Upgrading program for improving the cryogenic stability of LHD helical coils by lowering the operating temperature

Imagawa, S.; Hamaguchi, S.; Yanagi, N.; Sekiguchi, Hiroyuki; Moriuchi, S.; Mito, T.; Komori, A.; Motojima, O.

doi:10.1016/j.fusengdes.2006.07.088

Cited by 11 publications

(5 citation statements)

References 12 publications

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“…Upgrading methods were studied for the cooling system of the helical coils with utilizing the superfluous capacity that is more than 2 kW at 4.4 K at steady operation [12]. It is known that the cryogenic stability is improved at the lower temperatures by being subcooled or by being evacuated.…”

Section: Methods To Improve Cryogenic Stabilitymentioning

confidence: 99%

Achievement of high availability in long-term operation and upgrading plan of the LHD superconducting system

et al. 2007

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The Large Helical Device (LHD) that has been demonstrating high performance of heliotron plasma is the world's largest superconducting system. Availability higher than 98% has been achieved in a long-term continuous operation both in the cryogenic system and in the power supply system. It will be owing not only to the robustness of the systems but also to efforts of maintenance and operation. One big problem is shortage of cryogenic stability of a pair of pool-cooled helical coils. Composite conductors had been developed to attain the sufficient stability at high current density. However, it was revealed that a normal-zone could propagate below the cold-end recovery current by additional heat generation due to the slow current diffusion into a thick pure aluminum stabilizer. Besides, a novel detection system with pick-up coils along the helical coils revealed that normal-zones were initiated near the bottom of the coil where the field is not the highest. Therefore, the cooling condition around the innermost layers, the high field area, will be deteriorated at the bottom of the coil by bubbles gathered by buoyancy. In order to raise the operating currents, methods for improving the cryogenic stability have been examined, and stability tests have been carried out with a model coil and small coil samples. The coil temperature is planned to be lowered from 4.4 K to 3.5 K, and the operating current is expected to be increased from 11.0 kA to 12.0 kA that corresponds to 3.0 T at the major radius of 3.6 m.

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Section: Methods To Improve Cryogenic Stabilitymentioning

confidence: 99%

Achievement of high availability in long-term operation and upgrading plan of the LHD superconducting system

et al. 2007

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“…Hence, liquid helium does not flow smoothly within the helical coils along the longitudinal direction. Specifications of the helical coils are described in the [1]- [3].…”

Section: A Details Of Lhd Helical Coilsmentioning

confidence: 99%

“…Thermal resistance was given at the boundary between each region to take into account the pool-cooled coil. Based on the experimental result [4], the heat transfer coefficient of subcooled helium was used in (3). In this calculation, the influence of natural convection is not considered in the subcooled helium.…”

Section: B Numerical Model and Governing Equationsmentioning

confidence: 99%

Performance Tests of the Subcooling System for the LHD Helical Coils

Obana

Imagawa

Hamaguchi

et al. 2008

IEEE Trans. Appl. Supercond.

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Performance tests of the subcooling system for the helical coils in the Large Helical Device (LHD) have been performed. For the performance of the system, it is necessary to supply the helical coils with a subcooled helium mass flow of 50 g/s at 3.2 K. From the test results, the performance was able to fulfill the design requirements of the system. Additionally, the coil temperature has been evaluated with the quasi-one dimensional numerical calculation, in which the numerical model simplifying the helical coil configuration is used. The coil temperature would be 3.6 K under a steady heat load of 100 W from outside of the coil case in the system which maintains the above performance, as the result of the calculation.

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“…The Large Helical Device (LHD) is an experimental device for fusion plasmas and the LHD helical coils are large scale superconducting coils to provide heliotron magnetic field [1,2]. In 2006, a helium subcooling system was installed to the helical valve box in the cooling system of the LHD helical coils in order to enhance the magnetic fields and improve the cryogenic stability of the coils by lowering the operating temperature of the coils [3][4][5][6]. Figure 1 is a photograph of the LHD and the helical valve box with the helium subcooling system.…”

Section: Introductionmentioning

confidence: 99%

Operations of the Helium Subcooling System for the LHD Helical Coils during Ten Plasma Experimental Campaigns

Hamaguchi

Imagawa

Obana

et al. 2018

Plasma and Fusion Research

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The Large Helical Device has a helium subcooling system with two cold compressors for helical coils to enhance the magnetic fields and improve the cryogenic stability of the coils by lowering the coil temperature. The system was installed in 2006 and then it has stably supplied 3.2 K subcooled helium at the nominal mass flow rate of 50 g/s to the coils during ten plasma experimental campaigns. The running time of the cold compressors exceeds 30,000 hours and the total time of subcooling operations exceeds 20,000 hours. In the system, the supplied helium is subcooled in a heat exchanger of a saturated helium bath. The bath pressure and temperature are reduced by a series of two centrifugal cold compressors with gas foil bearing. In the steady state subcooling operation, the bath temperature is stabilized within range of 0.02 K with automatic flow control of helium gas through the cold compressors by a heater in the bath. The control method is also useful to protect the system by mitigating large disturbance of the pressure and the mass flow rate. In the present study, the thermal hydraulic behavior of the system and the operational performance during ten plasma experimental campaigns are reported.

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Upgrading program for improving the cryogenic stability of LHD helical coils by lowering the operating temperature

Cited by 11 publications

References 12 publications

Achievement of high availability in long-term operation and upgrading plan of the LHD superconducting system

Achievement of high availability in long-term operation and upgrading plan of the LHD superconducting system

Performance Tests of the Subcooling System for the LHD Helical Coils

Operations of the Helium Subcooling System for the LHD Helical Coils during Ten Plasma Experimental Campaigns

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