Fusion research has reached the development stage of an experimental reactor. The amount of radioactive materials handled, such as tritium, has become large, compared with that of plasma experimental machines, and even more attention is being paid to safety. In order to mitigate abnormal events, it is very important to shut down the plasma, cool down the reactor structure and confine its tritium. Although plasma shutdown is the first step in the mitigation, a concept for this is still unclear. It is the subject of this paper.

From the viewpoint of safety, a plasma shutdown system has been studied for the fusion experimental reactor. Role and position of the active plasma shutdown system have been clarified. The system has a role of plasma shutdown within 1 second after measuring an abnormal event. An impurity pellet injection system was proposed as the active plasma shutdown system [1]. It uses the density limit disruption. Pellet size and its injection velocity were obtained by using an evaluation model of the tearing mode and a model of pellet ablation.

Before studying plasma-pellet interaction by injecting the pellet into plasma, it is necessary to establish the impurity pellet injection system. At the first step, it is very important to verify main functions of the impurity pellet injection system, in which a pneumatic method is proposed. These functions are to keep the vacuum during the reactor life time, to inject the pellet of a proper velocity with a high pressure and to transfer the pellet in a drift tube with keeping the proper velocity. These functions were checked principally by a trial manufacture of the impurity pellet injection system. The test stand of the trial manufacture consists of the electromagnetic valve, pellet box, velocity measuring instrument, gas collector and acceleration tube. Damage of the plasma facing component was also checked by using carbon tiles and stainless steel tiles with carbon pellets.

The concept for this plasma shutdown system is extended for ITER. It is necessary to use a 2 m tube and 8 MPa propellant gas to obtain the injection velocity of 500 m for the 5 mm radius pellet of ITER.

[1] T. Okazaki et al., Plasma Shutdown System for Fusion Experimental Reactors, in print of Fusion Technology.