Spectroscopic study of low-pressure microwave-induced helium discharge in the extreme ultraviolet wavelength range

In this work, as a promising laboratory-based extreme ultraviolet (EUV) radiation source, microwave (MW)-induced helium discharge is studied and analyzed based on spectroscopic measurements at low pressures. The helium emission spectrum in the EUV wavelength range is presented to show all the relatively strong EUV lines. A Maxwellian shape is assumed for the electron energy distribution, and a corona model is applied to evaluate the plasma parameters under low-pressure conditions. The intensities of a pair of emission lines at 30.38 and 58.43 nm, which are the strongest in the spectrum and of great astrophysical interest, are studied under gradient discharge powers and pressures. After correcting for plasma reabsorption, the intensity ratio of the two lines is used to obtain the electron temperature (T-e), which is found to vary within the range of 3.7-5.6 eV. Electron density (n(e)) is deduced from a global discharge model, which is of the order of magnitude of 10(10) cm(-3). From experimental determination and mechanism analyses, the optimal discharge pressure is found to be within 1.45-2.18 mbar for the 34.38 nm line and the vicinity of 3.45 mbar for the 58.43 nm line. This work explores the dependency of physical behavior of discharge on different working conditions based on a EUV spectroscopic study and theoretical analyses and determines the optimal working condition to produce the strongest EUV emission lines of the low-pressure MW-induced helium discharge.