Effect of trace CF₄ electronegative gas on electron density and temperature diagnosed by Thomson scattering in argon cascade arc plasma

In this study, the high-resolution Laser Thomson Scattering (LTS) technique was employed to carry out, for the first time, non-intrusive and accurate measurements of electron density (ne) and electron temperature (Te) in a cascaded arc argon plasma doped with the trace electronegative gas carbon tetrafluoride (CF4). The experimental results indicate that, at a background pressure of 250 Pa and a discharge current of 120A, the electron density (ne) decreases significantly from 1.24×1020 m-3 to 7.32×1019 m-3 when the CF4 ratio is increased from 0% to 5%. Meanwhile, the electron temperature (Te) remains essentially constant. This is due to the compensation of multiple physical mechanisms: on the one hand, the electronegative gas (CF4) preferentially attaches low-energy electrons, retaining the high-energy tail of the electron energy distribution; furthermore, the axial electric field slightly increases due to the reduced electron density in the constant-current discharge, both of which tend to increase the electron temperature. On the other hand, CF4 molecules possess large vibrational excitation cross-sections and high dissociation energies, and electrons rapidly lose energy through excitation and dissociation processes during collisions with CF4, thereby suppressing the increase in electron temperature. Under the combined effects of the above mechanisms, the increasing and decreasing trends of electron temperature compensate each other, and the electron temperature finally exhibits little variation with the proportion of added CF4. This study provides direct experimental evidence for understanding the influence of electronegative gas doping on plasma parameters.