Estimation of electron temperature and density of the decay plasma in a laser-assisted discharge plasma extreme ultraviolet source by using a modified Stark broadening method

In order to investigate the plasma expansion behaviors and the electrical recovery process after the maximum implosion in our tin fueled laser-assisted discharge plasma (LDP) 13.5 nm EUV source, we developed and evaluated a cost-efficient spectroscopic method to determine the electron temperature Te and density ne simultaneously, by using Stark broadenings of two Sn II isolated lines (5s²4f²F°5/2 – 5s²5d²D3/2 558.9 nm and 5s²6d²D5/2 – 5s²6p²P°3/2 556.2 nm) spontaneously emitted from the plasma. The spatial-resolved evolutions of Te and ne of the expansion plasma over 50 to 900 ns after the maximum implosion were obtained using this modified Stark broadening method. According to the different ne decay characteristics along the Z-pinch axis, the expansion velocity of the electrons was estimated as ∼1.2 × 10⁴ ms^- 1 from the plasma shell between the electrodes towards the cathode and the anode. The decay time constant of ne was measured as 183 ± 24 ns. Based on the theories of plasma adiabatic expansion and electron-impact ionization, the minimum time-span that electrical recovery between the electrodes needs in order to guarantee the next succeeding regular EUV-emitting discharge was estimated to be 70.5 μs. Therefore, the maximum repetition rate of our LDP EUV source is ∼14 kHz, which enables the output to reach 125 W/(2πsr).