Cryogenic System for the Large Helical Device

Cryogenic System

Abstract

The cryogenic system and cooling schemes for the Large Helical Device (LHD) are described. The cryogenic system for the LHD consists of a helium refrigerator/liquefier, the LHD cryostat in which the superconducting helical coils, poloidal coils and coil supporting structures are installed, and the peripheral equipment such as superconducting bus lines, control-valve boxes and cryogenic transfer lines. The helium refrigerator/liquefier has cooling capacities of 5.65kW at 4.4K and 20.6kW from 40 to 80K, and 650l/h liquefaction. Three different cooling schemes are utilized for each cooling object: a pool boiling for the helical coils (cold mass of 240t), a forced flow of supercritical helium for the poloidal coils (182t), a forced flow of two-phase helium for the coil supporting structure (390t) and the superconducting bus lines (total length of 463m). The heat in-leaks calculated from the final design of the cryogenic system become about half of the initial design values because of the extensive efforts of designing and developing each component. Many technical data were gathered during research and development, and were utilized to modify the configurations and operational conditions of the cryogenic system. To operate the complicated LHD cryogenic system automatically, we developed a new cryogenic control system which is highly flexible and functional in terms of both hardware and software.