Effect of inhomogeneous magnetic field on blue core in Ar helicon plasma

The effect of the inhomogeneous magnetic field on blue core phenomena in helicon plasma is investigated in this work. The permanent magnets (PMs) are used to provide the magnetic field required for generation of helicon plasma, and three different types of the inhomogeneous magnetic field are constructed by changing the permanent magnets positions, which are PM-top/bottom, PM-top, and PM-bottom, respectively. The maximum magnetic field strengths in these three types of inhomogeneous magnetic fields are at both ends (case A), at the upper end (case B), and at the lower end (case C) of the discharge tube, respectively. Nikon camera, an intensified charge coupled device, optical emission spectrometer (OES), and Langmuir probe are used to diagnose the blue core phenomenon in helicon plasma. The electronic excitation temperature is calculated based on optical emission lines captured by OES. Helicon wave discharge is affirmed by mode transition with three discharge stages corresponding to E-, H- and W-modes, respectively. The blue core could occur in the maximum magnetic field strengths located at both ends at around 200 W, at the upper end at around 300 W, and at the lower end at around 400 W. The formation of blue core in the present work can be attributed to the non-uniformity of the inhomogeneous magnetic field. Meanwhile, the radial distribution of plasma density and electron temperature in blue core is different. The plasma density still maintains the on-axis peak, and electron temperature transforms from on-axis peak to off-axis peak due to the skin effect.

Automated summary

In this study, the effect of inhomogeneous magnetic field on blue core phenomena in helicon plasma was investigated by changing the positions of permanent magnets. The formation of blue core was attributed to the non-uniformity of the magnetic field, with different characteristics observed at different locations and power conditions.