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, N 2 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, N 2 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 N 2 and Air helicon plasma are mainly dominated by the TG wave.

We present the axial profiles of argon helicon plasma measured by a local optical emission spectroscope (OES) and Langmuir RF-compensated probe. The results show that the emission intensity of the argon atom lines (750 nm, 811 nm) is proportional to the plasma density determined by the Langmuir probe. The axial profile of helicon plasma depends on the discharge mode which changes with the RF power. Excited by helical antenna, the axial distribution of plasma density is similar to that of the external magnetic field in the capacitive coupled mode (E-mode). As the discharge mode changes into the inductively coupled mode (H-mode), the axial distribution of plasma density in the downstream can still be similar to that of the external magnetic field, but becomes more uniform in the upstream. When the discharge entered wave coupled mode (W-mode), the plasma becomes nearly uniform along the axis, showing a completely different profile from the magnetic field. The W-mode is expected to be a mixed pattern of helicon (H) and Trivelpiece-Gould (TG) waves.

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The electric double layer in argon helicon plasma by using floating electrostatic probe and local optical emission spectroscopy under different external magnetic field was investigated. Results show that electrons generated in the plasma source can be magnetized by the magnetic field and transported by the gradient of diverging magnetic field in the diffusion chamber, giving rise to the hollowed distribution of electron temperature. The electron temperature on axis has a sudden decrease, leading to the sudden decrease of plasma density as well as the plasma potential, forming a double layer structure. The position of double layer moves along with the position of diverging magnetic field, and the potential drop increases along with the strength of magnetic field. The structure of the external magnetic field plays the decisive factor for the double layer in helicon plasma.

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