Dynamics of laser-ablated molybdenum plasma in vacuum: a novel spectral matching algorithm based on Saha–Boltzmann equilibrium for ne and Te determination in fusion wall diagnostics

Abstract

Understanding the spatio-temporal evolution of plasma parameters is critical for advancing the accuracy of laser-induced breakdown spectroscopy (LIBS) in fusion reactor wall diagnostics. This study investigates the electron temperature (T_e) and electron density (n_e) dynamics of molybdenum (Mo) plasma—a key plasma-facing material in the Experimental Advanced Superconducting Tokamak (EAST)—generated by a nanosecond pulsed laser (6.0 GW cm⁻²) under vacuum (3 × 10⁻⁵ mbar). By integrating spatially resolved optical emission spectroscopy with fast ICCD imaging, we resolve the temporal scales of continuum radiation (5–60 ns), ionic lines (50–300 ns), and atomic lines (80–800 ns), revealing distinct spatial expansion profiles (up to 10 mm for neutral species). To overcome limitations of traditional Stark broadening methods (e.g., spectral overlap, missing electron collision parameters), we propose a spectral matching algorithm (SMA) based on Saha ionization equilibrium and Boltzmann distribution under local thermodynamic equilibrium (LTE). This method simplifies the calculation process of plasma parameters and achieves high correlation coefficients (>0.9) between simulated and experimental spectra. The results present a rapid cooling and decay behaviour of T_e (from 4.0 eV to 0.66 eV) and n_e (from 1.63 × 10²⁶ m⁻³ to 8.53 × 10²¹ m⁻³) within the time window of 50 ns to 310 ns in Mo plasma under present experimental conditions, driven by adiabatic expansion and three-body recombination. The spatial heterogeneity of T_e and n_e highlights non-equilibrium plasma behaviour. This work presents a method for plasma parameter (T_e and n_e) determination in calibration-free LIBS (CF-LIBS) applications for tokamak wall diagnostics, while simultaneously providing direct empirical validation data for laser ablation plasma dynamics simulations.