  A 3D numerical model for the ablation of pellets and shattered pellet
injection (SPI) fragments in tokamaks in the plasma disruption mitigation and
fueling parameter space has been developed based on the Lagrangian particle
code [R. Samulyak, X. Wang, H.-S. Chen, Lagrangian Particle Method for
Compressible Fluid Dynamics, J. Comput. Phys., 362 (2018), 1-19]. The pellet
code implements the low magnetic Reynolds number MHD equations, kinetic models
for the electronic heating, a pellet surface ablation model, an equation of
state that supports multiple ionization states, radiation, and a model for
grad-B drift of the ablated material across the magnetic field. The Lagrangian
particle algorithm is highly adaptive, capable of simulating a large number of
fragments in 3D while eliminating numerical difficulties of dealing with the
tokamak background plasma. The code has achieved good agreement with theory for
spherically symmetric ablation flows. Axisymmetric simulations of neon and
deuterium pellets in magnetic fields ranging from 1 to 6 Tesla have been
compared with previous simulations using the FronTier code, and very good
agreement has also been obtained. The main physics contribution of the paper is
a detailed study of the influence of 3D effects, in particular grad-B drift, on
pellet ablation rates and properties of ablation clouds. Smaller reductions of
ablation rates in magnetic fields compared to axially symmetric simulations
have been demonstrated because the ablated material is not confined to
narrowing channels in the presence of grad-B drift. Contribution of various
factors in the grad-B drift model has also been quantified.
