Investigation Of Low-field Peaks In Helicon Plasma

Helicon wave whose frequency???lies between the ion and electron cyclotron frequencies,is a kind of electronmanetic waves that propagates in magnetized plasmas.Helicon plasma has great potential applications in semiconductor processing and plasma thruster due to the high ionization and plasma density.In helicon discharge,the plasma density should generally increase linearly with the external magnetic field,but this relation is not true at the low magnetic field,characterized by appearance of density peaks?or low-field peaks?.Understanding of the generation mechanism of low-field peaks is important for both the helicon plasma itself and the application of helicon plasma sources.In this dissertation,the discharge characteristics of helicon plasma and the generation mechanism of the low-field peaks with Boswell antenna and M=-1 helical antenna have been investigated experimentally and numerically under different axial magnetic field profiles.Work in the dissertation includes three parts:Firstly,the discharge characteristics with Boswell antenna and M=-1 helical antenna under uniform magnetic field are investigated experimentally.The effect of the magnetic field on plasma density,antenna effective resistance and the radial glow distribution is studied.The results show that two low-field peaks generate in the low magnetic field less than 250G in helicon plasma with Boswell antenna and M=-1 helical antenna,accompanying with the peaks of antenna effective resistance.The plasma discharge sustains in the wave-coupled mode when the low-field peaks appear.The discharge occurs at the near-antenna field region and at the edge of the discharge tube,which implies the two low-field peaks are relevant with Trivelpiece-Gould?TG?waves.Secondly,the effects of non-uniform axial magnetic field profile on the helicon plasma characteristics with Boswell antenna and M=-1 helical antenna are studied experimentally.The results show that a pure helicon mode discharge can be obtained by locating the peak of the magnetic field along the axis(B_0P)near the tube end sealed by a dielectric plate or connected with a diffusion chamber.The discharge is sustained by the traveling wave(when B_0P locates near the endplate)or standing wave(when B_0P locates near the diffusion chamber)though collisional or collisionless heating mechanisms.The antenna effective resistance does not change with the magnetic field in Boswell antenna,but achieves one or two low-field peak in M=-1 helical antenna according to the B_0P direction.When B_0P locates at the middle of antenna,the discharge is governed by TG waves and low-field peaks appear which is related to the antenna type or the B_0P direction.Finally,a two-dimensional HELIC Code is used to investigate the helicon plasma and the formation of low-field peaks.The numerical results illustrate that the two low-field peaks are caused,to some extent,by the excitation of TG wave through non-resonance conversion.The peaks of plasma resistance increase and shifts to high-end of magnetic field as the RF frequency increasing.The electrical property of the end-plate and the location of antenna also affect the characteristics of the low-field peaks.