Influence of CO2-laser pulse parameters on 13.5 nm extreme ultraviolet emission features from irradiated liquid tin target

TL;DR

This study uses simulations to analyze how CO2 laser pulse parameters affect 13.5 nm EUV emission from liquid tin targets, revealing the dynamics of plasma formation and optimizing emission efficiency.

Contribution

It introduces a detailed RMHD simulation approach to study the influence of laser pulse duration and energy on EUV emission from liquid tin plasma.

Findings

EUV conversion efficiency is around 4% for 30 ns pulses at 14 GW/cm2.

Longer pulses slightly reduce EUV yield due to increased radiation re-absorption.

Optimal collector configurations can be derived from emission direction diagrams.

Abstract

Laser-produced plasma (LPP) induced during irradiation of a liquid tin droplet with diameter of 150 um and 180 um by CO2 laser pulse with various pulse durations and energies is considered. The two-dimensional radiative magnetohydrodynamic (RMHD) plasma code is used to simulate the emission and plasma dynamics of multicharged ion tin LPP. Results of simulations for various laser pulse durations and 75-600 mJ pulse energies with Gaussian and experimentally taken temporal profiles are discussed. It is found that if the mass of the target is big enough to provide the plasma flux required (the considered case) a kind of dynamic quasi-stationary plasma flux is formed. In this dynamic quasi-stationary plasma flux, an interlayer of relatively cold tin vapor with mass density of 1-2 g/cm3 is formed between the liquid tin droplet and low density plasma of the critical layer. Expanding of the tin vapor from the droplet provides the plasma flux to the critical layer. In critical layer the plasma is heated up and expands faster. In the simulation results with spherical liquid tin target, the CE into 2${\pi}$ is of 4% for 30 ns FWHM and just slightly lower - of 3.67% for 240 ns FWHM for equal laser intensities of 14 GW/cm2. This slight decay of the in-band EUV yield with laser pulse duration is conditioned by an increasing of radiation re-absorption by expanding plasma from the target, as more cold plasma is produced with longer pulse. The calculated direction diagrams of in-band EUV emission permit to optimize a collector configuration.