Characteristics and mechanism of low-field peak in argon helicon plasma of single loop antenna

Low magnetic field density peak (LFP) is a typical nonlinear phenomenon in helicon wave discharge, which is characterized by the nonlinear increase in electron density with the magnetic field in lower magnetic fields. In this paper, the characteristics and generation mechanism of LFPs of argon helicon wave plasma excited by m = 0 single-loop antenna are studied by experiment and numerical simulation. Experimental results show that plasma density shows two peaks at increasing magnetic field in the range of 0-100 G. The first peak appears around 10 G, and the second one appears between 30 and 50 G. The peak density is related to gas pressure, radio frequency power, and tube dimension. From B-dot measurement, there exists obvious helicon wave structure in plasma at field strength around the LFP, with component of standing wave. Theoretical analysis demonstrated that the first density peak occurs on the demarcation line in density-magnetic field map where the H-wave limited by radial boundary condition begins to propagate, while the second peak is due to the fact that the axial wavenumber of H-wave decreases gradually with the increased magnetic field and the heating effect by standing wave resonance coupling is weakened above a critical magnetic field, leading to a sudden decrease in plasma density. Simulation by HELIC code shows that the change of radial distribution of power deposition reflects the conversion of heating mechanism from single TG-wave mode to H-TG wave coupled mode heating in low magnetic fields. The axial wavenumber with the maximum absorbed power decreases with the increased magnetic field, corresponding to the change of wave structure.