Hirotaka Chikaraishi

Asymmetrical normal-zone propagation observed in the aluminum-stabilized superconductor for the LHD helical coils

Transient normal-transitions have been observed in the superconducting helical coils of the Large Helical Device (LHD). Stability tests have been performed for an R&D coil as an upgrading program of LHD, and we observed asymmetrical propagation of an initiated normal-zone. In some conditions, a normal-zone propagates only in one direction along the conductor and it hence forms a traveling normal-zone. The Hall electric field generated in the longitudinal direction in the aluminum stabilizer is a plausible candidate to explain the observed asymmetrical normal-zone propagation.

Development of UPS-SMES as a protection from momentary voltage drop

We have been developing the UPS-SMES as a protection from momentary voltage drop and power failure. The superconducting system is suitable as electric power storage for large energy extraction in a short time. The most important feature of superconducting coil system for the UPS-SMES is easy handling and maintenance-free operation. We have selected low temperature superconducting (LTS) coils instead of high temperature superconducting (HTS) coils from the viewpoint of cost and performance. However, it is difficult for the conventional LTS coils to fulfill maintenance-free operation since the cooling methods are either pool boiling with liquid helium or forced flow of supercritical helium. Thus, a conduction cooled LTS pulse coil has been designed as a key component of the UPS-SMES. The development program of 1 MW, 1 sec UPS-SMES is explained.

Development and Test of JT-60SA Central Solenoid Model Coil

A central solenoid (CS) model coil (CSMC) was manufactured by using real manufacturing jigs and procedure to validate the CS manufacturing processes for JT-60SA. The winding accuracy and the temperature control precision during the heat treatment met the requirements. The vacuum pressure impregnation process was also successfully finished. The cold test of the CSMC was performed as a final check of the manufacturing process. The joint resistance, the Ic, and the pressure drop measurements were conducted as the verification test. The results of verification test satisfied the design requirements. These results indicate that the manufacturing processes of the JT-60SA CS has been established. The manufacturing of real CS pancakes just started after finishing the CSMC test.

Coupling losses in cable-in-conduit conductors for LHD poloidal coils

Abstract Coupling losses in Large Helical Device (LHD) poloidal coils have been measured during operations with three different waveforms. The superconductors of the poloidal coils are cable-in-conduit conductors (CICC) cooled by supercritical helium. In the experiments, the operating currents were simultaneously changed with a given waveform, and the enthalpy increase due to the losses was observed at the inlet and outlet of the helium coolant. Inter-strand coupling currents through resistive contact points mainly caused the losses. Time constants of the coupling currents are estimated by using analytical expressions with a circuit model. The results indicate a broad distribution of the time constants from the order of 10–1000 s.

Results of stability test in subcooled helium for the R&D coil of the LHD helical coil

Helical coils of the Large Helical Device are pool-cooled superconducting magnets. The operating current is restricted below about 90% of the design current because a normal-zone has propagated dynamically at several times at almost the same current. In order to estimate the effect of lowering temperatures on the cryogenic stability, an R&D coil was made of the same conductor. The cryogenic stability of the R&D coil was examined in saturated and subcooled helium. A normal-zone was initiated by a heater inserted between the conductor and the layer to layer spacer. The propagation was detected by voltage taps. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to begin propagation is 10.7 to 10.8 kA. It becomes higher at the lower temperature, and it exceeds 11.7 kA in subcooled helium of 3.5 K as a temperature inside the R&D coil.

Analysis of the normal transition event of the LHD helical coils

Normal transitions and a subsequent quench were experienced with the pool-cooled helical coils of the Large Helical Device (LHD) during its excitation test. Although the initiated normal zone once started to recover, a disruptive transverse propagation followed and triggered an emergency discharging program. The cryogenic stability of the composite-type superconductor has been studied by sample experiments as well as by numerical calculations. Due to the rather long magnetic diffusion time constant in the pure Al stabilizer, transient stability of the conductor seems to play an important role for driving finite propagation of a normal zone. The cause of the final quench is also discussed from the viewpoint of cooling deterioration due to a possible accumulation of He bubbles.

