Line ratio diagnostics in helium and helium seeded argon plasmas

Abstract

We investigate the potential use of line ratio diagnostics to evaluate electron temperature in either helium or helium seeded argon plasmas. Plasmas are produced in a helicon plasma source. A rf compensated Langmuir probe is used to measure both the electron temperature and plasma density while a spectrometer is used to measure He I line intensities from the plasma. For all plasma densities where the electron temperature remains at 5 ± 1 eV, three He line ratios are measured. Each experimental ratio is compared with the prediction of three different collisional radiative models. One of these models makes uses of recent R-matrix with pseudo-states calculations for collisional rate coefficients. A discussion related to the different observations and model predictions is presented.

Relations between light emission and electron density and temperature fluctuations in a helium plasma

“…In Fig. 3(a), for n e < 10 17 m À3 the line emission is almost linearly proportional to n e , since for such relatively low densities the population exclusively excited from the ground state is balanced by the spontaneous radiation decay (the corona model [24][25][26][27], i.e., Cð1; pÞn e n 1 ¼ P q<p Aðp; qÞnðpÞ, where C(1, p) is the electron impact excitation rate coefficient from ground state to level p. For n e > 10 17 m À3 the slope of the emission curves decreases due to the increased importance of the secondary physical processes such as excitation and de-excitation from neighboring levels and metastable contributions. In this case, the excited level populations can no longer be approximated by the corona model.…”

“…For helium plasmas at low density (n e < 10 17 m À3 ), the population in the excited level can be described by a simple corona model, [24][25][26][27] which only considers the electron impact excitation from the ground state and the spontaneous radiative decay. At higher densities, however, other processes such as excitation from metastable states, collisional excitation and de-excitation, and cascading become important and can no longer be neglected.…”

“…[22][23][24] It is generally considered that the 2 3 S state has a much higher population than the 2 1 S metastable because a large portion of the population de-excited from the triplet levels to the 2 3 S state is quickly and predominantly reexcited back. 25 Thus, the population contributed from the 2 3 S state to the triplet levels is relatively large and the population fluctuation in the triplet level is also significantly affected.…”

“…Unfortunately, some experiments indicate that under certain plasma conditions the 2 3 S population should be much lower than the value estimated by the quasi-static approximation. 23,24 This leads to a large uncertainty by using spectral lines emitted from the triplet levels for the characterization of plasma parameters. Meanwhile, we notice that the He I 728.1 nm line is almost not affected by either of the two metastables.…”

“…In recent years, accurate cross section data for neutral helium have been assessed thoroughly. 16 The model for helium has been widely used for the measurement of n e and T e by the line intensity ratio method, [17][18][19][20][21][22][23][24][25] which gives consistent results compared with those from other diagnostics. [19][20][21][22] Based on a CR model for neutral helium together with a linear approximation technique, we investigated the relationship between fluctuations in three helium lines, He I 667.8 nm (3 1 D !…”