First-principles simulation of shocked H-He mixture along the principal Hugoniot

Abstract

Recent laser-shock experiments on an H--He mixture containing 11~$\%$ helium (atomic fraction) have suggested the presence of an immiscibility region inside Jupiter. Reflectivity measurements were used as the primary diagnostic of H--He demixing, with discontinuities in the optical reflectivity proposed as a signature of phase separation under conditions relevant to Jupiter's interior. Here, we investigate shock-compressed H--He using \textit{ab initio} molecular dynamics simulations with optical properties evaluated within the Kubo--Greenwood formalism. The equation of state and ionic configurations were obtained using the thermal Tr$^2$SCANL meta-GGA exchange--correlation (XC) functional, while optical properties were computed using the recently developed RS-KDT0 range-separated thermal hybrid XC, which provides state-of-the-art accuracy for band-gap predictions in the warm dense matter regime. The calculated reflectivity shows overall good agreement with experimental measurements; however, no discontinuity is observed at elevated temperatures. Moreover, the reflectivity predictions for the mixed system are consistent with the experimental measurements in the temperature range where the mixture is inferred to be demixed. These results suggest that reflectivity alone may not provide a unique or sensitive diagnostic of H-He demixing at low helium concentrations under these conditions.