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EUV spectra from laser-induced argon/oxygen mixed plasmas - Gas mixture dependence of EUV emission-

A laser detonation atomic oxygen (AO) source, which has widely been used as AO environmental simulator, uses high-power CO2 laser to create laser-sustained oxygen plasma. The basic configuration of laser detonation AO beam source is similar to that of extreme ultraviolet (EUV) light source using xenon gas. However, EUV emission from laser-detonation AO beam source has not been evaluated. Thus, EUV spectra from Ar+O2 plasma was investigated related to the very-low altitude orbital applications. The laser detonation AO beam source at Kobe University was used in this study [1]. A pulsed CO2 laser (5 J/pulse) and a pulsed supersonic valve (PSV) are used in this system. The PSV introduces pure oxygen gas into the nozzle and the laser light is focused on the oxygen gas in the nozzle. The AO beam, thus generated, was characterized by a time-of-flight (TOF) distribution measured by a quadrupole mass spectrometer (QMS) installed in the beam line. Premix gases of Ar and O2 was used to produce Ar+AO beams with various compositions. A flat-field EUV spectrometer was used to measure EUV spectra. Figure 1 compares the EUV spectra from laser-sustained plasmas of 100 % O2, 100% Ar, and 50%Ar+50%O2. In the experiment with mixed gas target, translational energies of AO and Ar components were 5.0 and 10.8 eV, respectively. It is clearly obvious that EUV emission intensity from Ar plasma is higher than that from O2 plasma. Ar-originated emission has not been observed from the laser-sustained plasma consisting of 50%Ar+50%O2 gas mixture, i.e., EUV lines originated from oxygen are observed. Emission properties from an Ar+O2 plasma have been studied with relating to the grow discharge optical emission spectroscopy (GD-OES). It was reported that the only a few sub-percent oxygen gas is able to make a great alteration in the emission intensities of the Ar plasma. Wagatsuma reported that the O I emission enhanced 40 % by the addition of Ar up to 50 % in the glow discharge, whereas OII intensities was hardly affected [2]. The results obtained in this study are explained by the excitation of O2 dissociation by electrons produced by Ar ionization. It is suggested that the similar mechanism will be applicable in laser-induced plasma. This work was supported by the Collaborative Research Support from ILE, Osaka University. A part of this research was also supported by the Grant-in-Aids for Scientific Research from JSPS under contract Nr. 25289307, 26289322, 15K14252 and 15K14253. Financial support through the Coordination Funds for Promoting Aerospace Utilization from the MEXT was also appreciated.