  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.
