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The first large well vacuum/atmosphere-protection heat treatment system, specialized for heat treatment of Nb3Sn superconducting magnet coils, has been recently installed in the High Magnetic Field LAb (HMFL) in Hefei, China. This heat treatment system is designed for the heat treatment of Nb3Sn coils made from the Cable in Conduit Conductor (CICC) to form the Nb3Sn superconducting magnet. The system can also be used for high temperature heat treatment of other large-scale coils.Currently, only a few countries, including USA, Japan, and Republic of Korea, have similar equipments. The successful Research & Development of this equipment breaches a gap in superconducting magnet technology in China and provides the basis for further development and production of large-scale Nb-Sn superconducting magnets.
The heat treatment system, jointly developed by CHMFL and BEIJING SEVENSTAR ELECTRONICS CO., LTD, composed mainly the furnace, the purging gas supply system, the control system, and the gas impurities monitoring system. The purging gas supply system includes the gas flowing pipes system for the conductor, the vacuum system for the conductors, the vacuum system for the furnace, etc. The gas flowing pipes system can be used for the cleaning of conductor’s internal part, as well as the realization of automatic control of the gas flux in the conductor during the atmosphere protected process. The gas flowing pipes system provides 7 pipe lines for gas flowing inside the conductor, which allows for simultaneous heat treatment of 7 groups of conductors. In order to reduce the cleaning time of the conductor, the vacuum system is used to clean the internal part of the Nb-Sn conductor until a relatively high base vacuum is achieved. The vacuum system for the furnace adopts two vacuum units, each consistes of a oil diffusion pump, a roots pump and two mechanical pump. The vacuum degree is higher than 1×10-3 Pa in the furnace.
Installation and callibration of the heat treatment system is focused on the important technical parameters, and all parameters meet or exceed specifications. The cold state vacuum degree is higher than 8×10-4 Pa, and the hot state vacuum degree exceeds 1×10-3 Pa, with a pressure rising rate under 0.005 Pa/h. Under atmosphere protection, the furnace temperature homogeneity (inside a cylindrical region of 2.5 m diameter and 2.5 m height) is less than ±5℃. For vacuum condition, temperature homogeneity isless than ±3℃. Temperature homogeneity is the most important factor for the heat treatment of large-scale Nb-Sn coils. Our test results suggest that the temperature homogeneity, either under Ar gas protection or vacuum condition, reaches a very high level.
The heat treatment system design incorporated many innovative techniques. For example, it can be operated in two modes, which can be switched directly from one to the other during the heat treatment process. In order to achieve higher temperature homogeneity, soaking and the ventilation equipment are used in the furnace to allow for a homogeneity on par with international standard. Successful R&D of this equipment will guarantee the smooth construction of the "Static High Magnetic Field Device" under the national grand scientific project of the Eleventh-Five-Year Plan. Additionally, it will provide an important technological equipment for the future development and applications of Nb-Sn superconducting magnets in China.
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