On the wave mode transition of argon helicon plasma

In this paper, multiple wave modes and transitions of argon helicon plasma excited by a half right-helical in a system without any reflection endplate are investigated experimentally and theoretically at increasing radio frequency (RF) powers and external magnetic fields. Experiments show that above a critical magnetic field strength and pressure (about 250 G and 0.3 Pa in this work), two to four distinct wave coupled modes and transitions were observed at increasing RF powers and/or magnetic fields. Theoretical analysis based on dispersion relationship show that in high magnetic field helicon wave of the lowest order of axial eigenmode is always excited firstly, then the higher order axial or radial mode, hence the plasma density increases after mode jumping. There are two mechanisms responsible for the wave mode transitions in the present system, i.e., axial and radial mode transitions owning to the change of axial and radial wavenumbers from a lower eigenmode to a higher one. Higher plasma density and magnetic field are helpful for achieving more higher-order modes of helicon waves.