Search

Diffusion welding of metallic materials requires contact of clean surfaces by removing surface films. However, there is microscopic unevenness on the surfaces and they are covered with oxide layers, absorbed gas layers, contaminating substance layers, etc. Welding is prevented by this unevenness and by these surface films and it is the stability of the surface films, in particular, which affects welding. Aluminium is regarded as a particularly difficult material for diffusion welding since it has a highly stable strong oxide film. Hence, to weld aluminium, it is necessary to remove this stable film so that welding can be performed. This article describes will describe the result of the investigation into the effect of twist angle, welding pressure and temperature, film thickness and surface roughness on the joint strength of the material.

The BigRIPS fragment separator in the RIKEN RIBF project is characterized by fourteen superconducting triplet quadrupoles (STQ's) with large apertures and high-field gradients. While the five STQ's on the first stage of the BigRIPS are cooled by a liquid helium cryogenic plant, the nine STQ's on the second stage are cooled by small cryocoolers on their cryostat. This first stage serves to produce and separate RI beams, so that the first two STQs are exposed to the high radiation from the production target and beam dumps, which causes radiation heat loads as well as radiation damages. Measurements of the radiation heat loads to the cryostats of the first two STQs during commissioning operations of the RIBF facility are reported.

Development of Cryogenic Process Real-time SimulaTor (C-PREST) has been finalized to provide a platform to study the entire cryo-plant processes of Large Helical Device (LHD). Although LHD utilizes three different cooling schemes, a pool boiling, a forced-flow supercritical helium cooling and a two-phase flow cooling, C-PREST successfully demonstrated its capability to simulate the LHD cooldown process and filling five Liquid helium (LHe) reservoirs independently. To understand the dynamic behaviors of the cryo-plant, the simulations have been carried out for a transient operating condition as introducing the thermal disturbance(s) to the 20,000 liter LHe reservoir of the refrigerator. This paper describes simulations of a large cryogenic plant and discusses dynamic behavior of helium refrigerator/liquefier under various thermal disturbances. Computational speeds associated with these operating conditions were also discussed.

Dynamic behavior of a helium liquefier has been studied in detail with a Cryogenic Process REal‐time SimulaTor (C‐PREST) at the National Institute for Fusion Science (NIFS). The C‐PREST is being developed to integrate large‐scale helium cryogenic plant design, operation and maintenance for optimum process establishment. As a first step of simulations of cooldown to 4.5 K with the helium liquefier model is conducted, which provides a plant‐process validation platform. The helium liquefier consists of seven heat exchangers, a liquid‐nitrogen (LN2) precooler, two expansion turbines and a liquid‐helium (LHe) reservoir. Process simulations are fulfilled with sequence programs, which were implemented with C‐PREST based on an existing liquefier operation. The interactions of a JT valve, a JT‐bypass valve and a reservoir‐return valve have been dynamically simulated. The paper discusses various aspects of refrigeration process simulation, including its difficulties such as a balance between complexity of the adopted models and CPU time.

The Large Helical Device (LHD), an experimental fusion apparatus, has conducted a second experiment cycle. The infrastructure of the LHD cryogenic control system (TESS) is a combination of a distributed control system with an integrated controller linked by multiple LANs. The integrated controller shares information with four subsystems: the helium refrigerator/liquefier, the helical coil, the poloidal coil and the superconducting busline, via reflective memories. Each subsystem has a VME bus and a workstation for process control and programming. Cooldown processes of each subsystem are implemented independently by the temperature levels. Sequence programs are developed to change cooling modes for subsystems corresponding to their temperatures. Program Control Units (PCUs) proceed cooldown and warmup process in terms of gaseous-helium supply temperature and mass-flow rates. Temperature distributions in the subsystems are monitored and process will be interrupted if temperature gradients exceed 50 K. In the case of coil quench, emergency sequence programs are executed to prevent any damage to the apparatus. The paper describes the performance of LHD-TESS during cooldown, steadystate operation, and the case of emergencies.