil and coolant during energizing and after quenching are presented.The analysis shows the design of coils of this type is possible using hysteresis loss and heat conduction analysis.     

Triple-phase phenomena during quenches of superconductors cooled by pressurized superfluid helium II

New fluid phase transition phenomena involving He II are reported for isobaric, quasi-steady conditions during heat supply from solids at elevated pressures below the thermodynamic critical pressure. Multi-fluid regimes include the three phases liquid He II, He I and vapour. Conclusions concerning optimum coolant states are presented for cryostatic stabilization of superconducting composites.     

Using the Vincenta code to analyse pressure increases in helium during the quench of a superconducting magnet

The Vincenta code is used to simulate the pressure increases in helium in case of a quench in the superconducting coils. We focus on two classes of coil in which helium is in direct contact with the conductor: coils consisting of cable-in-conduit conductors (as in ITER or JT-60SA), in which supercritical helium is forced through long channels; and bath-cooled coils, in which static helium is confined in short channels perpendicular to the conductor and opening into a bath (as in Tore Supra or Iseult). Various physical phenomena are responsible for the pressure increases in helium, which is subjected to strong heat flux in the conductor during a quench: at the local level, i.e. in the heated channels, the inertial forces that must be overcome to expel the fluid and the friction forces due to the induced velocity; at the global level, i.e. throughout the cryogenic system, the adiabatic compression of non-heated volumes hydraulically connected to the heated channels. Here we analyse the thermohydraulic behaviour of helium to highlight the dominant phenomena, according to the geometry of the helium flow paths. The results are applied to numerical simulation of the pressure rise in case of quench in a JT-60SA cable-in-conduit conductor (CICC) and in the bath-cooled Iseult coil.     

Temperature rise during the tensile test in superfluid helium

Measurements were made on the temperature rise of tensile test specimens undergoing plastic deformation and discontinuous flow in superfluid helium. Compared with the results in normal liquid helium, the frequency of load drop increased, and the temperature rise was suppressed.     

The stability of Nb3Sn-composite conductors cooled by normal and superfluid helium

The stability of Nb₃Sn superconductors, which were additionally stabilized by soldering copper to the prereacted cable, was investigated. Cooling channels in a sample coil provided access to LHe in the 4.2 K and the 1.85 K temperature range. The measurements show that a heat flow rate of 0.6 W cm^?2 at 4.2 K in the Nb₃Sn composite conductor guarantees a stable operation even at local heat inputs of about 0.5 J onto the surface of the conductor. In superfluid helium, recovery up to the take-off values of current in magnetic fields between 8 T and 13 T was found. A theoretical analysis was performed to explain the experimental results with respect to the cooling channel geometry, magnetic fields, and the local energy inputs to initiate a normal conducting region.     

A large superconducting dipole cooled by forced circulation of two phase helium

A large superconducting dipole cooled by forced circulation of two-phase helium is described. Details are given of the magnet design and construction. A short discussion of some cryogenic problems are presented together with the first results of the dipole's operation.     

超导磁体失超引起的氦泄放过程分析

为了合理设计系统的低温容器和安全装置(排气管、安全阀等),需要了解失超后容器内氦工质的状态变化过程.首先对失超后超导磁体与氦工质问的传热过程进行了分析,进而研究了安全阀开启前后容器内的热力学变化过程和氦流在管道内的流动过程.基于质量守恒定律、能量守恒定律、熵方程和热力学状态方程建立了相应的数学模型,数值模拟了一浸泡磁体失超后容器内压力和排气管道内质量流量随时间的变化关系.计算结果表明:低温容器的最高工作压力约为5.8 ×101.325 kPa;管道内的质量流量最大为0.645 kg/s.综合考虑排气管的口径和容器的设计压力可以降低容器的最高工作压力.     

AMS-02超导磁体中超流氦加注过程研究

论述了AMS-02超导磁体中超流氦的加注过程,进行了各个环节压降和温降的计算分析,并用图表表示了在不同质量流量加注超流氦工况下管路系统中氦工质的速度、压力、温度和含气量的变化情况.计算结果表明,氦工质从液氦主杜瓦加注到磁体杜瓦的过程中,其压力和温度不断降低,而含气量不断增加.同时也表明在节流阀VVP9中实现了常流氦向超流氦的转变和质量流量的控制.     

Peak and recovery heat flux densities in bath of subcooled superfluid helium

This paper describes the measurement of the peak and recovery heat flux densities (q_P, q_R) in a bath of subcooled superfluid helium. A RhFe wire (d = 51 μm, L = 34.8 mm) was horizontally immersed in a He Ilp cryostat, which served as the sample, the heater and the temperature sensor. For the steady state measurement a programmed voltage method with a triangular waveform of 20 s was applied. The measured q_P and q_R values cover the temperature range 1.81–2.1K at a pressure of 0.1 MPa. On the basis of the heat and mass transfer through phase interfaces, theoretical relations (without geometrical parameters) of q_P and q_R are derived, which have very wide application range due to the relative critical heat flux density method used for the calculations. The theoretical results fit the experimental results well for both q_P and q_R.     

