Optical emission spectroscopy (OES) is a common method for characterizing radio frequency (RF) discharge plasmas. Particulary, helicon plasma is featured by its high plasma density among all RF-excited plasmas. In order to obtain the spatial-resolved information of a helicon plasma, local optical emission spectroscopy (LOES) with a 3 mm spatial resolution was proposed and carried out to evaluate the local electron density and temperature. The plasma emission intensity via LOES was measured and compared with the electron density obtained by a RF-compensated Langmuir probe (LP) in Ar, N 2 and Air helicon plasmas, respectively. The results revealed that there existed a functional relationship between some specific lines (LOES) and electron density (LP). Further, helicon plasma characteristics under capacitive (E) , inductive (H), and helicon (W) modes were systemetically investigated based on LOES. Besides, two-dimensional (2D) contour maps for plasma distributions were made via LOES as well. It was found that in E-and H-modes, axial profiles of plasma density and electron temperature were consistent under two opposite magnetic field directions. However, in W-mode, the plasma presented an asymmetric axial profile along the tube. As for radial profiles, plasma distribution varied under three discharge modes due to different heating mechanisms in Ar, N 2 or Air helicon plasma. A deeper analysis indicated that the bulk absorption comes from the coupling of the helicon wave in Ar helicon plasma while the power depositions in N 2 and Air helicon plasma are mainly dominated by the TG wave.

Blue core (BC) is a special spectral phenomenon in argon helicon plasma, with intense blue lights from ion emission around the radial center of discharge tube. In this paper, the characteristics of BC in argon (Ar) helicon plasma were investigated experimentally from aspects of discharge mode transitions, plasma spatial distributions, and spectral features. It is found that the BC generally appears at strong magnetic field (480 G or above in this work) with high plasma density in wave mode, accompanied by exponentially rising of ion line intensity. The electron density and temperature, the neutral density and temperature, and the line emission intensity show a radial profile with a central peak in BC mode. The steep gradient of ion line intensity (corresponding to the ion density) defines a clear boundary of the core. Further, a pressure balance model was developed to investigate the influence of neutral depletion on BC formation. The neutral density is depleted significantly from 7.24×1013 to 0.38 × 1013 cm−3 at magnetic field of 600 G in BC mode, while to 3.13 × 1013 cm−3 at magnetic field of 250 G in normal wave (NW) mode. The ionization rate in BC reaches as high as 70% compared with 9.6% of that in NW mode. The ionization rate and the ion line intensity show similar radial profiles, indicating the BC phenomenon is closely related to the distribution of peaked ion density and hollowed neutral density. Fundamentally, the central electron heating and strong magnetic field contribute to the centrally local high ionization rate and strong neutral heating. The severe neutral depletion with prominent central heating is considered to be the immediate cause of appearance of blue core.

We present the axial profiles of argon helicon plasma measured by a local optical emission spectroscope (OES) and Langmuir RF-compensated probe. The results show that the emission intensity of the argon atom lines (750 nm, 811 nm) is proportional to the plasma density determined by the Langmuir probe. The axial profile of helicon plasma depends on the discharge mode which changes with the RF power. Excited by helical antenna, the axial distribution of plasma density is similar to that of the external magnetic field in the capacitive coupled mode (E-mode). As the discharge mode changes into the inductively coupled mode (H-mode), the axial distribution of plasma density in the downstream can still be similar to that of the external magnetic field, but becomes more uniform in the upstream. When the discharge entered wave coupled mode (W-mode), the plasma becomes nearly uniform along the axis, showing a completely different profile from the magnetic field. The W-mode is expected to be a mixed pattern of helicon (H) and Trivelpiece-Gould (TG) waves.

Nitrogen (N2) helicon plasma is produced with radio frequency (RF) right-helical antenna at low pressures. Several wave modes and their transitions of N2 helicon discharge are observed experimentally. Blue Core phenomenon is achieved at high magnetic fields and high RF powers, with strong local blue light emissions of N II lines and high electron temperature inside the core area. Based on actinometric ratio and pressure balance model, species kinetics of N2 helicon plasma are analyzed. It is shown that about 79% of N2 molecules are dissociated and about 49% of the neutrals are ionized inside the Blue Core in high magnetic field of 850 G and RF power of 2200 W. Nearly 99% of N2 molecules inside the core area are depleted considering the neutral density before and after discharge, from 7.3×1013 cm-3 to 6.5×1011 cm-3. Serious neutral depletion of N2 and N neutrals and high electron temperature are suggested to be the dominant causes for significant enhancement of central N II emissions (blue lights). Meanwhile, evolution of reaction processes indicates that N ionization and N+ excitation become dominant in BC mode. Besides, external magnetic field is an important factor to control the discharge mode transitions as well as the radial distributions of plasmas. From the calculated results of dispersion relation, the cavity mode resonance, rather than antenna coupling resonance, of helicon waves plays a dominant role on the wave mode formation and RF energy coupling between RF antenna and plasma. The mode transition results from excitation of helicon wave of higher axial eigenmode. N2 helicon plasma shows different characteristics from argon in mode transition, spectral emission and Blue Core formation. It is due to the high dissociation energy of N2 molecules (9.8 eV) and extensive dissociation and ionization processes. This results in a higher RF power as well as magnetic field for helicon wave coupled mode in N2 helicon plasma than that in Ar plasma.

In this paper, we present an experimental study on the self-pulsing phenomenon in a flow-induced atmospheric pressure plasma jet (APPJ) in a hollow electrode configuration driven by dc voltage supply. The current-voltage curve, the typical waveforms of current and voltage of self-pulsing, the time-resolved images, and the repetition frequency were measured under different experimental conditions. The results show that the APPJ of a hollow electrode can sustain in a stable, repeatable self-pulsing regime. The waveform of the pulsed current is very stable with nearly constant rising time and decay time at different discharge averaged currents. Although the pulsing frequency increases linearly with the averaged current and the gas flow rate, it decreases with the electrode gap. An equivalent electric circuit consisting of a capacitor and two resistors was used to model the self-pulsing discharge plasma. The simulation results and the time-resolved images recorded using an ICCD camera show that the pulsed process of the hollow electrode APPJ contains the evolutions of gas breakdown, discharge development, and decay of a glow plasma. A weak discharge is maintained during the time interval between two pulses, indicating that the self-pulsing in this APPJ is a mode transition between glow and weak discharge.