Diagnosis of helicon plasma by local OES

Optical emission spectroscopy (OES) is a common method for characterizing radio frequency (RF) discharge plasmas. Particulary, helicon plasma is featured by its high plasma density among all RF-excited plasmas. In order to obtain the spatial-resolved information of a helicon plasma, local optical emission spectroscopy (LOES) with a 3 mm spatial resolution was proposed and carried out to evaluate the local electron density and temperature. The plasma emission intensity via LOES was measured and compared with the electron density obtained by a RF-compensated Langmuir probe (LP) in Ar, N₂ and Air helicon plasmas, respectively. The results revealed that there existed a functional relationship between some specific lines (LOES) and electron density (LP). Further, helicon plasma characteristics under capacitive (E), inductive (H), and helicon (W) modes were systemetically investigated based on LOES. Besides, two-dimensional (2D) contour maps for plasma distributions were made via LOES as well. It was found that in E- and H-modes, axial profiles of plasma density and electron temperature were consistent under two opposite magnetic field directions. However, in W-mode, the plasma presented an asymmetric axial profile along the tube. As for radial profiles, plasma distribution varied under three discharge modes due to different heating mechanisms in Ar, N₂ or Air helicon plasma. A deeper analysis indicated that the bulk absorption comes from the coupling of the helicon wave in Ar helicon plasma while the power depositions in N₂ and Air helicon plasma are mainly dominated by the TG wave.