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The boundary region between the solar radiation zone and the convection zone (T ~ 180 eV, ne ~ 9×1022 cm-3) is a critical interface where energy transport in the solar interior transitions from radiationdominated to convection-dominated regimes. This region also serves as a natural laboratory for studying hot dense plasma. The physical properties of this zone are essential for the reliability of stellar evolution models and the stability of energy transport mechanisms. One of major unresolved issue is how electron collision ionization affects the density of free electrons and radiation properties in this plasma, while accurately describing the impact of hot-dense environments on electron impact ionization (EII) (such as electron screening, ion correlation). To fill this gap, we systematically calculate EII cross sections for C, N, and O ions under realistic solar boundary conditions, with a focus on hot-dense environment impacts. We develop a novel computational framework that merges hot-dense environment effects into atomic structure calculations: the Flexible Atomic Code (FAC) for atomic structure is combined with the Hypernetted-Chain (HNC) approximation to capture electron–electron, electron–ion and ion-ion correlations, enabling self-consistent treatment of electron screening and ion correlation. Atomic wave functions are derived by solving the Dirac equation within the ion-sphere model, using a modified central potential that incorporates both free-electron screening and ion–ion interactions. EII cross sections are then computed via the Distorted-Wave (DW) approximation in FAC. The results demonstrate that hot-dense environment effects significantly enhance the electron-impact ionization cross sections of C, N, and O compared to those calculated under the free-atom model. Additionally, a notable reduction in the ionization threshold energy is observed. These effects are attributed to the overlap of atomic potentials due to strong ion coupling and the shift in bound-state energy levels caused by free-electron screening. For instance, under solar boundary conditions, the ionization cross section of C+ increased by up to 50%, with the ionization threshold decreasing from about 24 eV (isolated) to 18 eV (with screening). Similar enhancements were observed for nitrogen and oxygen ions across various charge states. By providing updated ionization cross sections for C, N, and O ions under realistic solar interior conditions, this work offers essential parameters for improving radiation transport models, ionization balance calculations, and equation-of-state models in stellar interiors. The results underscore the necessity of including hot-dense environment effects in atomic process calculations for hot dense plasmas, with implications for astrophysics and inertial confinement fusion research.
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Keywords:
- radiation/convection zone boundary /
- environment effect /
- electron impact ionization /
- hypernetted-chain approximation
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