Development of a variable quench pressure relief valve for superconducting magnet system

A new variable quench pressure relief valve (VQRV) for a superconducting magnet system has been developed at the High Energy Accelerator Research Organization (KEK). The VQRV is designed that the setting of the blowout pressure can be freely controlled and be maintenance-free for long-term operation.A prototype VQRV was tested under a high radiation environment up to 2.5 MGy. The heat load of 1.5 W at 4.2 K and a seat leakage rate of 4.5 × 10⁻⁷ kg/s at 4.2 K of the VQRV were confirmed. It has enough performances for the cryogenic system operation in the Japan Proton Accelerator Research Complex (J-PARC) neutrino beam line.The design and test results of the VQRV are described in this technical note.     

Temperature rise in a superconducting cable during overload

Approximate calculations are presented for the temperature rise and temperature fluctuation in an ac superconducting cable during current overload. The calculations are for a cable, which it is intended will remain superconducting throughout the overload. These show that because of the low value of T_c and low J_c niobium-titanium is not a suitable material to use. Niobium-zirconium with a 1 mm copper backing will carry continuous overloads of up to × 9 and a first cycle overload of up to × 11 provided it is fully stabilized against flux jumps. (Normal surface current rating = 4 × 10⁴ A m^?1). Niobium-tin can in principle carry very large overloads, but the limiting factors are its poor mechanical properties, the difficulty in producing thick layers of the material, and stabilization.     

Acoustic emission during quench training of superconducting accelerator magnets

Acoustic emission (AE) sensing is a viable tool for superconducting magnet diagnostics. Using in-house developed cryogenic amplified piezoelectric sensors, we conducted AE studies during quench training of the US LARP’s high-field quadrupole HQ02 and the LBNL’s high-field dipole HD3. For both magnets, AE bursts were observed, with spike amplitude and frequency increasing toward the quench current during current up-ramps. In the HQ02, the AE onset upon current ramping is distinct and exhibits a clear memory of the previously-reached quench current (Kaiser effect). On the other hand, in the HD3 magnet the AE amplitude begins to increase well before the previously-reached quench current (felicity effect), suggesting an ongoing progressive mechanical motion in the coils. A clear difference in the AE signature exists between the untrained and trained mechanical states in HD3. Time intervals between the AE signals detected at the opposite ends of HD3 coils were processed using a combination of narrow-band pass filtering; threshold crossing and correlation algorithms, and the spatial distributions of AE sources and the mechanical energy release were calculated. Both distributions appear to be consistent with the quench location distribution. Energy statistics of the AE spikes exhibits a power-law scaling typical for the self-organized critical state.     

The role of quench back in quench protection of a superconducting solenoid

This report demonstrates the role of quench back in the quench protection of high current density superconducting solenoid magnets with well-coupled shorted secondary circuits. The phenomenon of ‘quench back’ can be used to greatly reduce the size of an external quench protection resistor or even to eliminate the need for an external quench protection system altogether. A comparison is made with conventional magnet quench protection systems with and without a closely coupled secondary circuit.     

Demagnetization of a Bi-2223 high-temperature superconducting coil in RT-1 through spontaneous temperature rise

The Ring Trap 1 (RT-1) device produces a magnetospheric configuration for the confinement of a high-β plasma with a Bi-2223 high-temperature superconducting magnet. Here we report the results of emergency demagnetization of the superconducting coil, where we could not connect current leads, temperature measurement connectors, and connectors for a persistent-current switch (PCS) heater to the coil. The spontaneous warming of the coil caused a rise in the flux-flow resistance of the superconducting coil, and the persistent current slowly decreased as coil resistance increased. Approximately 98% of the total stored magnetic energy was safely released before the quenching of the PCS, and there was no substantial damage to the superconducting coil.     

Characteristics of a silicon pressure sensor in superfluid helium pressurized up to 1.5 MPa

It is known that a piezo-resistive pressure sensor, FPS-51B manufactured by Fujikura Ltd., is available for in situ pressure measurement in superfluid helium. The sensor covers a pressure range of zero to 103.4 kPa. The maximum rated pressure is 202.6 kPa at room temperature. The characteristics of the pressure sensor in a pressure range up to approximately 0.2 MPa were reported in detail for use in superfluid helium. We measured the pressure characteristics of this sensor up to 1.5 MPa to determine its availability to be used under much higher pressure. Measurements were taken using a cryostat, which can be pressurized up to 1.5 MPa at room temperature and superfluid helium temperatures. It was found that the sensor could be used in a superfluid helium environment at pressures up to 1.5 MPa, without any damage and with good reproducibility.     

