Development of a double-barreled Tracer Encapsulated Solid Pellet (TESPEL) injection system for LHD

Abstract

For performing a study of impurity transport efficiently by using a Tracer-Encapsulated Solid Pellet (TESPEL) in Large Helical Device (LHD), a new double-barreled TESPEL injection system was recently developed and installed on the LHD. The TESPEL in each injection line can be injected with different timing and acceleration pressure which can be set individually. To prevent the high-pressured gas for the TESPEL acceleration from getting into the vacuum vessel of the LHD, a 3-stage differential pumping scheme was applied. Since the available space is very limited, the 3-stage differential pumping system is shared with each TESPEL injection line. And for the same reason, one of the guide tubes in one injection line of the new TESPEL injection system was purposely bent (bending radius of about 40 m). The guide tubes in the other injection line of the new TESPEL injection system are straight. We performed a laboratory test to check the TESPEL transferring capability with the guide tubes in the new injection system. The points of TESPEL impact on the target foil are found to be within the circles defined by a possible pellet divergence full angle, 2 degrees. Therefore, we confirmed that the TESPEL can pass through both the slightly bent and the straight injection lines of the new injection system without any problem. The blank shot test shows that the increment of the total pressure even by using both the injection lines simultaneously is much below the base pressure of the LHD vacuum vessel during the discharge. This result successfully demonstrated the capability of the applied 3-stage differential pumping scheme in the new injection system.