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Dynamic simulation of a helium liquefier has been conducted; using a Cryogenic-Process Real-time SimulaTor (CPREST), which has been jointly developed by NIFS and NSC (Nippon Sanso Co.) as a part of the National Laboratory and Industry Collaboration program. The C-PREST is primarily used to analyze a helium-refrigeration-plant operation. It is also designed to utilize as a platform for the plant process-analysis tool, to study coupled cryogenic phenomenon and to train operators.To demonstrate the capability of C-PREST, a dynamic simulation of a helium liquefier has been conducted. The discrepancy at TI 26 is due to the simplification of the modeling. The priority for C-PREST is to achieve the real-time dynamic simulation so that the CPU time must not be impacted by too much modeling detail. In this case, the GHe temperature increase in·the transfer line and the dewar are neglected. As a result, the model fails to predict operation disturbances as shown in the Fig. 2.We demonstrated a liquefaction process of a helium liquefier with C-PREST. This is the first time to perform the liquefaction with a dynamic simulation. Eyen though some simplification of modeling each component such as, a heat exchanger, a dewar, the results are accurate enough to study plant process and train operators.The results encourage us to proceed· to model a LHD helium refrigerator/liquefier for the next project, which consists of 7 expansion turbines and 14 heat exchangers. RCV3

The Large Helical Device (LHD), built in the 1990s, is a heliotron-type fusion plasma experimental device with the world's first fully superconducting magnetic confinement system. The LHD cryogenic system operated stably for 18 years from 1998 to 2015 with high availability exceeding 99%. Unfortunately, in August 2015, a fire occurred in the cold box of the He refrigerator during maintenance, and nonmetallic components such as multilayer insulation films, temperature sensors, and measuring instruments were burnt down. Repair work started in November 2015 and completed at the end of July 2016. In August 2016, a test operation of the He refrigerator was conducted, and the refrigeration power was compared with that measured in the initial performance test conducted in 1995. The measured equivalent refrigeration power at 4.4 K was 9.19 kW, representing a decrease ∼2% from the value of 9.38 kW measured in 1995. We attributed this slight decrease in refrigeration power to performance deterioration owing to aging over 18 years and not to the fire. The LHD restarted operation in January 2017, and its 19th operational cycle for a deuterium plasma experiment was conducted successfully up to August 2017. This paper reports the operational history and restart of the LHD superconducting magnet and cryogenic system.

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