Blue Core Characteristics Of Helicon Plasma In Inhomogeneous Magnetic Field Using Permanent Magnets

Helicon refers to the helical nature of the waveform of a bounded whistler wave.Helicon can be used to generate plasma with high density and low electron temperature under a wide range of flexible external conditions.Compared with electron cyclotron resonance(ECR)plasma,helicon plasma need lower magnetic field This make it,has a wide range of application prospects in the fields of thin film deposition,etching,space particle acceleration and plasma propulsion.Based on practical application,this thesis designed a helicon plasma source under inhomogeneous magnetic field using permanent magnets,studied its discharge characteristics,and especially focused on the Blue core characteristics in the helicon plasma.First,experiments were carried out on the effect of inhomogeneous magnetic field on Blue core phenomena in helicon plasma.The permanent magnets(PMs)were used to provide the magnetic field required for generation of helicon plasma,and three different types of inhomogeneous magnetic field were constructed by changing the permanent magnets positions,which were named as PM-Top/Bottom,PM-Top and PM-Bottom,respectively.The maximum magnetic field strengths in these three types of inhomogeneous magnetic fields were at both ends(Case A),at upper end(Case B)and at lower end(Case C)of the discharge tube,respectively.Nikon camera,intensified charge coupled device(ICCD),optical emission spectrometer(OES),and Langmuir probe were used to diagnose the Blue core phenomenon in helicon plasma.The electronic excitation temperature wa calculated based on optical emission lines captured by OES.Helicon plasma discharge was 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,upper end at around 300 W and lower end at around 400 W.The formation of Blue core in present work can be attributed to the non-uniformity of inhomogeneous magnetic field.Meanwhile,the radial distribution of plasma density and electron temperature in Blue core are different.The plasma density still maintains the on-axis peak and electron temperature transforms from on-axis peak to off-axis peak.Then,the effect of the neutral pressure on blue core phenomena in helicon plasma was investigated.The neutral pressure was set to 0.08 Pa,0.36 Pa and 0.68 Pa.Helicon wave discharge was affirmed by mode transition before power rise to 200 W.The plasma density increases and maintains the central density peak with the increase of neutral pressure.However,the brightness of the blue core gradually decreases.It demonstrates that the intensity of Ar II spectral lines and the ionization rate in the central area are reduced.ICCD photos results showed the situation of dark center and bright edges,the radial distribution curve of electron temperature becomes flat and even hollow in the center.Finally,combining the diagnostics results of the ICCD and Langmuir probe,it was assured that the peak positions of plasma density and electronic temperature were inconsistent in the blue core of the inhomogeneous magnetic field helicon plasma.It was proved that neutral gas pressure plays an important role here.The skin effect and TG(Trivelpiece-Gould)waves at the edge increase with the neutral gas pressure.Therefore,an increase in neutral pressure leads to a decrease in the central heating efficiency dominated by the helicon wave,and an increase in the edge heating efficiency dominated by the TG wave and skin effect.This paper successfully demonstrated how both the inhomogeneous magnetic field and the neutral pressure significantly influence the characteristics of the blue core in helicon plasma.