Optimizing fuelling pellet injection geometry for COMPASS Upgrade using HPI2

COMPASS Upgrade (COMPASS-U) Komm et al. (2024) is a high-magnetic field (5 T) tokamak, currently being built at the Institute of Plasma Physics in Prague. The HUN-REN Centre for Energy Research plans to develop a pellet injector for COMPASS-U. This work aims to optimize the pellet injector parameters - pellet velocity, size, injection direction, and to investigate the pellet-plasma interaction in terms of fuelling efficiency in various plasma scenarios using the HPI2 code (Koechl et al., 2014). HPI2 is a widely used pellet injection simulation code that can simulate pellet ablation and the evolution of the pellet plasmoid, using an enhanced version of the Neutral Gas and Plasma Shielding (NGPS) model. The pellet size and velocity were determined by plasma scenario requirements and injection technology constraints. For the desired material deposition, pellets with a radius of 0.75 mm should be injected into the plasma. Higher injection velocity is advantageous, but based on technical considerations, simulations were run using an injection velocity of 500 m/s. The properties of different injection directions were explored, in several scenarios, from early L-mode to the high-performance H-mode scenario, based on which the optimal injection geometry was defined. While LFS injection is the easiest to realize technically, it results in the least efficient fuelling, as expected, since drift phenomena quickly accelerate the pellet cloud out of the plasma, achieving less than 20% fuelling efficiency. The internal, HFS injection is the most efficient, where the drift direction points inward, into the plasma, achieving complete material deposition, but due to accessibility constraints, pellets cannot be delivered here. A good compromise is the vertical (VHFS) injection, from an existing port, which achieved full fuelling efficiency with an injection location R < 0.8 m . The exact injection location and angle was optimized, and the sensitivity of the deposition to small variations in the launch parameters was investigated, with deposition found to be robust to variations of up to ± 5 cm in launch position and ± 3 ° in angle. A set of pellet guide tube geometries with minimal curvatures were determined, with minimum curvature radii larger than the r c u r v > 0.28 m threshold.