A fast response superconducting thin-film probe for detection of second-sound shock waves in superfluid helium

Fast response probes are needed for studying the formation and propagation of second-sound shock waves (and for applying such waves to special measuring purposes) in superfluid helium. Newly developed superconducting thin-film probes enable shock-front rise times of down to 0.3 μs to be detected at signal-to-noise ratios higher than about 100. Using high vacuum evaporation techniques, such probes are relatively easy to produce. Their main body consists of a cylindrical quartz glass rod 1.5 mm in diameter with one end face polished to a plane of optical quality. The sensor strip is deposited onto this plane face as a two-component film of 0.02 mm width and 1 mm length. The temperature variations due to second sound cause changes in the resistance of the film and thus, at constant bias current, variations of the voltage drop across it. The temperature where the film undergoes its steep transition to superconductance and where, therefore, the probe works at its greatest sensitivity, is primarily fixed by the ratio of the two components (tin and gold) of the film, but can be adjusted to special values via the magnetic field produced by the adjustable bias current. The high resolution in time which is achievable by this probe makes it useful for accurate measurement of even small variations of the running time of second-sound shock waves. Such variations may be caused by flows, as is shown in the case of a flow produced by a rotating vane; their measurement may, therefore, serve as a tool for flow investigation.     

Experimental investigation of heat transfer through porous media in superfluid helium

An experimental investigation of heat transfer through porous media in superfluid helium has been conducted in the framework of the development of porous electrical insulations for superconducting magnet cables cooled by superfluid helium. Several types of porous media with different characteristics were tested and, in particular, samples with pore size diameters of 0.1 μm, 1 μm, 2 μm, 10 μm and 20 μm. Temperature and pressure were measured between an insulating inner bath and the cryostat bath, communicating only through the porous medium. The cryostat bath is held constant all along the measurement and, for each sample, the tests are performed for bath temperature from 1.4 K to 2.1 K with 0.1 K increment. Depending on the porous medium average pore size diameter, different flow regimes are observed: for porous media with a pore diameter of 0.1 and 1 μm, only the Landau regime is observed whereas for porous media with a pore diameter of 2 μm, we observed the Landau regime and the Gorter-Mellink regime. For samples with a pore diameter of 10 and 20 μm, measurements only permitted to detect the Gorter-Mellink regime. In the laminar regime, the permeability of the samples was determined and it was found that the permeability is constant for bath temperature above 1.9 K whereas it increases as the bath temperature decreases from 1.8 K to 1.4 K. For samples with a pore size diameter of 10 and 20 μm, measurement permits only to observe the turbulent regime and the analysis exhibits a constant average tortuosity for each samples, independently of the bath temperature.     

Longitudinal propagation of a normal phase along a superconducting wire and its transient heat evacuation to a normal helium bath

The longitudinal destruction of the superconducting state along a wire by an electrical subcritical current can occur if a strong enough thermal disturbance appears somewhere along the wire. The longitudinal expansion velocity of the normal phase along the wire is then strongly dependent on the different heat production and evacuation processes. As for a wire in a superfluid helium bath, a steady boundary thermal conductance across the wire wall and the normal helium bath can be deduced by adjusting the theoretical data with the experimental data. For the temperature range considered in this paper, the steady boundary thermal conductance seems to be independent of the helium bath temperature but not of the wire surface morphology. It is also shown that the experimental and theoretical data are in strong disagreement unless a transient term in the heat transferred across the wire wall and the liquid He I separating interface is introduced in the theoretical model. A transient heat transfer coefficient is defined, and its dependence on the different wire characteristics is investigated. An empirical relationship between the transient heat transfer coefficient and the wire characteristics is deduced for a given bath temperature. The transient heat transfer coefficient is shown to depend on the maximum thermal flux released into the bath across the wire wall and not on the wire surface morphology.     

Scattering of light by thermal ripplons on superfluid helium

It is well known that thermally excited waves on the free surface of a liquid can cause light to be diffusely scattered. In order to investigate the possible effects of the superfluid transition in liquid helium, we have measured the intensity of the surface scattering from the free surface of liquid helium as a function of temperature (1.3–2.1 K) and as a function of the scattering angle. The surface was illuminated vertically from above with a He-Ne laser. A measuring technique was devised which could not only distinguish among background scattering, scattering from the bulk liquid, and scattering from the surface, but could determine the ratio of surface to bulk scattering also. We have found that our results can be described by the theory of Mandelshtam, which is valid for classical liquids. According to this theory, the surface-scattered intensity is given by the ratio of temperature to surface tension. The angular dependence is determined by the characteristic wave vector dependence of the mean square amplitude of the fluctuations. The ratio between surface- and bulk-scattered intensity is calculated on the basis of the classical theory. The data indicate that in the frequency range around about 1 MHz no influence of the two-fluid nature of the superfluid is detectable.