Unstable expansion of plasma foils irradiated by circularly-polarized laser pulses in non-transparent regimes*

A new instability has been observed in one-dimensional (1D) particle-in-cell (PIC) simulations. Plasma foils, irradiated by circularly-polarized (CP) laser pulses, were observed to expand even when a_I < ζ_e, which is the conventional condition of the stable laser-foil interactions. We derived a modified condition a_I < ζ_e / (1 + k_L²d_e²)^1/2, where d_e is the thickness of the foil, by assuming slab-like plasma distributions. The modified stability condition shows excellent agreement with the simulation results; stable interactions satisfied the new condition while the instability didn't. In this talk we present the derivation of the equation, and its comparison with 1D PIC simulations. We also suggest a criterion of the pulse duration to prevent the unstable expansion. Furthermore, thresholds for the instability and the self-induced transparency of the foil, and its physical meaning will be discussed.