Refractive index measurement of hydrogen isotopologue mixture and applicability for homogeneity of hydrogen solid at cryogenic temperature in fusion fuel system

Deuterium (D)-Tritium (T) nuclear fusion reaction has potential as an energy source in the future. In both magnetic confinement and inertial confinement fusion reactors, solid D-T will generally be supplied as fusion fuel. The efficiency of the nuclear fusion reaction depends on the quality of solid D-T fuel, which is related to the composition, homogeneity, helium-3 (3He) content, and so on. However, there is no technique for in-situ examination of solid D-T fuel. In this study, we consider a simple and precise method for the characterization of solid hydrogen isotopologues at cryogenic temperature using refractive index measurement, and evaluate the distribution of hydrogen isotopologue composition and homogeneity. To evaluate without the effect of tritium decay, the homogeneity of the hydrogen (H2)-deuterium (D2) mixture is measured at first. By the in-situ refractive index measurement at cryogenic temperature, the homogeneity of solid H2-D2 mixture is roughly quantified. The phase diagram of the H2-D2 mixture shows a solid solution type. D2-rich crystal first appears from the liquid phase as a primary crystal. The composition of D2 in liquid phase ias homogeneous, whereas it reduces by obeying the liquidus line in the phase diagram with the crystallization. On the other hand, the composition of the H2-D2 mixture in solid phase is inhomogeneous because the mobility of H2 and D2 in solid phase was too slow to be homogeneous and solid. The compositions of H2-D2 mixture in liquid and solid phases could be evaluated by the in-situ refractive index measurement in time. Consequently, the refractive index measurement shows great potential as an inspection method of solid D-T fuel in fusion reactors.Zhang J., Iwamoto A., Shigemori K., et al. Refractive index measurement of hydrogen isotopologue mixture and applicability for homogeneity of hydrogen solid at cryogenic temperature in fusion fuel system. Nuclear Fusion 63, 076020 (2023); https://doi.org/10.1088/1741-4326/acd015

Effects of spatially limited external magnetic fields on short sample tests of large-scale superconductors

For short sample tests of large-scale superconductor coil conductors, it is difficult to get sufficient spatial uniformity using external magnetic fields because of the size limitations of test facilities. The effects of spatially limited external magnetic fields on short sample tests are discussed by comparing the test results for narrow and broad external magnetic fields. The authors tested short samples of pool-cooled 10 kA class superconductors using two kinds of split coils which are different in bore size. The measured recovery currents for the narrow external field are more than twice those for the broad field. It shows that the insufficient spatial distribution of the external field biases the stability measurements of superconductor

Mechanical issues of FIREX target under cryogenic environment

A typical FIREX target is assembled with a 500 ?m diameter PS shell, a glass fill tube and a gold cone guide. Each part is glued together by an epoxy resin. To date, several assembled targets have been cooled down to cryogenic environment. However, they could not survive a cool down process and appeared to rupture around the glued boundary between the PS shell and cone guide. To reveal a crucial factor of the target destruction, its thermal stress after cool down was calculated using the ANSYS code. A two dimensional axisymmetric calculation model is composed of the PS shell and cone guide glued by an epoxy resin which is covered on by an epoxy fillet. A cool down process from 293 K to 10 K was simulated. The calculation showed that the rupture of the shell would start from the PS shell and gold cone guide boundary and the target validity depended on how the epoxy fillet could reinforce the PS shell

Refractive index measurements of solid deuterium–tritium

Physical properties of tritium (T) and deuterium (D) have been of great interest as a fuel for nuclear fusion. However, several kinds of the physical properties in a cryogenic environment have not been reported. Optical properties in liquid and solid phases are indispensable for the quality control of the DT fuel. We study the dependence of the refractive index of solid DT on temperature. A dedicated cryogenic system has been developed and forms a transparent solid DT in a prism cell. Refractive index measurements based on Snell’s law were conducted. The refractive indexes of solid DT are from 1.1618 ± 0.0002 to 1.1628 ± 0.0002 in the temperature range of 19.40 K to 17.89 K.Iwano K., Zhang J., Iwamoto A., et al. Refractive index measurements of solid deuterium–tritium. Scientific Reports 12, 2223 (2022); https://doi.org/10.1038/s41598-022-06298-1

Lessons learned from twenty-year operation of the Large Helical Device poloidal coils made from cable-in-conduit conductors

The Large Helical Device (LHD) superconducting magnet system consists of two pairs of helical coils and three pairs of poloidal coils. The poloidal coils use cable-in-conduit (CIC) conductors, which have now been adopted in many fusion devices, with forced cooling by supercritical helium. The poloidal coils were first energized with the helical coils on March 27, 1998. Since that time, the coils have experienced 54,600 h of steady cooling, 10,600 h of excitation operation, and nineteen thermal cycles for twenty years. During this period, no superconducting-to-normal transition of the conductors has been observed. The stable operation of the poloidal coils demonstrates that a CIC conductor is suited to large-scale superconducting magnets. The AC loss has remained constant, even though a slight decrease was observed in the early phase of operation. The hydraulic characteristics have been maintained without obstruction over the entire period of steady cooling. The experience gained from twenty years of operation has also provided lessons regarding malfunctions of peripheral equipment

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

Development of FAIR conductor and HTS coil for fusion experimental device

This study is aimed at the development of high-temperature superconducting (HTS) magnets for application in a fusion experimental device next to the Large Helical Device (LHD). By applying the features of an HTS, high current density and high stability can be balanced. As a candidate conductor, REBCO tapes and pure aluminum sheets are laminated and placed in the groove of an aluminum alloy jacket with a circular cross-section, after joining a lid to the jacket using friction stir welding, and twisting the conductor to homogenize its electrical and mechanical properties. The FAIR conductor derives its name from the processes and materials used in its development: Friction stir welding, an Aluminum alloy jacket, Indirect cooling, and REBCO tapes. Initially, the degradation of the critical current of the FAIR conductor is observed, which was eventually resolved. The development status of the FAIR conductor has been reported

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

Improvement of Ic degradation of HTS Conductor (FAIR Conductor) and FAIR Coil Structure for Fusion Device

As a high-temperature superconducting (HTS) conductor with a large current capacity applicable to a nuclear fusion experimental device, REBCO (REBa 2 CuO y ) tapes and high-purity aluminum sheets are alternately laminated, placed in a groove of an aluminum alloy jacket having a circular cross section, and the lid is friction-stir welded. To make the current distribution and mechanical characteristics uniform, the conductor is twisted at the end of the manufacturing process. In the early prototype conductor, when the I c was measured in liquid nitrogen under self-magnetic field conditions, I c degradations were observed from the beginning, and the characteristic difference between the two prototype samples under the same manufacturing conditions were large. Furthermore, I c degradation was progressed by repeating the thermal cycle from room temperature to liquid nitrogen temperature. This I c degradation did not occur uniformly in the longitudinal direction of the conductor but was caused by local I c degradation occurring at multiple locations. If the conductor was not manufactured uniformly in the longitudinal direction, the difference in thermal shrinkage between the REBCO tape and the aluminum alloy jacket caused local stress concentration in the REBCO tape and buckling occurred. Element experiments to explain this mechanism were conducted to clarify the conditions under which I c degradation due to buckling occurs. Then prototype conductors were tested with improved manufacturing methods, and as a result, I c degradation could be suppressed to 20% or less. We have achieved the prospect of producing a conductor with uniform characteristics in the longitudinal direction