Laser intensity calibration by sequential double ionization of Mg in circularly polarized intense fields

We propose a method for determining intensity of circularly polarized laser fields based on sequential double ionization of Mg. The measured Mg²⁺/Mg⁺ ratio is fitted using the Perelomov-Popov-Terent'ev (PPT) simulations, in which we employ an ion imaging technique to determine the spatial boundary for intensity averaging. This approach partially addresses the focus volume effects and improves intensity calibration accuracy. We further show that the Ammosov-Delone-Krainov (ADK) model fails to describe the sequential double ionization of Mg. Our approach is applicable to few-cycle circularly or multi-cycle elliptically polarized fields, where accurate intensity calibration is essential for attoclock-based measurements of tunneling ionization delay and related nonadiabatic dynamics.