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

摘要

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 T_e and density n_e simultaneously, by using Stark broadenings of two Sn II isolated lines (5s~24fF~2F°_(5/2)-5s~25d~2D_(3/2) 558.9 nm and 5s~26d~2D_(5/2)5s~26p~2P°_(3/2) 556.2 nm) spontaneously emitted from the plasma. The spatial-resolved evolutions of T_e and n_e 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 n_e decay characteristics along the Z-pinch axis, the expansion velocity of the electrons was estimated as ~1.2 × 10~4 ms~(-1) from the plasma shell between the electrodes towards the cathode and the anode. The decay time constant of n_e 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/(27rsr).