ITER Fuelling System Design and Challenges - Gas and Pellet Injection and Disruption Mitigation

Full text: The ITER fuelling system plays a key role in plasma operation, ensuring density control, ELM frequency control, radiative cooling enhancement, plasma detachment control, disruption mitigation, etc. It consists of 3 major sub-systems: the Gas Injection System (GIS), Pellet Injection System (PIS) and Disruption Mitigation System (DMS). This paper describes the design status of the three systems and the challenges associated with each. The ITER fuelling system is capable of delivering fuel particles (H2, D2 and DT) at average and peak throughputs of 200 Pa·m3s-1 and 400 Pa·m3s-1 respectively in the form of gas or pellets, as well as impurities such Ne, Ar and N2 with average and peak throughputs of 10 Pa·m3s-1 and 100 Pa·m3s-1. The current GIS consists of 4 upper port level and 3 divertor port level injections. During burning plasma operation, boundary plasma simulations suggest that gas fuelling from the edge will be inefficient for core fuelling, even for main chamber injection locations where fuelling efficiencies are usually higher. The upper port injections are thus foreseen as vehicles for possible He ash removal (increasing the SOL density independently of the core) or for coupling improvement of RF heating systems. The divertor injection points are envisaged mainly for extrinsic seeding of impurities to effect detachment control through volumetric radiative cooling. The PIS provides core plasma density control from high field side (HFS) injection and ELM pacing from low field side (LFS) pellet introduction. The current flight tube configuration for HFS pellet injection allows maximum pellet speeds of 300 ms-1 to be achieved. The very high plasma stored energies of which ITER will be capable mean that mitigation of thermal and electromagnetic loads due to disruptions, vertical displacement events and runaway electrons is indispensable for machine protection. Physics studies to define the requirements for the DMS are currently running in parallel with a detailed engineering assessment of candidate systems. (author)