This paper presents the experimental and numerical results about the influence of nitrogen (N₂) and oxygen (O₂) admixture on the development of a helium (He) atmospheric-pressure plasma jet (APPJ) in a long dielectric tube. The results revealed that the jet length and the propagation velocity are strongly affected by introducing N₂ or O₂ into the He flow. Specifically, it was observed that a higher N₂/O₂ admixture led to the decrease in the density of both energetic and relative low-energy electrons outside the grounded electrode, which corresponds to the shortening of the jet length. In the He/O₂ mixture, the electrons are easily captured by O₂/O in the region of the plasma bulk. In the He/N₂ mixture, the jet propagation characteristics will change since N₂ has many low-level excitation states that consumed a large number of energetic electrons. The simulation shows that the magnitude of the axial electric field in the jet head depends strongly on the amount of N₂ and/or O₂ in the gas flow. In both cases, the peak electric field is on the order of 5 kV/cm, which is significantly higher than that in pure helium of 3 kV/cm even if the admixture is low (less than 4% N₂ or 2% O₂ in this work). Positive charges of higher density in the jet head are needed to induce a stronger electric field for the jet propagating in N₂(x%)/He and O₂(x%)/He mixtures compared with that in pure He.

In this paper, a two-dimensional fluid model was developed to study the radio frequency (RF) hollow cathode discharge (HCD) in argon at 1 Torr. The evolutions of the particle density distribution and the ionization rate distribution in RF HCD at 13.56 MHz indicate that the discharge mainly occurs inside the hollow cathode. The spatio-temporal distributions of the ionization rate and the power deposition within the hollow cathode imply that sheath oscillation heating is the primary mechanism to sustain the RF HCD, whereas secondary electron emission plays a negligible role. However, as driving frequency decreases, secondary electron heating becomes a dominant mechanism to sustain the discharge in RF hollow cathode.

We present here a comparison study on self-pulsing phenomena in negative corona, hollow cathode discharges (HCD) and parallel-plate discharge in air. The voltage-current (V-I) curve, the waveforms of self-pulsed currents, and the time-resolved images of the pulsed discharge are measured under various operating conditions. It is experimentally evidenced that the Trichel pulse in a negative corona and the self-pulsing in HCD and/or parallel-plate discharge have similar features as well as spatial-temporal developing process. It is suggested that they should have a similar mechanism that the pulsing reflects the mode transition of discharge between the low-current Townsend and the high-current normal glow. The pulse rising corresponds to the breakdown and formation of temporal glow discharge in a background of low-current Townsend discharge, while the decay edge relates to the transition back to Townsend discharge. The pulse interval is the re-building process of the space charge layer of high density to ensure the glow breakdown.

In this paper, the helium plasma jet generated by micro-hollow cathode discharge (MHCD) was studied. The MHCD was driven by a square-wave pulsed power source, and the characteristics of discharge and plasma jet were measured experimentally. The influences of the gas flow rate on the MHCD and the plasma jet were investigated. And the propagation mechanisms of the plasma jet were analyzed. The results show that within 100–1000 sccm of the gas flow rate, the breakdown delay time of the MHCD increases with the helium flow increasing. It is considered that the gas flow affects the density of seed electrons and thus the breakdown delay time. With the helium flow rate increasing, the whole plasma jet length increases firstly and then decreases. A detailed investigation shows that during one discharge pulse, two distinguishable propagation processes of the plasma jet are observed. It is found that the jet of the first stage is formed during the rising edge of the current pulse, while the other is generated after the discharge current becomes stable. The propagation velocity of jet in the first stage is on the order of several km s ⁻¹ , which is similar to that of the discharge evolution obtained by simulation. And the propagation speed of the jet in the second stage is on the order of several hundred m s ⁻¹ , which is close to the velocity of gas flow. The spatial–temporal distributions of light emission show that high-energy electrons can only be observed during the jet propagation in the first stage, and low-energy electrons can be detected in both the first and second stages. The results show that the electric field plays an important role on the jet propagation in the first stage, and the jet propagation during the second stage is mainly promoted by the thermal gas expansion.

This paper deals with an experimental study regarding the spatial–temporal evolution of single wire explosions in air and water at atmospheric pressure. Experiments were carried out with a microsecond timescale pulsed current source under 500 J stored energy. The morphology of the exploding wire with different discharge type, wire material, insulating coat, and ambient medium was intensively observed via self-emission images. The results revealed that the plasma radiation of wire explosion in air mainly contained two stages: initial intense radiation from optically thick plasma, and later decaying radiation from expanded arc-like plasma. A hollow structure was observed in a Cu exploding wire in air, and the plasma channel tended to develop inside it. As for W case, there would always be a core-corona structure, resulting in lower expansion rate and shock wave strength. Moreover, the plasma radiation of the underwater wire explosion was mainly determined by two factors: expansion rate and chemical reaction. The expansion rate of the radiant region in water was only 10 ^–1 mm μ s ^–1 level (1 mm μ s ^–1 level for air), leading to the lagging radiation. No noticeable difference in morphology was found between Cu and W wire explosions in water. Particularly, the Al wire explosion generated a bright light emission due to chemical reactions. Finally, radiative characteristics in an exploding wire were preliminarily concluded and existing problems proposed.

In this paper, we present an experimental and numerical study on micro-hollow cathode discharge (MHCD) in helium at atmospheric pressure. The discharge in the micro-hollow cathode is driven by a −700 V pulsed power supply with the μs width and kHz frequency. The electrical characteristics were obtained by the experimental measurement and a self-consistent fluid model. The waveforms of the voltage and discharge current obtained by simulation are highly consistent with those obtained by experiments. The electrical characteristics, the spectra, and the distribution of the discharge plasma indicate that an abnormal glow discharge was formed in the MHCD. The spatial-temporal evolutions of the discharge show that the maximum plasma density can attain about 10¹⁵ cm⁻³. The averaged plasma density is over 10¹⁴ cm⁻³ during the discharge, and the plasma can be maintained for a long time during the afterglow period. The discharge current density can reach 30 A cm⁻². All results show that a strong ionization and excitation can be generated by pulsed micro-hollow cathode discharge which is suitable for chemical analysis.