Norimasa Yamamoto

Status of a 200-Meter DC Superconducting Power Transmission Cable After Cooling Cycles

We constructed a facility of a 200-m HTS power transmission test cable (CASER-II) in 2010. Generally, an HTS cable contracts about 0.3% when it is cooled from room temperature to liquid nitrogen (LN2) temperature. The contraction of the 200-m HTS cable corresponds to 0.6 m. In order to realize the HTS power transmission system, it is an essential issue to absorb the mechanical stress of the HTS cable during the cycles of cooling-down and heating-up. The CASER-II uses smooth pipes as the cryogenic pipe for the cable line to reduce the pressure drop of the liquid nitrogen flow, whereas the other HTS cables use corrugated pipes to absorb the mechanical stress. The CASER-II employed (1) the movable terminals at the cable end, and (2) the extendable bellows inserted in the cryogenic pipe, to absorb the contraction of 0.6 m in cooling cycles. Even at the 4th cooling-down test, no damage was observed in the CASER-II. Use of the smooth cryogenic pipe enabled low pressure drop with low LN2 flow rate, and negative pressure drop appeared at less than 5 L/sec of the LN2 flow rate. This negative pressure drop was caused by the LN2 flow assisted by siphon effect due to the difference of LN2 density along the cryogenic pipe line with elevation of 2.6 m.

Cutting Speed of Electric Discharge Machining for SiC Ingot

A new method based on electric discharge machining (EDM) was developed for cutting a silicon carbide (SiC) ingot. The EDM method is a very useful technique to cut hard materials like SiC. By cutting with the EDM method, kerf loss and roughness of sample are generally smaller than those obtained by cutting with a diamond saw. Moreover, the warpage is smaller than that by the diamond saw cutting, and the cutting speed can be 10 times faster than that of the diamond saw at the present time. We used wires of 50 mm and 100 mm diameters in the experiments, and the experimental results of the cutting speed and the kerf losses are presented. The kerf loss of the 50 mm wire is less than 100 mm, and the cutting speed is about 0.8 mm/min for the thickness of a 6 mm SiC ingot. If we can maintain the cutting speed, the slicing time of a 2 inches diameter ingot would be about seven hours.

Slicing of Rotating SiC Ingot by Electric Discharge Machining

A new method of electric discharge machining (EDM) is proposed for slicing a large silicon carbide (SiC) ingot in order to realize low kerf loss and fast cut. This principle is based on the rotating ingot, and it is called the rotating slicing method (RSM). It would be defecate the cutting chip effectively and one-point discharge. In this paper, we reported results of examinations of the RSM experimentally. Unstable discharge was not observed. Discharge damages on the wire surface were fewer than those of the conventional method. Net cutting speed was almost the same as the present method for the 2-inches ingot. The rotation axis of the ingot should be perpendicular to the feed direction of the wire, and it is important to fix the performance of the EDM such as the kerf loss. Roughness of the cutting surface was 3.4 µm of Ra

Refrigeration Process to Realize a Multistage and Gas-Cooled Current Lead

Heat leak from the current lead is the major source of power loss for superconducting (SC) magnets, motors and generators, and short distance SC power transmission lines. Therefore, reducing the heat leak from the current lead can improve the economy of the SC system and allow the use of SC systems commercially. The Peltier current lead was proposed and developed at Chubu University to reduce the electrical power consumption of the refrigerator. The heat leakage of the Peltier current lead saves almost 40%, but the power requirement of the refrigerator is still high. In order to realize greater power savings, the multistage current lead (MCL) and the gas-cooled MCL are proposed. The heat leakage reduction of the gas-cooled MCL depends on the characteristics of the cryogen and its pressure and the realization of high cost of performance and a high-temperature refrigerator. Here, we show the calculation results of the high-pressure effect of cryogen and discuss the performance of nitrogen trifluoride as cryogen.

Intensity ratio of EUV emission lines in Fe XV studied with electron beam ion traps

We present emission spectra of highly charged Fe ions in the extreme ultraviolet range obtained with two electron beam ion traps. The intensity ratio between the 3s3p3P2 - 3s3d 3D3 (233.9 A) and 3s3p 1P1 - 3s3d 1D2 (243.8 A) transitions in Fe XV is given for various electron beam parameters. The experimental ratios are compared with theoretical model calculation as well as our previous experiment, where significant discrepancy with theoretical model was found. The present result, which has been obtained with a higher resolution spectrometer to investigate possible line blending, confirms that the discrepancy is not due to line blending.

Effects of Machining Fluid on Electric Discharge Machining of SiC Ingot

Recently, ingots of silicon carbide have been adapted to be sliced by the wire-cut electrical discharge machining. Fast slicing, and the reduction in the loss are important for slicing of the wafer. In this paper, characteristic features of the electric discharge machining in the ion-exchange water and the fluorine-based fluid were compared for these improvement. The discharge was caused by a pulse voltage applied to a ingot of silicon carbide and the wire in machining fluid, and the slicing was proceeded. As a result, improvement of surface roughness and kerf loss was confirmed, for the first time. In addition, the improving methods for fast slicing were considered.

Observation of Impurity Accumulation After Hydrogen Multi-Pellet Injection in Large Helical Device

Impurity accumulation is studied for neutral beam-heated discharges after hydrogen multi-pellet injection in Large Helical Device (LHD). Iron density proflles are derived from radial proflles of EUV line emissions of FeXV-XXIV with the help of the collisional-radiative model. A peaked density proflle of Fe 23+ is simulated by using one-dimensional impurity transport code. The result indicates a large inward velocity of i6 m/s at the impurity accumulation phase. However, the discharge is not entirely afiected by the impurity accumulation, since the concentration of iron impurity, estimated to be 3.3£10 i5 to the electron density, is considerably small. On the other hand, a ∞at proflle is observed for the carbon density of C 6+ , which is derived from the Zefi proflle, indicating a small inward velocity of i1 m/s. These results suggest atomic number dependence in the impurity accumulation of LHD, which is similar to the tokamak result.