Results on the superconducting magnet system for the Large Helical Device

The Large Helical Device (LHD) is a plasma physics experimental device with eight superconducting coils. Design and construction of LHD started in April 1990. The trial operation and the first plasma discharge of the eight-year Phase I project for LHD were finished on the last day of March 1998 as initially planned. The second experimental campaign and several excitation tests were conducted from August 1998 to January 1999. Major test results on the superconducting magnet system for LHD are as follows: (1) The cooldown time of the LHD coils and support structure was 23 days. The LHD cryogenic system succeeded in 6400-hour operation and proved its high reliability. (2) Although the inside coil lead and the inner block of one helical coil caused two-stage successive normal propagation at a helical current of 11.45 kA, they were energized to the same current in the superconducting state in the next excitation test. (3) All six poloidal coils were excited stably. (4) Nine flexible superconducting bus-lines with a total length of 497 m were operated stably and safely. In the third experimental campaign many plasma discharge tests up to a plasma field of 2.9 T were carried out.

Superconducting current feeder system for the large helical device

A flexible superconducting (SC) busline was developed as a current feeder system for the fusion experimental device, LHD. An aluminum stabilized NbTi/Cu compacted strand cable was developed to satisfy the fully stabilized requirements at a rated current of 31.3 kA. A pair of SC cables was electrically insulated and installed in a cryogenic transfer line. Measured breakdown voltage in the 77 K helium gas is 8.33 kV. Nine sets of SC current feeders with 45-65 m lengths are installed for LHD. The total heat loads into 80 and 4.2 K levels are estimated to be 2.12 and 1.02 kW, respectively. The SC current feeder system is designed to maintain its rated capacities for 30 minutes, whenever the coolants supplied to the current feeder system are accidentally stopped.

Winding techniques for conduction cooled LTS pulse coils for 100 kJ class UPS-SMES as a protection from momentary voltage drops

In order to develop the 100 kJ class UPS-SMES as a protection from momentary voltage drops, design of the conduction cooled LTS pulse coil was carried out and special winding machine has been developed. Such coil is required to simultaneously attain low AC loss and high stability and the distributions of temperature in the coil are sensitively controlled. For this purpose, an aluminum stabilized conductor with circular cross-section composed of a Cu stabilized NbTi Rutherford cable was used as the winding conductor, and in the winding process the twist angle of the conductor around its axis was controlled to adjust the direction of edge-on orientation to the Rutherford cable to direction of local transverse magnetic fields applied to the conductor in winding area of the coil. The developed winding machine is used for this winding method. As a result, conduction cooled LTS pulse coil can be expected to operate stably in adequate temperature margin.

Summary of a 1 MJ Conduction-Cooled LTS Pulse Coil Developed for 1 MW, 1 s UPS-SMES

The development study of a 1 MJ conduction-cooled low temperature superconducting (LTS) pulse coil used for a 1 MW, 1 s UPS-SMES is summarized. We have developed a conduction-cooled LTS pulse coil as a key technology for the UPS-SMES. The AC loss reduction and the high stability are required for the SC conductor for a LTS pulse coil because of a limited cooling capacity of 4 K cryocooler. The conductor of a NbTi/Cu compacted strand cable extruded with an aluminum was designed to have the anisotropic AC loss properties to minimize the coupling loss. The coil was wound, utilizing a specially developed automatic winding machine which enables an innovative twist-winding method. The Dyneema FRP (DFRP) spacers and the Litz wires (braided wires of insulated copper strands) were inserted in each layer in order to enhance the heat transfer in the coil windings. The coil was installed in the test cryostat and was connected to three GM cryocoolers, which have a total cooling capacity of 4.5 W at 4 K and 240 W at 50 K. The coil was cooled conductively without liquid helium by attaching the end of the Litz wires directly to the cold heads of the cryocoolers. The cooling and excitation test of the 1 MJ coil has been done successfully. The test results validated the high performance of the conduction-cooled LTS pulse coil, because the high thermal diffusivity resulted in the rapid temperature stabilization in the coil.