Helicon plasma density may show a non-monotonic dependence on the magnetic field at low strength, so-called “low-field peak (LFP).” We presented the multiple LFPs and the formation mechanism in argon helicon plasmas in this paper. Propagating conditions of helicon (H) and Trivelpiece–Gould (TG) waves in collisional plasmas were calculated based on the dispersion relation. It is demonstrated that there are two mechanisms during mode transition responsible for LFP, i.e., resonance of H- and TG-waves and anti-resonance of TG-wave. Especially, H-TG resonance of the highest axial mode in the helicon plasma results in a density jump rather than a density peak due to the mode transition from non-wave to co-H/TG-wave mode. Higher plasma density in lower magnetic fields is helpful for achievement of multiple LFPs in argon helicon plasmas.

In this paper, the air plasma jet produced by micro-hollow cathode discharge (MHCD) is investigated. The discharge is powered by a positive nanosecond pulse high voltage supply. The waveforms of the discharge, the images of the jet, the evolution of the plasma bullet and the reactive species are obtained to analyze the characteristics of the MHCD plasma jet. It is found that the length of the plasma jet is almost proportional to the air flow rate of 2–6 slm. Two plasma bullets appear one after another during a single period of the voltage waveform, and both of the two plasma bullets are formed during the positive pulse voltage off. The propagation velocity of the two plasma bullets is on the order of several hundred m/s, which is approximate to that of the air flow. These results indicate that the gas flow has an important influence on the formation of this MHCD plasma jet.

The effect of neutral pressure on blue core in Ar helicon plasma under inhomogeneous magnetic field was investigated in this work. The neutral pressure was set to 0.08 Pa, 0.36 Pa and 0.68 Pa. Nikon camera, intensifies charge coupled device (ICCD), optical emission spectrometer (OES), and Langmuir probe were used to diagnose the blue core in helicon plasma. Helicon plasma discharges experienced density jumps from E-, H- to W-mode before power just rose to 200 W. The plasma density increased and maintained the central peak with the increase of neutral pressure. However, the brightness of the blue core gradually decreased. It demonstrated that the relative intensity of Ar II spectral lines and the ionization ratio in the central area were reduced. Radial electron temperature profiles were flattened and became hollow as neutral pressure increased. It demonstrated that increasing the neutral pressure weakened the central heating efficiency dominated by helicon wave and strengthened the edge heating efficiency governed by the TG wave and skin effect. Therefore, the present experiment successfully reveals how the neutral pressure affects the heating mechanism of helicon plasma in inhomogeneous magnetic field.

Formation and propagation of surface plasmons (SPs) of microwaves (MWs) along a dense non-uniform plasma column were investigated by employing the finite-difference time-domain method and evidenced by experiment of argon helicon plasma. The effects of plasma density, incident frequency, and radius of plasma column on the SP features were discussed. Results showed that when the MW frequency and the density distribution of plasma column satisfy suitable conditions, SPs can be excited on the surface of a uniform plasma column when the frequency is 2–5 times of the MW, while inside a nonuniform plasma column when the plasma frequency is only a little higher (about 5%–20%) than the MW. The SP wavelength depends on the plasma density for a given incident MW frequency. The forward scatter MW field will be enhanced significantly when SP appears along the nonuniform plasma column.