Recently several investigators reported on various means of generating cold plasma jets at atmospheric pressure. More interestingly, these jets turned out to be not continuous plasmas but trains of small high velocity plasma packets/bullets. However, until now little is known of the nature of these ‘bullets’. Here we present experimental insights into the physical and chemical characteristics of bullets. We show that their time of initiation, their velocity and the distance they travel are directly dependent on the value of the applied voltage. We also show that these bullets can be controlled by the application of an external electric field. Using an intensified charge coupled device camera we report on their geometrical shape, which was revealed to be ‘donut’ shaped, therefore giving an indication that solitary surface ionization waves may be responsible for the creation of these bullets. In addition, using emission spectroscopy, we follow the evolution of various species along the trajectory of the bullets, in this way correlating the bullet propagation with the evolution of their chemical activity.

In this paper He-discharge (plasma jet/bullet) in atmospheric pressure air and its progression phenomenon has been studied experimentally using ICCD camera, optical emission spectroscopy (OES) and calibrated dielectric probe measurements. The repetitive nanosecond pulse has applied to a plasma pencil to generate discharge in the helium gas channel. The discharge propagation speed was measured from the ICCD images. The axial electric field distribution in the plasma jet is inferred from the optical emission spectroscopic data and from the probe measurement. The correlation between the jet velocities, jet length with the pulse duration is established. It shows that the plasma jet is not isolated from the input voltage along its propagation path. The discharge propagation speed, the electron density and the local and average electric field distribution along the plasma jet axis predicted from the experimental results are in good agreement with the data predicted by numerical simulation of the streamer propagation presented in different literatures. The ionization phenomenon of the discharge predicts the key ionization parameters, such as speed, peak electric field in the front, and electron density. The maximum local electric field measured by OES is 95 kV/cm at 1.3 cm of the jet axis, and average EF measured by probe is 24 kV/cm at the same place of the jet. The average and local electron density estimated are in the order of 1011 cm-3 and it reaches to the maximum of 1012 cm-3

The influence of the N 2 admixture ratio on a He-N 2 uniform atmospheric pressure glow discharge (APGD) generated using an asymmetric 20 µs, 10 kHz bipolar applied voltage pulse is systematically investigated, using electrical and optical emission spectroscopic measurements as diagnostic tools. We focused our investigation on understanding the discharge characteristics of the plasma ignited after an off-voltage period of 80 µs duration. The measured breakdown voltages for various N 2 admixture ratios of up to 3% are compared with a theoretically calculated breakdown voltage, taking into account the secondary electron emission coefficient of the dielectric surface. The results indicate that the value of the secondary emission coefficient should increase from 0.1 to 0.5 to realize the experimentally obtained breakdown voltage variation within a 3% N 2 admixture ratio. Using experimentally obtained values of the breakdown voltage, discharge current densities for different N 2 admixture ratios, and 66 reactions among electrons, helium and N 2 , the variations of the N 2 (A), N 2 (C) and N + 2 (B) state densities with N 2 admixture ratio are calculated. The N 2 admixture ratio dependence of the calculated N 2 (C) and N + 2 (B) state densities are similar to the N 2 admixture ratio dependences of the emission intensities of the 337 nm band of the N 2 second positive system (SPS) and the 391.4 nm band of the N + 2 first negative system, respectively. Experimental observation and numerical investigation indicates that below a 1% N 2 admixture ratio, the He (2 3 S) metastable state has a strong influence on the discharge characteristics, and above a 2% N 2 admixture ratio, the N 2 (A) metastable state influences the discharge behavior. The N 2 (A) metastable state density is estimated experimentally using the emission intensity ratio of the 337 nm band of N 2 SPS and the 247 nm band of a NO-γ system. The experimentally estimated and theoretically calculated value of N 2 (A) metastable state density is of the same order of 10 13 cm −3 . The estimated N 2 (A) metastable state density shows a rapid increase above a 2% N 2 admixture ratio, which supports the idea that the secondary emission coefficient increases due to an increase in N 2 (A) flux.

Using an electrical discharge to control airflow has recently been an active area of research. This is mostly because of the interest in the manipulation of free airflow for aerodynamic applications. Corona discharges are well suited for these applications. In this paper, we present photographs illustrating the ion wind effect in a clear visual manner. The device used is a positive corona discharge between the end of sharp wires and a grounded mesh electrode. Measurements of the average wind speed as a function of the applied voltage for two different gap distances are presented.

To understand the self sustained propagation of the plasma jet/bullet in air under atmospheric pressure, the ignition of the plasma jet/bullet, the plasma jet/bullet ignition point in the plasma pencil, the formation time and the formation criteria from a dielectric barrier configured plasma pencil were investigated in this study. The results were confirmed by comparing these results with the plasma jet ignition process in the plasma pencil without a dielectric barrier. Electrical, optical, and imaging techniques were used to study the formation of the plasma jet from the ignition of discharge in a double dielectric barrier configured plasma pencil. The investigation results show that the plasma jet forms at the outlet of the plasma pencil as a donut shaped discharge front because of the electric field line along the outlet's surface. It is shown that the required time for the formation of the plasma jet changes with the input voltage of the discharge. The input power calculation for the gap discharge and for the whole system shows that 56% of the average input power is used by the first gap discharge. The estimated electron density inside the gap discharge is in the order of 10 11 cm −3 . If helium is used as a feeding gas, a minimum 1.48×10 −8 C charge is required per pulse in the gap discharge to generate a plasma jet.