Two density peaks in low magnetic field helicon plasma

In this paper, we report two density peaks in argon helicon plasma under an axial magnetic field from 0'G to 250'G with Boswell-type antenna driven by radio frequency (RF) power of 13.56'MHz. The first peak locates at 40-55'G and the second one at 110-165'G, as the RF power is sustainably increased from 100'W to 250'W at Ar pressure of 0.35'Pa. The absorbed power of two peaks shows a linear relationship with the magnetic field. End views of the discharge taken by intensified charge coupled device reveal that, when the first peak appeared, the discharge luminance moves to the edge of the tube as the magnetic field increases. For the second peak, the strong discharge area is centered at the two antenna legs after the magnetic field reaches a threshold value. Comparing with the simulation, we suggest that the efficient power absorption of two peaks at which the efficient power absorption mainly appears in the near-antenna region is due to the mode conversion in bounded non-uniform helicon plasma. The two low-field peaks are caused, to some extent, by the excitation of Trivelpiece-Gould wave through non-resonance conversion.