Full understanding of the basic physical mechanism behind the low magnetic field density peak phenomenon is crucial to realize a simple, low-cost and high-performance radio frequency helicon plasma source. Although research on this nonlinear phenomenon has continued for many years, there is still no unified conclusion among researchers on its explanation, and further research remains to be conducted both theoretically and experimentally.
As a further theoretical attempt to fully figure out the basic physical picture behind the low magnetic field density peak phenomenon, in this paper, the influence of neutral gas pressure on Trivelpiece-Gould (TG) wave energy deposition characteristic in low magnetic field density peak phenomenon is investigated. Within the typical parameter range of low magnetic field density peak phenomenon, i.e., wave frequency ω/2π=13.56 MHz, low magnetic field (B₀ ≤ 100 G), moderate plasma density (n₀ =1×10¹² cm^-3), low neutral gas pressure (p≤5 mTorr ), ratio of axial ion temperature to axial electron temperature T_i,z/T_e,z =0.1, by employing the general warm plasma dielectric tensor model and eigenmode dispersion relationship of electromagnetic waves in magnetized plasma, the mode coupling characteristic between helicon and Trivelpiece-Gould (TG) waves, and power deposition properties of helicon and TG waves for the first three azimuthal modes (m =0, -1, +1) are theoretically analyzed. Analytical results suggest that: 1) the intensity of neutral gas pressure not only has a significant impact on the mode coupling characteristics between the electromagnetic and electrostatic branches of Whistler waves, but also significantly alters the mode coupling characteristics between the helicon wave and the TG wave, mode coupling surface (MCS) position and amplitude of the perpendicular wave number of helicon and TG waves. 2) Regarding the first three azimuthal modes (m =0, -1, +1) of helicon and TG waves, under low-pressure conditions, as pressure increases, energy deposition by helicon wave gradually weakens while increases by the TG wave; the m =+1 mode dominates energy deposition for helicon wave, whereas the m =0 mode dominates energy deposition for TG wave; as pressure increases, the peak energy deposition regions for both the m =+1 mode of helicon wave and m =0 mode of TG wave moving toward the edge region of the plasma column; the TG wave m =0 mode plays a dominant role in whole energy deposition process, more specifically, energy deposition induced by collisional and Landau damping dominates for different electron temperature ranges, as pressure increases, collisional damping induced energy deposition intensifies until the pressure value surpasses a critical threshold, above which the collisional damping becomes the absolute dominant mechanism in whole energy deposition process. 3) For the m =0 mode of TG wave, under low neutral gas pressure conditions, energy deposition resulting from collisional damping and Landau damping peaks at a magnetic field of approximately 50 G and an electron temperature of a few electron-volts.
These analysis results provide theoretical support for understanding the low magnetic field density peak phenomenon and offer new evidence for a more comprehensive understanding of the helicon discharge process.