Spectral Evolution of an Inertial Electrostatic Confinement Discharge with Pressure and Voltage

This work presents a qualitative optical emission spectroscopy (OES) analysis of an argon Inertial Electrostatic Confinement (IEC) discharge operated between 3 and 12 mTorr, with up to −5 kV cathode voltage. Spectra were normalized to integration time, background-subtracted, and scaled to the Ar I 763.5 nm line to enable comparison across operating conditions. All cases showed the same set of Ar I and Ar II features, including the prominent Ar II lines at 484.8 and 487.9 nm, and strong Ar I lines between 700.0 and 850.0 nm, with additional weak H I (486.1 nm) and N I (772.7 nm) contributions. Pressure sweeps at fixed voltage demonstrated that the 3 and 5 mTorr cases exhibit comparatively higher normalized intensities in the 400.0 to 500.0 nm ionization-associated region, while the 7 mTorr case shows higher emission across the long-wavelength Ar I excitation lines. Voltage sweeps at 6 mTorr revealed that increasing the magnitude of cathode voltage raises the normalized intensity of most isolated peaks. The largest spectral differences occurred between jet and spray modes, with the jet mode producing higher Ar II intensity in the 484.0 to 488.0 nm interval, while spray mode produced higher Ar I intensities throughout the 700.0 to 850.0 nm region.