The transition threshold from relativistically transparent regime to opaque regime in the interaction of an ultra-intense laser pulse and a bulk plasma with mobile background ions is investigated. The threshold corresponds to the onset of laser hole-boring. We show that for an ultra-intense laser, the threshold depends on the ion composition of the plasma, and the corresponding plasma density is significantly lower than that with ion immobile plasma. These are supported by 1D PIC simulations by using hydrogen plasma and fully ionized carbon plasma. The movement of background ions modifies the dynamics and distribution of electrons in the plasma, which results in the Doppler-red-shift of the incident laser and the increase of the effective plasma density. An intuitive model, which gives the dependence of the transparency-opaqueness threshold on laser intensity and frequency, as well as the ion composition, is established. It is tested by multi-dimensional PIC simulations with hydrogen plasma.
This paper investigates the transition from a relativistically transparent regime to an opaque regime in the interaction between an ultra-intense laser pulse and a bulk plasma with mobile background ions. The threshold for this transition depends on the ion composition and is lower than for a plasma with immobile ions. The movement of background ions modifies the electron dynamics, leading to a Doppler redshift of the incident laser and an increase in the effective plasma density.