Kinetic modeling of ion acceleration in laser-driven tin plasma EUV sources*

Laser-produced tin plasmas are enabling the continuation of Moore's law through the use of 13.5 nm narrow-band EUV radiation for next generation lithography. A major challenge for their industrial application is damage to the sensitive optics from energetic ions produced during the laser-plasma interaction. In this work we use fully kinetic PIC simulations to study the ion acceleration mechanisms in tin plasma EUV sources, utilizing an inverse-bremsstrahlung heating operator to model the interaction of a tin target with an Nd:YAG laser and a Monte-Carlo Coulomb collision operator to model thermal conduction. These simulations capture the global source evolution while allowing for detailed analysis of energetic ion trajectories. Benchmarking tests against analogous single-fluid radiation hydrodynamics simulations show qualitative agreement in most of the domain. However, the long timescales for thermal equilibration in the ablated plasma allow for the development of two-temperature features in the PIC simulation. A collimated population of energetic ions is produced in the PIC simulation with a significant enhancement at the highest energies compared to the fluid simulations. The dominant acceleration mechanism is a large-scale electric field supported by the electron pressure gradient, which becomes stronger in the kinetic simulations due to the increased electron temperature. We discuss the implications for advancing the modeling of these sources and developing debris mitigation schemes.