# Influence of Pre-Ionized Plasma on the Dynamics of a Tin Laser-Triggered Discharge-Plasma
## 4 Experimental Results and Discussion
It can be found that the 13.5 nm intensity increases about 120% when the delay increases from 0.7 to 5 \(\upmu\)s. The EUV conversion efficiency (CE) is increasing from 0.5% to 1.1%. The EUV CE is determined by the ratio of the energy radiated in a 2% bandwidth around 13.5 nm to the input electrical energy [5]. As shown in Figure 7, the EUV CE is increasing with the laser-current delay, but the increase does not have a linear relationship. In contrast, the stability of the light decreases, which is harmful to the source. Moreover, the 13.5 nm intensity is almost the  
Figure 5: Extreme ultraviolet (EUV) spectra and 13.5 nm intensity in 2% band under different laser-current delays. **(a)** Original spectra between 12.0 and 16.0 nm, the spectra marked between the two black lines are the 13.5 nm emission in the 2% band. **(b)** Normalized for each comparison. Figure 6: Comparison of theoretical spectra (blue, dash line) with the experimental spectra (black, solid line) at electron temperatures 32 eV **(a)**, 20 eV **(b)**, and 12 eV **(c)** for 4 of 1.5 \(\upmu\)s. The fractions of the contributing ion stages for each electron temperature are given in the inset. same as that of 4 \(\upmu\)s when the delay is longer than 4 \(\upmu\)s. When the delay is too large, the density of the pre-ionized plasma is too little, and the voltage becomes hard to be broken down as well. For the EUV source, the 13.5 nm emission intensity in the 2% band is defined by the lifetime and density of the plasma which can generate spectra around 13.5 nm. The plasma lifetime and density are decided by the recombination and ionization of the ions. Hence, we should keep the plasma, which is useful for emitting 13.5 nm emission, as long as possible. According to Figures 5 and 6, it can be found that the mean temperature is increasing with the delay, which means that the plasma is in a high-temperature state for a longer time during the Z-pinch process. In this experiment, the optimal delay for 13.5 nm emission in the 2% band, which is mainly generated by the Sn\({}^{84}\)-Sn\({}^{13+}\) 4_d_-_4\(f\) and 4_p_-_4\(d\) transitions, has not been obtained. The density of these ions is decided by the density of the plasma. Moreover, it is also decided by the abundance ratio between the density of Sn\({}^{84}\)-Sn\({}^{13+}\) ions and the total density of the plasma. Longer laser-current delay leads to a lower density. On the other hand, the increasing of Sn\({}^{10+}\) and Sn\({}^{12+}\) ions' intensity with the delay from 0.7 to 5.0 \(\upmu\)s means that the ratio should increase. In other words, a limited current leads to a limited Z-pinch Lorentz force. When the Z-pinch Lorentz force is limited, the pre-ionized plasma cannot be pinched accurately as well. As a result, the electron temperature is not high enough to get enough Sn\({}^{84}\)-Sn\({}^{13+}\) ions. In this way, we will try to increase the discharge current to reduce the optimal laser-current delay, and it will increase the EUV CE as well. Moreover, finite laser power will restrict the density of the pre-ionized plasma as well, so we will study the influence of laser-current delay under different laser power by changing the focus distances.