Bright extreme ultraviolet (EUV) light source in the wavelength around 13.5 nm is indispensable for the next generation lithography process in semiconductor device fabrication technology. Though laser-produced xenon (Xe) plasma is one of the promising candidates for the light source, construction of theoretical model for detailed atomic process and benchmark with various experimental data are required to achieve the best performance. However, the laser-produced plasmas are unsuitable for benchmarking since detailed measurements of plasma density and temperature profiles are generally difficult due to their sharp spatial and temporal variations. On the other hand, magnetically confined plasmas utilized for fusion research have relatively mild temperature and density gradients measured by reliable diagnostic tools. In this study, we have carried out EUV spectroscopic measurements in Xe plasmas produced by a medium-size fusion-oriented experimental torus device, Compact Helical System (CHS), to contribute to the development of reliable spectroscopic data of Xe ions EUV spectra. The first results obtained by an existing spectrometer and comparison with the model calculation are reported in this article.
We have used a flat field grazing incidence spectrometer (model JYF-306) in the wavelength region of 10-110 nm already installed in CHS for impurity monitoring. Groove density and focal length of the grating are 450 g/mm and 306 mm, respectively. A Chevron MCP of 40 mm diameter equipped with a phosphor screen is placed along the focal plane, and the spectral image reduced to 25 mm is detected by a fiber-coupled CCD linear image sensor (Hamamatsu, S7010) with the minimum readout time of 10 ms. Overall spectral resolution is about 0.5 nm determined mainly by the MCP. The plasmas are produced by electron cyclotron heating (ECH) using a 53 GHz gyrotron with a duration of 100 ms. Line integrated emissivity along the equatorial plane is measured within the horizontally elongated cross section. We have firstly studied pure Xe plasma, and characteristic bright spectral lines around 10-20 nm have been observed and some lines around 50-80 nm have been also found. Since the electron temperature in the pure Xe plasma was too low for Thomson scattering diagnostic, we have also tried hydrogen plasma with Xe gas puff. In this case line-averaged electron density and central electron temperature are 8×10¹⁸ m^-3 and 1 keV, respectively, at several tens of milliseconds after Xe gas puff. Observed spectral lines are compared with the model calculation by HULLAC code. The charge states of Xe ions are identified for most of the prominent peaks. A new spectrometer with higher resolution will be installed and utilized for more detailed spectral measurements in the near future.