Design of a collective Thomson scattering system for diagnosing mid-infrared laser-produced plasmas for extreme...

Collective Thomson scattering (CTS) is an essential diagnostic technique for measuring the electron density (ne) and electron temperature (Te) of laser-produced plasmas used as extreme-ultraviolet (EUV) light sources. However, as these plasmas reach higher electron densities (1020 cm-3) with the introduction of next-generation 2 μm driver lasers and requirement of higher spatial resolution, conventional CTS systems employing a 532 nm probe laser can cause significant electron heating through inverse bremsstrahlung absorption. In this study, we quantitatively evaluated the electron temperature increase induced by a 532 nm CTS system in plasmas with electron densities of ∼1019 cm-3 and established an empirical scaling model describing the probe-induced heating dynamics. Based on this predictive framework, we designed a high-resolution CTS spectrometer using a 266 nm ultraviolet probe laser (the fourth harmonic of an Nd:YAG laser) to minimize probe-induced electron heating. The proposed system employs a cascaded configuration of six reflective diffraction gratings and an inverse slit, providing effective stray-light rejection while achieving a spectral resolution of 3.4 pm. Extrapolation using the empirical model indicates that when the probe energy is limited to less than 2.5 mJ, the 266 nm CTS system can be used to diagnose dense plasmas with electron densities up to ne = 2.6 × 1020 cm-3. Under these conditions, the measurement-induced electron temperature increase is suppressed to less than 2 eV, while maintaining a sufficient signal. These results establish a practical and robust framework for minimally perturbative diagnostics of high-density EUV source plasmas.