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

Abstract

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.

Recent Fusion Research in the National Institute for Fusion Science

“…In FY2006, the cryogenic system was reconstructed for the sub-cooling of helical coils, which realizes a higher magnetic field. Both inlet and outlet temperatures were successfully decreased to 3.2 K and 3.8 K, respectively, and the maximum magnetic field of 2.96 T became available [5]. This sub-cooling system has been safely running for over 7000 hours.…”

“…In the past 12 years, the superconducting coils have been excited 1,380 times without any serious problem and the confining magnetic field has been provided with up to 2.96 T in steady-state as shown in Fig. 1 (a), which realized 101,000 plasma discharges [5]. Three kinds of heating devices are equipped with LHD, that is, neutral beam injection (NBI) [6], electron cyclotron heating (ECH) [7] and Ion cyclotron heating frequency (ICRF) [8].…”

Operation and Control of Helium Subcooling System of LHD Helical Coils During Change of Rotational Speed of Cold Compressors

“…The cryogenic system of the LHD was upgraded in 2006 in order to improve the cryogenic stability of the helical coils thanks to reduction of the coil temperature and disappearance of helium gas by using subcooled helium as the coolant [3]. In the upgrade, the subcooling system for the helical coils was installed to a valve box and then it has been possible to supply subcooled helium of 3.2 K, 120 kPa at the inlet of the helical coils [4]- [6].…”

Performance Tests of the Subcooling System for the LHD Helical Coils

“…T HE cooling system for the helical coil of the Large Helical Device (LHD) has been upgraded to generate higher magnetic fields for improving the performance of plasma experiments [1]. In the upgraded system, which is called the "subcooling system", the liquid helium from the existing cooling system of the LHD is cooled down by an additional heat exchanger to generate subcooled helium of 3.0 K. In addition, heaters were attached on the outlet pipes of the coils to recover gaseous helium by evaporating the subcooled helium in the cooling system, which is not provided with return lines of liquid.…”