Matter injection technology for DEMO, state of the art

•Mass losses in matter injection systems are to be considered.•Pellet injection to the magnetic high field side is essential for core particle fueling.•Geometry of transfer line is crucial for efficiency.•Gas Injection System valves are switching between tokamak bypass and vacuum vessel providing continuous flow from Tritium plant and enables fast switching times. DEMO will be a burning fusion device, most likely a tokamak type and thus, the plasma needs to be re-fuelled. A project was launched in the European DEMO Programme to develop concepts for the matter injection. This approach and the actual status of the project are presented. In a first step, requirements for actuators on plasma core density were elaborated. Related modelling activities indicated that only sufficiently deep fuel deposition can achieve target operational parameters. Hence, suitable techniques had to be identified and evaluated with respect to their availability and capability. Several techniques for pellet injection have been benchmarked in view of the defined requirements. Finally, cryogenic pellet injection was chosen as the main actuator on plasma core density. From further modelling activities, assuming for the pellet mass the ITER reference value, required launching speeds were derived, with respect to different injection geometries. Gas puffing and the respective technical system are necessary for pre-fill, ramp-up and plasma confinement enhancement. The ITER gas injection system (GIS) is assessed in view of suitability for DEMO. The piping system and the manifold concept can be adopted. The Gas Valve Box (GVB) is considered not to be an optimum solution for DEMO. Instead of this GVB, a pressure based RUN/VENT flow regulation and injection system is proposed in order to meet DEMO requirements. The principle of this system is described as well as some considerations about injection locations; further orienting gas flux numbers are provided.