MHD Simulation Study on Impurity Assimilation Efficiency and Disruption Dynamics during Shattered Pellet Injection

TL;DR

This work addresses how shattered pellet injection (SPI) impurities interact with 3D MHD dynamics during tokamak disruptions. Using 3D nonlinear MHD simulations with the NIMROD-KPRAD framework and a particle-based SPI model on a J-TEXT-like equilibrium, it systematically varying fragment velocity, fineness, injection level, impurity composition, injection geometry, resistivity, and parallel conductivity. It finds a trade-off between impurity assimilation and core penetration: slower fragments boost assimilation and MHD activity, finer fragments enhance ablation and cooling, and multi-pellet, toroidally uniform injections improve impurity penetration while reducing radiation asymmetry; higher parallel conductivity and resistivity alter transport and post-TQ dynamics. These insights inform SPI optimization strategies for future devices, including ITER, by balancing effective core density buildup, radiation symmetry, and disruption mitigation efficiency.

Abstract

Shattered Pellet Injection (SPI) has become a critical technique for mitigating plasma disruptions in fusion devices, yet optimizing its efficiency demands a proper understanding of the interaction between impurity dynamics and MHD response. We perform 3D nonlinear MHD simulations of SPI-induced disruption in a J-TEXT-like tokamak using the NIMROD code, systematically examining key parameters: fragment velocity and fineness, injection quantity, impurity composition, injection location and multiple injectors, resistivity, and parallel thermal conductivity. We find that slower fragment velocity enhances impurity assimilation and amplifies MHD activity. Finer fragments significantly increase impurity ablation and cooling efficiency. Mixed deuterium-neon pellets effectively elevate electron density without compromising radiative cooling efficiency. Plasma poloidal rotation affects ablation and cooling efficiency, whereas toroidally uniform multi-pellet injection enhances impurity ablation by nearly a factor equal to the number of pellets and lowers radiation asymmetry. Higher plasma parallel thermal conductivity results in higher radiation cooling efficiency in parallel directions, enhances impurity transport, and reduces toroidal peaking factor (TPF) of radiation. Variations in resistivity significantly influence Ohmic heating, impurity deposition and current dynamics after TQ, with higher resistivity leading to stronger magnetic perturbations and more pronounced current spikes. These findings provide physical bases for optimizing SPI schemes in future tokamak devices.

Figures (22)

• Figure 1: Initial equilibrium profiles as functions of the normalized poloidal flux. The temperature profile is shown in blue and the density profile in red, both normalized to their core values (left y-axis). The safety factor profile is shown in green (right y-axis).
• Figure 2: Simulation mesh grid and initial fragment positions (red dots).
• Figure 3: (a) Plasma current, (b) normalized magnetic energies of toroidal components $\delta B/B = \sqrt{\left ( W_{mag,n} / W_{mag,n=0} \right ) }$, (c) core electron temperature at $\phi=0$ (the solid blue line) and thermal energy (the solid yellow line), and (d) sum of dilution power and ionization power (the solid blue line), Ohmic heating power (the solid green line) and total radiation power (the solid yellow line) as functions of time. The green, red, and blue vertical lines correspond to 0.5 ms, 0.9 ms, and 1.25 ms, respectively.
• Figure 4: The time evolution of (a) impurity deposition density, (b) temperature, and (c) pressure profiles as functions of major radius.
• Figure 5: Poincaré plots and impurity deposition distribution in poloidal plane at various times: (a) t=0.5ms, (b) t=0.6ms, (c) t=0.7ms, (d) t=0.9ms, (e) t=1.1ms, and (f) t=1.4ms.