75 research outputs found
National Institute for Fusion Science
"This annual report summarizes research activities at the National Institute for FusionScience (NIFS) between April 2007 and March 2008. The main objective of NIFS issymbolically expressed as the realization of a sun on the earth, generating a new, futuristicsource of energy. Human beings face a serious crisis concerning the change of the globalenvironment. It is obvious that the highest priority for us is to reduce the output of carbondioxide by developing a new energy source before a climate crisis occurs.
Magnetic Field and Force of Helical Coils for Force Free Helical Reactor (FFHR)
The electromagnetic force on a helical coil becomes smaller by decreasing the coil pitch parameter which is the angle of the coil to the toroidal direction. This makes it possible to enlarge the central toroidal field or to simplify the supporting structures of the coil. The plasma minor radius, however, becomes smaller with the pitch parameter, and a higher field is necessary to attain the same plasma performance. Another important item in a helical reactor is the distance between the helical coil and the plasma to gain enough space for blankets. In order to reduce the mass of the coil supports, a lower aspect ratio is advantageous, and an optimum value of the pitch parameter will exist around 1.2 and 1.0 for the helical systems of the pole numbers of 2 and 3, respectively
Operational status of the superconducting system for LHD
Large Helical Device (LHD) is a heliotron-type experimental fusion device which has the capability of confining current-less and steady-state plasma. The primary feature on the engineering aspect of LHD is using superconducting (SC) coils for magnetic confinement: two pool boiling helical coils (H1, H2) and three pairs of forced-flow poloidal coils (IV, IS, OV). These coils are connected to the power supplies by SC bus-lines. Five plasma experimental campaigns have been performed successfully in four years from 1998. The fifth operation cycle started in August 2001 and finished in March 2002. We have succeeded to obtain high plasma parameters such as 10 keV of electron temperature, 5 keV of ion temperature and beta value of 3.2%. The operational histories of the SC coils, the SC bus-lines and the cryogenic system have been demonstrating high reliability of the large scale SC system. The operational status and the results of device engineering experiments are summarized
Observation of charge-exchange spectra on C6+ +H in low-energy collision
The extreme ultraviolet spectra of C VI have been studied for a Neutral Beam Injection (NBI) plasmas in Large Helical Device (LHD). A strong distortion in the population distribution over the excited levels was observed and we conclude that is caused by charge-exchange recombining (CXR) processes between C^6+ ion and recycling neutral hydrogen. Spatially resolved measurements show that the C^6+-H CXR processes take place in the plasma peripheral region in LHD. We have taken a CXR pan of C VI 1s-4p line using the result of a calculation code
Stable long-term operation of superconducting current-feeder system for the LHD
A superconducting (SC) current-feeder system is used as the current transmission lines for the experimental fusion device, LRD. It consists of nine flexible SC bus lines with total length of 497 m, and nine pairs of gas-cooled current leads. To avoid the propagation of the ice on the leads, the temperature of the terminals had been kept in the range between 5 and 20 degrees C by the heaters. The measured voltage drops of all leads were less than 20 mV. The liquid helium levels of the leads and the sub-cooler tank will equalize by the siphon method. The total time of the coil excitations exceeds 3000 hours. We have demonstrated successfully that the SC current-feeder system was stable and easy to handle, and is useful for the SC experimental fusion device
Analysis on the cryogenic stability and mechanical properties of the LHD helical coils
Transient normal-transitions have been observed in the superconducting helical coils of LHD. Propagation of a normal-zone is analyzed with a numerical simulation code that deals with the magnetic diffusion process in a pure aluminum stabilizer. During excitation tests, a number of spike signals are observed in the balance voltage of the helical coils, which seem to be caused by mechanical disturbances. The spike signals are analyzed by applying pulse height analysis and the mechanical properties of the coil windings are investigated
Stability test results on the aluminum stabilized superconductor for the helical coils of LHD
Stability tests have been carried out on short samples of the aluminum/copper stabilized composite-type superconductors developed and used for the pool-cooled helical coils of the Large Helical Device. The waveform of the longitudinal voltage initiated by resistive heaters shows a short-time rise before reaching a final value, which seems to correspond to the diffusion process of transport current into the pure aluminum stabilizer. The propagation velocity has a finite value even for the transport current being lower than the recovery current, and it differs depending on the direction with respect to the transport current
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
Stability and safety estimates and tests of a superconducting bus-line for large-scale superconducting coils
We have been developing a flexible superconducting bus-line as a unit electrical feeder between large-scale superconducting coils and their power supplies away from the coils. The designed superconducting bus-line consists of a pair of +/- aluminum stabilized NbTi/Cu compacted strand cables and a coaxial four-channel transfer line. A full-scale model of the SC bus-line (20 m long) has been constructed and tested successfully up to 40 kA without a quench under the short-circuit condition. Stability tests were also done by inducing a forced quench with heaters. A minimum propagation current larger than 32.5 kA was confirmed. Thus, the bus-line was cryogenically stabilized at the rated current of 30 kA. We have examined the test results and evaluated the stability and safety margins of this bus-line. The design criteria for a superconducting bus-line are also shown for large-scale superconducting coils with operating current as a parameter
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 bubble
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