Do Gyun Kim

A Study on the Optimization of an HTS Quadrupole Magnet System for a Heavy Ion Accelerator Through Evolution Strategy

This paper attempted to use a high-temperature superconductor (HTS) quadrupole magnet in a heavy ion accelerator. In such application, HTS quadrupole magnets are superior to conventional superconducting magnets because their heat loads can be removed more efficiently. Generally, the magnetic field quality of quadrupole magnets is assessed through harmonic analysis. In quadrupole magnets, the second (four-pole) component is the main magnetic field, and the field inhomogeneity is composed of the sixth (12-pole) component, the tenth (20-pole) component, and so on because of its symmetry. Therefore, the harmonic contents in a good field region can be defined to express the field quality of a quadrupole magnet by using higher order components (order > 2) with respect to the second component. In conventional analysis, multipole components can be obtained by a Fourier series of the radial field component. This paper analyzed the difference between radial and azimuthal multipole components. The HTS quadrupole magnet is wound with a rectangular tape and requires specific structures for refrigeration, which increases the harmonics. In this paper, the effects on the harmonic contents in relation to the HTS coil position and the shape of the iron pole were analyzed based on the HTS quadrupole magnet of the RAON system for the Rare Isotope Science Project in Korea. Several parameters were selected in designing the HTS quadrupole magnet system by using the evolution strategy method. Finally, all harmonic contents were optimized, and an HTS quadrupole magnet design was suggested.

Thermal and Mechanical Design of a HTS Quadrupole Magnet of In-Flight Fragment Separator for Rare Isotope Science Project (RISP)

An in-flight fragment (IF) system is used to produce and investigate an isotope beam of interest from heavy-ion accelerators, and it is composed of prestages and main stages. A quadrupole magnet of an IF preseparator is located in a high-radiation region, and the neutron radiation heat load is much larger than that of common superconducting magnets. Considering the efficiency of a cooling system, high-temperature superconducting (HTS) magnets are preferred to low-temperature superconducting magnets. To remove the large radiation heat load, the circulation of cold helium gas is used instead of conduction cooling. Pressurized helium gas circulates in cooling channels, and the optimized design of a cooling structure is required. Moreover, large Lorentz force is generated in the coil, and an appropriate supporting structure should be designed within the allowable conduction heat loss through the supporting structure. This paper describes the thermal and mechanical design of the REBCO HTS quadrupole magnet. Structural design is carried out considering the Lorentz force of the HTS magnet. Thermal analysis is also performed to estimate the heat loss using the designed structure. Finally, the design of a cooling system is carried out to keep the magnet temperature under 40 K against the estimated heat loss.

Huge Enhancement of Luminescence from a Coaxial-Like Heterostructure of Poly(3-methylthiophene) and Au

Recently, the light-matter interaction at nanoscale has attracted great interest from physicists, chemists and material scientists, as it gives peculiar optical properties that couldn’t be observed at the bulk scale. The synthesis and characterization of organic-inorganic heterostructures forming quantum dots, nanowires or nanotubes provide opportunities to understand their photophysical mechanism and to apply optoelecronic devices. Herein, we report a huge enhanced luminescence in a coaxial-like heterostructured poly (3-methylthiophene) (P3MT) with Au. We electrochemically synthesized P3MT nanowires (NWs) on a nanoporous template, and sequentially deposited Au on the surface of P3MT NWs. The diameter of heterostructured P3MT/Au NWs was about 200 nm, where the cladding-shape Au were about 10 nm. The visible range absorbance, with two new absorption peaks of P3MT/Au NWs, was significantly increased compared with that of P3MT NWs. Accordingly, the photoluminescence (PL) of a P3MT/Au NW was enormously increased; up to 170 times compared to that of P3MT NWs. More interestingly, an unexpected enhancement of PL was observed from cross-junction point of P3MT/Au NWs. The abnormal PL properties of P3MT/Au NWs were attributed to the charge transfer and the surface plasmon resonance between the cladding-shape Au and the core-shape P3MT, which resulted in the enhanced quantum yield. This incites us to reconsider the light-matter interaction in polymer-metal hybrid structures applicable for high-performance optoelectronic devices.

Test Results of Gas-Cooled Prototype Magnet for In-Flight Fragment Separator

The first quadrupole magnet at the in-flight fragment (IF) separator of heavy ion accelerator should withstand intense radiative heat load of neutron beam. Most of the HTS magnets are cooled by liquid cryogen or conduction cooling methods. However, the quadrupole magnet is exposed to the intense radiation heat load, and the quadrupole magnet can be cooled by the circulation of cold gaseous helium. The gaseous helium cooling system is composed of a liquid nitrogen precooler, a recuperator, a compressor at room temperature, and a single-stage GM cooler. Nichrome wire inside bobbin simulates the radiative heat load by neutron beam. The prototype magnet consists of two single pancake coils and has the half length of the full-scale HTS quadrupole magnet. HTS wire is wound on an aluminum bobbin without insulation layer. This paper describes the thermal stability test results of the prototype HTS magnet using additional heater to simulate the large radiation heat load. The operating current is transported while applying the additional heating on the bobbin surface, and the thermal characteristic is investigated. The experimental data will be used to design the thermal structure of the HTS magnets used in the first quadrupole magnet at the IF separator.