Conditions of structural transition from collisionless electrostatic shock to double-layer structure*

In unmagnetized plasmas, collisionless shocks can be sustained by electrostatic potential alone when electron temperature is significantly larger than ion temperature. These shocks, known as collisionless electrostatic shocks (CES), can be generated in laboratory settings through the interaction between intense lasers and near-critical density targets. Another structure having a similar presence of localized electrostatic potential is double-layer structure (DL) which has been frequently observed in space plasmas. Using particle-in-cell (PIC) simulations, we investigate the formation of CES at the interface between two plasma slabs with varying pressures and reveal a natural transition from CES to DL. From the numerical results, we identify that the transition to DL occurs when the initial density ratio between the two plasma slabs exceeds 40, independent of the temperature ratio. Motivated by this result, we propose a new model based on previous CES studies to also describe DL dynamics. In addition, our model successfully explains the transition observed in PIC simulations, providing new insights into the physics of CES and DL in space and laboratory plasmas.