Asma Begum

Atmospheric pressure He-air plasma jet: Breakdown process and propagation phenomenon

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.

A Positive Corona-Based Ion Wind Generator

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.

A Brief Study on the Ignition of the Non-Thermal Atmospheric Pressure Plasma Jet from a Double Dielectric Barrier Configured Plasma Pencil

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 outlets 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 1011 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.

Effects of fluid flow on the characteristics of an atmospheric pressure low temperature plasma jet

Recently interest in low temperature atmospheric pressure plasma jets has increased due to their unique capabilities and novel applications [1], such as biomedicine. Prior experimental results showed that low temperature plasma jets are in fact trains of plasma bullets/packets traveling at supersonic velocities. This is especially interesting because the plasma bullets travel in a region free of any external electric field. Although Lu and Laroussi [2] explained this phenomenon by photoionization, how the bullets form and how they reach such high velocities are still not well-understood issues. Additionally, some properties of the plasma jets, such as the plasma plume length, homogeneity, bullets shape etc., are directly affected by the fluid flow, the geometry of the electrodes, and the characteristics of the applied high voltage pulse. In this paper, the fluid dynamics and electrostatic simulations of the plasma pencil are performed by a commercially available, partial differential equation solver based on Finite Element Method. We found that if the fluid velocity value is more than 10 m/s, the plasma plume starts to fluctuate and becomes unstable. In addition, the length and homogeneity of the plasma plume are directly related to the applied high voltage (magnitude and pulse width) and the fluid velocity values. These numerical simulations contribute some insights for future theoretical explanations and allow us to determine the optimum operation conditions of the plasma pencil.

Investigations of the plasma bullet velocity by electrical and optical techniques

Summary form only given. The propagation velocity of the atmospheric helium pulsed plasma jet/bullet generated by the plasma pencil has been investigated through electrical and optical diagnostic techniques. The electrically-based technique measures the jet velocity from the time delay between the jet current peaks along the jet propagation axis. Using optical emission spectroscopy (OES) the temporal evolution of the reactive species along the plasma jet axis was observed and this evolution provided a measure of the average and instantaneous plasma bullet velocity. The measured velocities are compared to the velocity obtained from the images of the plasma jet taken at different times by using an ICCD camera. It is found that the magnitude of the velocity measured by the electrical technique is little higher than the velocity measured by imaging and spectroscopy techniques, which produced similar results. However, in all cases, the bullet velocity is in the same order of magnitude (*105 m/s). It is also shown that each pulse generates a single bullet and this bullet propagates leaving a low density plasma channel behind it.

Electrical characterization of the plasma jet generated by plasma pencil

Recent studies have shown that low temperature atmospheric pressure plasma jets are formed by the propagation of small plasma bullets traveling at very high velocities in ambient air. In this paper we report on the spatial evolution of the plasma jet properties investigated from the jet current density, where the jet current was measured by placing a lab-made probe along the jet propagation axis. The plasma jet was generated by a plasma pencil powered with high voltage short-duration pulses. The average current density, power density, electric field and electron density for every 0.5 cm plasma jet segment along the jet propagation trail were measured. The average peak current density, power density, and the electric field of the plasma jet were respectively 0.64 (A/cm2), 6.6 (kW/cm3) and 11.5 (kV/cm). The drift velocity of the electrons was calculated for the measured electric field along the jet axis and it showed a good agreement with the plasma jets front velocity measured by ICCD image tracking. It also shows that the helium gas plays an important role in the jet propagation. The estimated average electron density in the plasma jet is 4.8+1011 cm3.

Formation, propagation, and contraction of the plasma bullets emitted by a pulsed plasma jet

Recently non-thermal atmospheric pressure plasma jets have been playing an important role in plasma processing including biomedical applications [1]. This is due to the ability of providing plasmas not confined by electrodes. In this paper we report experimental investigations on the characteristics of the plasma jet emitted by a pulsed plasma generator, the “Plasma Pencil”. To study the formation and the discharge phenomenon of the plasma jet, we investigated the jet initiation point for three different electrodes configurations. The devices were operated by a high voltage pulse generator (up to 10 kV) with variable pulse widths and repetition rate. Using ICCD images we show that the plume is a series of plasma packets/bullets traveling at high velocities. The plasma bullet phenomenon was first observed by Teschke and co-workers for an RF jet (2005) and Laroussi and co-workers in the case of a nanoseconds pulsed jet (2006) [1]. Correlation between the discharge current and ICCD images reveals when and how the bullets are emitted from the device. The ICCD images of the jet formation point and the discharge current waveform showed that the jet generated in a discharge chamber with dielectric barrier method, begun as a surface discharge around the outlet hole of the barrier and propagated as a surface discharge, maintaining a donut shape. If the high voltage electrode is directly exposed to the ambient gas (no barrier) a filamentary discharge was found responsible for the jet formation and propagation. Depending on the input voltage and the configuration of the discharge chamber, the discharge process could be filamentary or glow-like. Even when the bullet first exhibited a donut shape, it eventually contracted and collapsed to a single head as it propagated away from the device.

Role of the species generated by plasma jets in the formation of the plasma bullets

Prior work on atmospheric pressure plasma jets has shown that the plasma plumes are in fact made of trains of small plasma packets/bullets1–4. These so-called bullets were found to travel at high velocities in ambient air. In this paper Optical Emission Spectroscopy (OES) is used to identify the chemical species that may play a role in the formation and propagation of the plasma bullets. The emission spectra under various operating conditions for a helium plasma plume in air are recorded by an Ocean Optics mini-spectrometer USB4000. It is found that the emission spectra are dominated by N2+ and by N2 excited states. Atomic oxygen, O, and OH lines are also present. To study the spatial variation of the intensity of the main emission lines (N2, N2+, O, OH, and He) an optical fiber is used to collect the localized emission along the axis of the plume. It is found that the N2+ density is a crucial parameter influencing the physical characteristics of the plasma plume. Helium emission lines seem to indicate that N2 ionization by helium metastable states may play an important role in the sustainment and propagation of the plasma bullets.

Experimental measurements of an ionic wind generated by a positive corona discharge

This paper reports on experimental measurements on an ionic wind generation device. Ionic wind is based on the entrainment of neutral atoms and molecules by ions being accelerated by an applied electric field. For a non-zero net average resultant force on the neutrals, single polarity ions are needed. In the case of a positive corona in air, molecular nitrogen positive ions dominate in the region between the cathode and the anode. The device used in ther experiments is a positive corona discharge in air between the end of sharp wires and a grounded mesh electrode. Measurements of the average wind speed as a function of different operating conditions are presented.

Investigation of the plasma bullets generated by low temperature plasma jets

It has recently been reported by several investigators that low temperature plasma jets are not continuous plasma plumes but in fact are made of small plasma packets/bullets traveling at high velocities . This is a rather interesting phenomenon especially in the case when the plasma jet is launched in a region where there is no externally applied electric field. ICCD images show that as it moves forward, the jet is indeed a small confined volume of plasma that is disconnected from the electrodes. The bullet travels for a certain distance and eventually the plasma quenches, ending the propagation process. In this paper, along with experimental results we propose a preliminary model that describes the bullet characteristics and its propagation dynamics. The model is based on a streamer approach with photoionization playing an important role in providing the seed electrons in front of the streamer head. Comparison between experimental results and the simulations will be presented and discussed.