Optimizing fuelling pellet injection geometry for COMPASS Upgrade using HPI2

COMPASS Upgrade (COMPASS-U) [1] 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 investigates the pellet-plasma interaction, using various plasma scenarios of the Compass Upgrade tokamak with the HPI2 code [2]. HPI2 is a widely used pellet injection simulation code that can handle 3D geometries and related secondary phenomena, like inhomogeneous pellet ablation.The dependence of material deposition and penetration depth on pellet size and velocity were examined, and 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 of the pellet injector was defined. While LFS injection is the easiest to realize technically, it results in the least efficient fuelling, as expected, as drift phenomena quickly accelerates the pellet cloud out from the plasma. The internal, HFS injection is the most efficient, where the drift direction points inward the plasma; due to accessibility constraints, pellets cannot be delivered here. A good compromise is the vertical (VHFS) injection, from an existing port. Relaxation of the plasma profiles after the injection were also investigated to characterise the pacing requirements of the injection.[1]: M. Komm et al, COMPASS Upgrade: a high-field tokamak for ITER- and DEMO-relevant research (2024) Nucl. Fusion 64 076028[2]: F. Koechl et al, Modelling of Pellet Particle Ablation and Deposition: The Hydrogen Pellet Injection code HPI2 (2014), EFDA–JET–PR(12)57, Nuclear Fusion