Electron density dependence of intensity ratio for FeXXII extreme ultraviolet emission lines arising from different ground levels in electron beam ion trap and large helical device

Abstract

Extreme ultraviolet (EUV) spectra of highly charged iron ions in a wavelength range of 100–300 Å have been observed from two different plasma sources of the Tokyo Electron Beam Ion Trap (Tokyo-EBIT) with a monoenergetic electron beam and a Large Helical Device (LHD) with Maxwellian electron energy. The excitation process of the spectral lines is compared between the two plasmas, and it is found that the excitation process for Fe XIX - Fe XXII ions is clearly different. Namely, the EUV emission lines from the EBIT plasma are only dominated by electron impact excitation connected to the ground state, but the excitation mechanism is not so simple in the LHD plasma. The difference in the excitation process is studied by measuring the intensity ratio of EUV emission lines (114.412 Å [1s22s2p2 2P3/2 → 1s22s22p 2P3/2]/117.144 Å [1s22s2p2 2P1/2 → 1s22s22p 2P1/2]) arising from different ground levels in the Fe XXII ions. The line intensity ratio has an extremely small value of 0.2 in the EBIT plasma with a low beam current of 30 mA and a beam energy of 2 keV, while the ratio varies with the electron density ne in the LHD plasmas, i.e., 0.35 for ne = 1 × 1013 cm−3 and 0.65 for ne = 4 × 1013 cm−3. Here, the electron density of the EBIT plasma is estimated to be smaller than 1012 cm−3 and the electron temperature of the LHD plasmas is 2 keV. The dependence of the line intensity ratio on the observed electron density is analyzed for both the EBIT and the LHD plasmas using several collisional-radiative (CR) models. The present experimental data can easily be reproduced by the analysis when the thermal proton impact excitation is taken into account. The importance of the proton impact excitation is also experimentally verified by injecting an iron pellet into the LHD plasmas and changing the ratio of the proton density to the electron density.