Rapid assimilation of high-Z impurity ions along the magnetic field line from an ablated pellet

TL;DR

This study uses kinetic simulations to show how high-Z impurity ions from ablated pellets rapidly integrate into hot plasma along magnetic field lines, driven mainly by ambipolar electric forces and influenced by impurity charge states.

Contribution

It introduces a first-principles kinetic model revealing the collisionless, electric-force-driven impurity assimilation process along magnetic field lines.

Findings

Impurity ions propagate steadily into plasma driven by ambipolar electric force.

Impurity front speed scales with the square root of plasma temperature and average charge.

Impurity charge state alignment is due to strong collisional friction among ions.

Abstract

The assimilation of ablated high-Z impurities into the hot surrounding plasma along the magnetic field is investigated by first-principles kinetic simulations. It is found that the assimilated impurity ions, primarily driven by the ambipolar electric force, propagate steadily into the surrounding plasmas. The high-Z impurities in different charge states are mostly aligned due to the strong collisional friction among them so that the averaged impurity ions charge $\bar{Z}$ is a deciding factor. Such assimilation is led by an impurity front that is behind the cooling front due to a smaller charge-mass-ratio of the impurity ions $\bar{Z}/m_I$. With the help of a self-similar solution, the speed of the impurity front $U_s$ is shown to be primarily set by the hot surrounding plasma temperature $T_0$ with a weak dependence on the pellet plasma temperature, underscoring the collisionless nature of the impurity assimilation process. Specifically, $U_s\sim \sqrt{\bar{Z}T_0/m_I}$. The ambipolar-constrained electron conduction flux from the hot plasma is primarily responsible for the collisionless impurity assimilation process.