# Spectroscopic Studies of Laser-Based Far-Ultraviolet Plasma Light Source
## 4 Results and Discussion
Figure 4a shows that all of the measured plasma spectral radiations have a continuum on which the emission lines are superimposed. Figure 4b presents the measured radiation conversion efficiencies (in units of % into 2\(\pi\)sr) over the wavelength region of 125-160 nm versus the peak laser irradiances in the range of \(\approx\)10\({}^{10}\) to 2.5 \(\times\) 10\({}^{11}\) W cm\({}^{-2}\). For these experimental conditions, maximum radiation conversion efficiencies in the range of 1-3%/2\(\pi\)sr were measured. All conditions that are known were kept constant during the experiments. However, note that at the low laser irradiance in contrast to the high region, around 60,000 pulses were accumulated to obtain each spectrum. All of the emission signals were corrected by the subtraction of the noise (dark) signal of the detector, which were separately measured for the same corresponding exposure time. Figure 4b shows the Si, Mo and Ta plasmas generate higher conversion efficiencies at the lowest laser irradiance of \(\approx\)10\({}^{10}\) Wcm\({}^{-2}\). We expect that low-ionized charge states of the heavy elements with the open d-sub-shell, for example, Mo (with the neutral ground configuration 4d\({}^{5}\)5s), Ta (with the neutral ground configuration 5d\({}^{3}\)6s\({}^{2}\)) and tungsten (with the neutral ground configuration 5d\({}^{4}\)6s\({}^{2}\)) [26] generate a strong continuum-like FUV spectra at the lower laser irradiance than \(\approx\)10\({}^{10}\) Wcm\({}^{-2}\). In particular, it is predicted through preliminary calculations that an optically thick Ta (or the tungsten) plasma radiates a strong broad quasi-flat FUV emission at a temperature region less than 5 eV. Indeed, more experiments needed to investigate the best target condition by considering the debris issue. Previous experiments demonstrated that a low-debris LPP plasma source is possible by using, e.g., gas [8] or droplet targets. For example, [PERSON] et al. [4] demonstrated that the realization of a high CE low-debris LPP Sn EUV source is possible by reducing the mass of tin target, which is accomplished by using tin-doped droplet targets. These features are now under investigation, especially the debris issue in high repetition-rates experiments at a low laser fluence region. Figure 4: (**a**) Measured spectral irradiances of the Si, Zn, Mo, Sn and Ta plasmas at a distance of 100 cm from the target over the wavelength region of \(\approx\)123–164 nm. Here, a plasma generated by a 1.0 μm, 100 Hz, 10 ns pulse at the laser irradiance of \(\approx\)2.5 \(\times\) 10\({}^{11}\) W cm\({}^{-2}\) (pulse energy \(\approx\) 230 mJ). The observed Si spectral lines are shown (on top) using the NIST database [22]; (**b**) measured radiation conversion efficiency (in units of % into 2\(\pi\)sr) over the wavelength spectral band of 125–160 nm for various laser irradiances (pulse energies) of \(\approx\)10\({}^{10}\) (\(\approx\)10 mJ), 5 \(\times\) 10\({}^{10}\) (\(\approx\)44 mJ), 1.0 \(\times\) 10\({}^{11}\) (\(\approx\)90 mJ), 1.5 \(\times\) 10\({}^{11}\) (\(\approx\)140 mJ), 2 \(\times\) 10\({}^{11}\) (\(\approx\)190 mJ) and 2.5 \(\times\) 10\({}^{11}\) Wcm\({}^{-2}\) (\(\approx\)230 mJ). Due to the lack of experimental information on the evolution of plasma parameters, a picture of the plasma dynamics may be obtained using the radiation-hydrodynamics model. Here, a radiation-hydrodynamics code, FLASH is used to investigate the plasma temperature and density at a given peak laser irradiance of \(\approx\)10\({}^{10}\) Wcm\({}^{-2}\). Figure 5a-c present typical 2D spatial distributions of the electron temperature, the plasma density, and electron density near the peak laser irradiance, respectively. Here, this code is employed to study the plasma dynamics for the case of a planar solid Si target (with the initial density of \(\approx\)2.33 g cm\({}^{-3}\)) irradiated by a 1.06 \(\upmu\)m, 10 ns FWHM Gaussian-temporal-spatial-shaped laser pulse with a peak irradiance of \(\approx\)10\({}^{10}\) Wcm\({}^{-2}\).