Satarou Yamaguchi

One dimensional simulation for Peltier current leads

Current leads, which connect superconducting magnets at the liquid helium temperature and power supplies at the room temperature, are the major source of heat leaking into cryostats, and therefore largely determines the running cost of magnet systems. Heat leak can be reduced by using high-temperature superconductors as the low-temperature (4 K-77 K) segments of current leads. Another method to reduce heat leakage, recently proposed, uses Peltier thermoelectric elements as the high-temperature (200 K-300 K) segments of current leads. These thermoelements effectively pump heat out of cryostats without using separate sources of electricity. The authors carried out experiments and numerical calculations with such Peltier current leads and found out that they reduce heat leak at 77 K by 20-30 percent.

Research activities of DC superconducting power transmission line in Chubu University

A experiment of a DC Superconducting power transmission line using HTS conductor was started in Chubu University, Japan in autumn 2006. The first cooling down began in October 2006. The coolant is liquid nitrogen, and the cooling system used a cryogenic cooler and cold pump. The operation temperature is the range of 72 K - 80 K. The power cable has a total length of 20 m, and composed of thirty-nine Bi-2223 HTS tape conductors with critical current approximately100 A. The power cable achieved a 2.2 kA. The insulation voltage of the cable is 20 kV. In order to reduce the heat leakage and to avoid the current imbalance in the HTS tapes, we installed the Pelteir Current Lead (PCL) for each of the nineteen HTS tapes of the cable, with remaining twenty HTS tapes connected individually by the usual copper leads (CCL). Depending on the visual observation, and the measurement of temperatures of the current leads, the heat leakage of the PCL is lower than that of CCL. We installed a current transformer for each individual HTS tape conductor circuit, and measured the current of each HTS tape conductor. We measured the critical current of each HTS tape after the cable was installed into the cryostat, and degradation was not observed. Since the variation of the current in each tape is less than 10%, we eliminated the problem of current imbalance. Computational Fluid Dynamics is used to estimate the pressure drop, showing that the straight-tube cryostat has the advantage against the bellows- and corrugated-tube cryostats to reduce the pressure drop of the circulation of coolant. We proposed further to use the siphon for circulating the coolant in order to reduce the circulation losses and costs. We proposed that the voltage of the system be kept below 30 kV in order to use low cost power inverters. This choice can increase the storage energy of the power transmission line itself if we do not use a co-axial cable system because of it allows use of large current. And the magnetic energy of power grid is estimated to 4.5MJ/km for ±30 kA.

Peltier current lead experiment and their applications for superconducting magnets

Many applications of superconductivity are studied because they are basically nondissipative systems; however, electric power is spent to keep them at low temperature. If heat leakage is high, the advantage of the superconductivity will be reduced or disappear, and therefore we must reduce the heat leakage as much as we can in the actual system. The Peltier current lead (PCL) was proposed to reduce heat leakage from the electric circuit of the superconducting magnet system. PCL is composed of thermoelectric semiconductors, metal leads, and high temperature superconducting (HTS) leads. Bismuth–tellurium alloys (BiTe) are used as the thermoelectric semiconductors in the experiment, and these are placed on the room temperature side. Optimum design of the PCL is conducted to solve a heat leakage equation, and depends on the transport parameters such as thermal conductivity, electric resistivity, and Seebeck coefficient of the materials of the PCL. The thermal conductivity of the BiTe is about 0.4% of the copp...

Numerical calculations for Peltier current lead designing

Abstract Peltier current leads (PCLs) are numerically analyzed. They consist of a thermoelectric element (TE) inserted into a pure copper lead at its room temperature end, with or without a high-Tc superconductor below 77 K. We solve a one-dimensional heat conduction equation to study their thermal behavior under the self-cooling condition. Each section of the lead is cooled with a helium gas under various degrees of cooling efficiency. Due to the Peltier effect, the PCLs are expected to exhibit less heat leak at the low temperature end than conventional current leads. Our calculations confirm that the PCLs show as much as 30% reductions of the heat leak when they are optimized. Temperature profiles and other physical quantities of interest are also calculated, which are useful for developing TEs and for designing PCLs.

