Lagrangian particle simulation of neon pellet ablation clouds for plasma disruption mitigation in tokamaks

摘要

A leading candidate for the ITER plasma disruption mitigation system is the Shattered Pellet Injection (SPI) [1] that performs fragmentation of a large, frozen, neon-deuterium pellet before its injection into a tokamak, and forms a stream of small fragments into plasma, causing a thermal quench. The pellet ablation problem in tokamaks can be studied on relatively small length scales compared to the tokamak size. By resolving all relevant physics processes such as pellet surface ablation, formation of a dense and cold ablation cloud, deposition of energy of hot electrons in the ablation cloud, heating, ionization and channeling of the ablated material along magnetic field lines by MHD forces, and radiation losses, such local models describe the evolution of the process in great detail and compute pellet ablation rates. Significantly different approximations are used in global pellet ablation models, which study the transport of the ablated material in the entire tokamak. Local studies have been performed using a number of theoretical models with analytical or semi-analytical solutions and one- and two-dimensional numerical simulations [2, 4, 5]. Global studies have been performed using typical MHD codes for tokamak plasmas with the addition of analytic source terms [6, 7]. However, local studies of a single pellet ablation in a spherical or axisymmetric approximation is not sufficient for the study of SPI, when a large number of gas/ plasma clouds, created by the ablation of pellet fragments, partially screen each other from the incoming electron heat flux. Our work intends to fill the gap in this area by developing accurate local 3D simulation models suitable for SPI. In the next phase of our work, our local models will be coupled with the well-known tokamak MHD codes NIMROD and M3D-C1.