Laser-induced fluorescence spectroscopy for kinetic temperature measurement of xenon neutrals and ions in the discharge chamber of a radiofrequency ion source

Abstract

Application of single-photon absorption laser-induced fluorescence (LIF) spectroscopy for non-intrusive measurement of neutral xenon and singly charged xenon ion kinetic temperatures in the discharge chamber of a gridded radiofrequency ion source is demonstrated. A LIF spectrum analysis approach including hyperfine structure reconstruction and inverse filtering (Fourier deconvolution) is outlined. Special focus is set on optimization of post-deconvolution filtering as well as retracing of deconvolution result imperfection due to hyperfine structure parameter uncertainty, incorrect natural linewidth, and saturation of the LIF signal. The corresponding contributions to the kinetic temperature estimation error are quantified via simulation of spectral lineshapes. Deconvolution of almost unsaturated LIF spectra recorded in the center of the ion source discharge chamber reveals that the neutral xenon and xenon ion kinetic temperatures range between approximately 500 and 700 K and, respectively, 700 and 1000 K depending on the radiofrequency power supplied to the discharge.

Zeeman effect of isotopes of Kr and Xe investigated at the linear plasma device PSI-2

“…Thus, the laser depopulates the metastable density of even-numbered isotopes (f JJ ′ ) significantly more than of the odd-numbered ones (f FF ′ ). For instance, [21] presents zero-field simulations of the Xe I 834.9 nm for different saturation and exhibits precisely the above-described behavior. Consequently, the line profile of the 760.4 nm Kr I line should change if the laser perturbs the population density of the 5s[3/2] 2 • level.…”

“…Thus, the magnetic field induces transitions between levels with |∆F| > 1 and |∆m F | ⩽ 1, increasing the overall number of lines with nonzero intensity, which is precisely the reason for the large number of additional components plotted in figure 4(a). Further increase of the magnetic field reduces the number of components again since the complete decoupling of I and J leads to a strong field pattern (the Paschen-Back effect) with 120 nonzero components, which is 2I + 1 times the number of FS magnetic sub-transitions according to selection rules (21) and (20), respectively.…”

“…Thus, the velocity of the neutral and singlyionized atoms is a crucial parameter accessed in the plasma plume via laser-induced fluorescence (LIF) or laser absorption spectroscopy. Typical lines accessed by the lasers are the Kr I 760.4 nm [18], Xe I 834.9 nm and Xe II 834.95 nm transitions [19][20][21]. For example, the Xe II line at 834.95 nm gives access to the velocity distribution function (VDF) in the plume of a Hall effect thruster at the maximal field of about 15 mT [19].…”