Experimental Observation of Anomalous Magneto-Resistivity in 10–20 kA Class Aluminum-Stabilized Superconductors for the Large Helical Device

Degradation of recovery current due to the unexpected enhancement of resistivity of aluminum stabilizers has been observed in pool-boiling-type superconductors that have been developed for the helical coils of Large Helical Device. Dependence of the measured resistivity on the magnetic field suggests that this is a kind of anomalous magnetoresistivity. The Hall effect in metal-metal composites is considered to be the most plausible candidate to explain this observation. We compared our data with the calculated values based on this model and confirmed that this model explains the experimental results well.

Characterization of Electric Discharge Machining for Silicon Carbide Single Crystal

In this study, we report electric discharge machining (EDM) as a new cutting method for silicon carbide (SiC) single crystals. Moreover, we discuss characteristics and usefulness of the EDM for the SiC. The EDM realized not only high speed and smooth cutting but also lower surface damage. Defect propagation in the EDM SiCs have been also estimated by etch pits observation using molten KOH, however, we confirmed the EDM has caused no damage inside the SiCs in spite of high voltage and high temperature during the machining.

Control of Toroidal Expansion of a High Beta Plasma Column in a Pinch Tokamak by Programmed Pulsed Vertical Field

Dynamics of high beta toroidal pinch plasmas in the major radius plane are calculated after a simple model and compared with experimental results of STP-2 screw pinch1) which is operated under the condition of Te20 eV and ne1.2×1015/cm3. It is demonstrated that the calculation agrees with the experimental results in the early phase of the discharge where the plasma keeps a high beta value (poloidal βp3) while the discrepancy between them begins to grow after 8 µs from the start of the main discharge because the βp value decreases considerably. In order not to deviate the plasma column from the center of the discharge tube a concept of the effective dissipative force generated by a proper programming of the pulsed vertical field is introduced. It is demonstrated that this concept is useful to keep the shock-heated high beta plasma in the center of the discharge tube since unfavorable toroidal expansion disappears completely in a few microseconds by a proper programming of the pulsed vertical field.

Robust Noise Modulation of Nonlinearity in Carbon Nanotube Field-Effect Transistors

Carbon nanotubes (CNTs) are one of the candidates for nanosize devices such as field-effect transistors. CNT field-effect transistors (CNTFETs) have very special properties sometimes caused by surface states. For example, they are also well known as noisy devices caused by the molecule adhesion on the surface. Nonlinear systems, however, have some advantages such as weak signal detection or enhancement in working with noise. The small signal enhancement was conventionally studied as stochastic resonance. Therefore, we study the modification of nonlinearity of the systems under noise. For actual applications, the noise is also generated from the devices. Thus, we combined the noise CNTFET and another CNT transistor for the trial nonlinear system. Then, the sine wave amplification in the transistor with 1/ f noise of CNTFETs was measured. We used two different combinations of CNTFETs for noise and nonlinear CNTFETs, and observed the robustness of the noise modification on the nonlinearity.

Reduction of heat leak in cryogenic system using Peltier current leads

Abstract Peltier current leads (PCLs) for cryogenic systems are investigated in regard to temperature dependence of thermoelectric materials. Due to the Peltier effect on the thermoelectric parts of the current lead, PCLs act as heat pumps. It is expected that PCLs will reduce the amount of heat leak from the room temperature side to the low temperature side of a cryogenic system. Six (three each for p and n type) hot-pressed BiTe samples for PCLs are selected to estimate PCL performance. Our experimental results and analyses indicate that PCLs show a capacity in the order of several hundred Amperes and as much as 20–30% reductions of heat leak.

Stability Characteristics of the Aluminum Stabilized Superconductor for the LHD Helical Coils

Stability tests have been carried out on the aluminum stabilized composite-type superconductors developed and used for the pool-cooled helical coils of the Large Helical Device (LHD). The longitudinal voltage of a normal zone shows a short-time rise before reaching a final value, which seems to be explained by the rather slow diffusion process of transport current into the pure aluminum stabilizer and the copper housing. The propagation velocity has a finite value even below the recovery current, and it differs depending on the direction of transport current. A numerical analysis dealing with transient thermal and electromagnetic relaxation processes in the aluminum stabilizer well explains the experimentally observed results.