Influence of species kinetics on discharge characteristics in oxygen helicon plasma

Oxygen (O₂) helicon plasmas in multiple wave modes were excited by a right-helical antenna with an upper metal endplate at low pressure. Mode transitions were observed at increasing input power or magnetic field, characterized by obvious jumps of plasma parameters. Blue Core appears at high magnetic fields (∼700 G) and input powers (∼1700 W), with a large radial gradient of plasma density, ion line intensity, and electron temperature. Emission spectra demonstrate that the blue lights originate from O II lines. We found that the intensity ratio of O II to O I of Blue Core in O₂ is lower by one order than that in N₂ or Ar despite their similar ionization rates and plasma densities in the Blue Core area. A high-temperature B-dot probe together with a waveform fitting procedure was used to present the measured oscillating waveforms of m = +1 helicon waves, showing distinct wave structures of different eigenmodes. Cavity mode resonance is suggested to be responsible for the formation of standing waves of discrete eigenmodes. A pressure balance model was developed to estimate the species densities around the central area in different modes, showing massive dissociation of O₂ molecules and high density of O atoms locally, so that O₂ helicon plasma behaves as a species feature of monatomic gas discharge. The obviously low intensity of the O II lines compared to the O I lines of Blue Core in O₂ is related to the quite high excitation threshold of O⁺ ions (∼30 eV) although electron density and temperature are relatively high. The combined effects of dispersed reaction energy distribution, massive molecule dissociation and negative ion creation are considered to be the main causes for the requirement of much higher RF power and magnetic field for Blue Core formation in O₂ helicon plasma than that in Ar. The calculated radial profiles of power deposition and the captured plasma morphology confirm that the dominant central electron heating is the essential reason for the large radial gradients of plasma density and electron temperature which contribute to the serious neutral depletion and Blue Core formation.