Effect of neutral pressure on the blue core in Ar helicon plasma under an inhomogeneous magnetic field

The effect of neutral pressure on the blue core in Ar helicon plasma under an inhomogeneous magnetic field was investigated in this work. The neutral pressure was set to 0.08 Pa, 0.36 Pa, and 0.68 Pa. A Nikon camera, intensified charge-coupled device (ICCD), optical emission spectrometer (OES), and Langmuir probe were used to diagnose the blue core in helicon plasma. Helicon plasma discharges experienced density jumps from the E mode, H mode to W mode before power just rose to 200 W. The plasma density increased and maintained a central peak with the increase of neutral pressure. However, the brightness of the blue core gradually decreased. It is demonstrated that the relative intensity of Ar II spectral lines and the ionization rate in the central area were reduced. Radial electron temperature profiles were flattened and became hollow as neutral pressure increased. It is demonstrated that increasing the neutral pressure weakened the central heating efficiency dominated by the helicon wave and strengthened the edge heating efficiency governed by the TG wave and skin effect. Therefore, the present experiment successfully reveals how the neutral pressure affects the heating mechanism of helicon plasma in an inhomogeneous magnetic field.