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In the 200 m high temperature superconducting cable test facility, BSCCO tapes with the critical current of 160 A in self field are used for the two superconducting conductors corresponding to each polarity of DC power transmission. In order to improve the cable property by tape configuration, we have investigated the critical current of a BSCCO tape with different structures. Previous study shows that the critical current of BSCCO tape decreases by 15% for the 3-layer stack structure. However, the stack structures are needed to obtain high current capacity of a cable. Therefore we study the performance of BSCCO tape in the stacked tapes conductor. The insulated tapes are arranged in a tape-above-tape structure and different currents are applied to each tape. The critical current of the target sample is measured by the four probe method by immersing the samples into liquid nitrogen bath. The experiments show that the improvement and degradation of the critical current of BSCCO tape in the stacked conductor are observed under different current feeding mode due to the magnetic field interactions between the tapes. The critical current of BSCCO in the stacked conductor is enhanced by 50% to more than 230 A under the opposite-direction current feeding mode.

A second cooling test of CASER2, the 200 m class superconducting direct current power transmission system at Chubu University, was performed from August to October 2010. During this time, the system was cooled down and the liquid nitrogen which cooled the superconducting power cable was circulated in the cryogenic pipe of CASER2. The heat leak of the cryogenic pipe was measured and the values of about 300 W was obtained. This result will be used further improvements of CASER2.

We designed and constructed the experimental device of 200-meter superconducting DC cable system in the beginning of 2010 and did the first experiment until March, 2010, and the second experiment has been done from August to October 2010. This experimental device is called CASER-2, and the main aim of the experiment is to demonstrate high performance of DC superconducting cable system for comparing the copper cable system. Thermal contraction and expansion during the thermal cycle are discussed in the paper because if the thermal stress of the HTS tape conductor is high, the critical current of the HTS tape is reduced. Therefore, it is the first priority to keep the system in high performance and safety. We also mentioned several experiments and ideas in the paper, such as the low heat leaks of the terminal cryostat and the cryogenic pipe, and the low circulation power, energy storage function of the cable system. In this paper, we present the experimental results of the CASER-2, and discuss and estimate the performance of the system and several ideas. These discussions include the specification of the future long cable system, and show the subjects to solve for the future system.

In the present experiments three straight high temperature superconducting BSCCO tapes were set parallel to each other on a board and the currents were fed to the tapes. The critical currents of the tape which was set between two tapes were measured with respect to the gap between the tapes. We observed that the critical current increases with decreasing the gap. The calculation of the magnetic flux lines around the tape by finite element method demonstrates the effect on self magnetic field by the adjacent tapes. We could provide some hints on how to arrange the tapes in the power cable for DC power transmission.

A liquid nitrogen (LN2) is usually used to keep the high-temperature superconducting (HTS) cable low temperature. A pump is utilized to circulate LN2 inside the cryopipes. In order to minimize heat leakage, a thermal siphon circulation scheme can be realized instead. Here, we discuss the effectiveness of thermal siphon with counter-flow circulation loop composed of cryogen flow channel and inner cable channel. The main feature of the system is the existence of essential parasitic heat exchange between upwards and downwards flows. Feasibility of the proposed scheme for cable up to 500 m in length has been investigated numerically. Calculated profiles of temperature and pressure show small differences of T and p in the inner and the outer flows at the same elevation, which allows not worrying about mechanical stability of the cable. In the case under consideration the thermal insulating properties of a conventional electrical insulating material (polypropylene laminated paper, PPLP) appear to be sufficient. Two interesting effects were disclosed due to analysis of subcooling of LN2. In case of highly inclined siphon subcooling causes significant increase of temperature maximum that can breakup of superconductivity. In case of slightly inclined siphon high heat flux from outer flow to inner flow causes condensation of nitrogen gas in outer channel. It leads to circulation loss. Results of numerical analyses indicate that counter-flow thermosiphon cooling system is a promising way to increase performance of short-length power transmission (PT) lines, but conventional subcooling technique should be applied carefully.

A compact electrostatic quadrupole lens system has been developed, and experiments have been conducted to study transport and focusing of intense He + beams with β=v/c =0.0029 and generalized perveance (Theory and Design of Charged Particle Beams, Wiley, New York, 1994) K=0.0013. Such beams are transported without any loss of beam particles and focused at a distance of about 0.6 m from the source with emittance growth less than a factor of about 1.8. This compact ESQ system has merits to satisfy a critical demand for developing intense He 0 beams for alpha-particle diagnostics in fusion-relevant machines.

The Large Helical Device (LHD) is the only machine that relies upon negative ion-based neutral beam injection system (NBI) as the principal heating power source. Since the first injection in 1998, NBI has been used to heat over 3000 shots for 2 years. Performance has progressed step by step to reach an injection power of 4.5 MW, with beam energy up to 165 keV, and pulse lengths up to 80 s at 0.5 MW. System operating level has reached ∼30% of the design power and ∼65% of the H− design current. This tangential high-energy NBI system based on negative ions has proved to be as efficient a heating system in a helical device as in a tokamak. The ratio of injection neutral beam power to high voltage drain power (injection power efficiency) was typically about 0.3. This is a low efficiency compared to the design one. For long pulse injection beam blocking (such as observed on PLT) was encountered, but it did not limit the operation. In order to achieve the design output of 15 MW, a great R&D effort is underway on the negative ion test stand to improve the performance of 40 A 180 keV negative hydrogen ion source.