Observation of Cryogenic Hydrogen Pellet Ablation with a Fast-frame Camera System in the TJ II Stellarator

摘要

Introduction Plasma core fuelling is critical for the development of steady state operation in large fusion devices, especially in helical type reactors [1]. The usual refuelling method, gas puffing, will not be useful for such large devices, since the fuel reaching the core is minimal. At present, cryogenic pellet injection is the most promising technique for efficient core fuelling. Moreover, pellets can be injected to suppress instabilities, to modify density and temperature profiles, and as an active diagnostic [2]. However, a complete comprehension of the physics of pellet ablation and subsequent particle transport (drift/diffusion) remains outstanding and thus ongoing studies are required. A pellet is ablated by background plasma particles so the ablated material forms an expanding cloud of neutrals that will become partially ionized eventually and will expand along the magnetic field lines. Moreover, the ablatant shields the pellet from ambient plasma in a self-regulated manner [3]. The understanding of pellet ablation can allow better mass deposition control and, therefore, improved plasma refuelling. In this work, the experimental set-up and capabilities of the TJ-II pellet injection system are briefly described. Next, fast-camera images are analysed for a range of pellet types injected into several plasmas. Finally, observed pellet toroidal deflection and future work are discussed.