Ruixue Wang

Comparison of Atmospheric-Pressure He and Ar Plasma Jets Driven by Microsecond Pulses

In order to compare the characteristics of plasma jets in He and Ar, the plasma jets are driven by a home-made microsecond pulse power, which provides 0-30-kV output voltage with a rise time of 0.5 μs and full-width at half-maximum of 4 μs, and the pulse repetition frequency can be changed from single shot to 5 kHz. The main electrical parameters, such as applied voltage, total discharge current, and jet current, are measured. Results show that the intensity and length of jets become stronger and longer as the voltage increases. The discharge current of He plasma jet is only a few millamperes, whereas Ar plasma is hundreds of millamperes. For both cases, the jet speed is on an order of ~104 m/s estimated by the intensified charge coupled device images. The analysis shows that the differences between He and Ar plasma jets are mainly due to the physical and chemical characteristics of Ar and He gases, especially the different ionization energies of particles. In addition, the optical spectrum results show that there are more excited OH radicals and N2 of the second positive system in Ar plasma jet while the He plasma jet contains more excited O and N2+ of the first negative system.

A comparison between characteristics of atmospheric-pressure plasma jets sustained by nanosecond- and microsecond-pulse generators in helium

Power source is an important parameter that can affect the characteristics of atmospheric-pressure plasma jets (APPJs), because it can play a key role on the discharge characteristics and ionization process of APPJs. In this paper, the characteristics of helium APPJs sustained by both nanosecond-pulse and microsecond-pulse generators are compared from the aspects of plume length, discharge current, consumption power, energy, and optical emission spectrum. Experimental results showed that the pulsed APPJ was initiated near the high-voltage electrode with a small curvature radius, and then the stable helium APPJ could be observed when the applied voltage increased. Moreover, the discharge current of the nanosecond-pulse APPJ was larger than that of the microsecond-pulse APPJ. Furthermore, although the nanosecond-pulse generator consumed less energy than the microsecond-pulse generator, longer plume length, larger instantaneous power per pulse and stronger spectral line intensity could be obtained in the nanosecond-pulse excitation case. In addition, some discussion indicated that the rise time of the applied voltage could play a prominent role on the generation of APPJs.

A repetitive microsecond pulse generator for atmospheric pressure plasma jets

Compared with other non-thermal plasma sources, the atmospheric-pressure plasma jet (APPJ) has advantages on simple structure, low temperatures, strong chemical activities, and convenient handling, all of which have attracted much attention. The power sources play an important role on the characteristics and the applications of the APPJ. In this article, a compact repetitive microsecond-pulse generator is designed for exciting the APPJ in helium and argon. The microsecond-pulse generator can produce repetitive pulses with output voltages of up to 20 kV, pulse width of ~8 μs, and pulse repetition frequencies (PRFs) of 1 Hz ~2 kHz. Using the designed repetitive microsecond-pulse generator, the characteristics of the APPJ are investigated by measuring the voltages and currents and obtaining images of the discharges. Experimental results show that the microsecond-pulse generator has been successfully used to sustain stable APPJs both in helium and argon. The shape of the output voltage pulses may change as the applied voltage increases. Nevertheless, the output voltages are stable at all PRFs when the applied voltage is fixed. Furthermore, the effects of flow rate, the applied voltage, and the PRF on the He/Ar APPJ are investigated. Results show that it is more likely to generate a He APPJ rather than an Ar APPJ under microsecond-pulse excitation. The length of the plasma plume is slightly affected by the PRF both in the He APPJ and the Ar APPJ.

Uniformity optimization and dynamic studies of plasma jet array interaction in argon

In this paper, the uniformity of a microsecond pulse driven argon plasma jet array is studied by optimizing experimental parameters, such as the ground electrode position d, gas flow rate, and pulse frequency. The results show that when d is less than 10 mm, there are always two plasma jets that cannot be ignited; while all jets are ignited with equal length at d = 12.5 mm. The plasma jet transition from laminar to turbulent at 6 l/min, presenting a “bead-like” structure. The jet length increases with the pulse repetition frequency and reaches a plateau at around 1.5 kHz. The ICCD images show that the plasma jets emerged preferably from the two quartz tubings located at the two ends of the linear array. The spatial-temporal resolved spectra show that the Ar emission (763.5 nm) and N2 emission (337.1 nm) at the end of the array (e.g., Jet #1) appeared earlier than that of the jet in the middle (e.g., Jet #2). On the contrary, the emission intensities of Ar in Jet #2 are higher than in Jet #1. This is due t...

Surface modification of epoxy using an atmospheric pressure dielectric barrier discharge to accelerate surface charge dissipation

In this paper, an atmospheric-pressure dielectric barrier discharge is used to modify the surface of the epoxy material and enhance the dissipation of surface charge to reduce the accumulation of surface charge. In the experiments, atmospheric-pressure air dielectric barrier discharge is driven by a microsecond pulse generator. Surface properties of epoxy before and after the plasma treatment are characterized by water contact angle, surface potential, and surface/volume conductivity measurements. Atomic force microscope and X-ray photoelectron spectroscopy are used to investigate the changes of the morphology and the chemical composition of the epoxy surface. Experimental results indicate that the surface of epoxy is etched by the plasma and the increase of the surface roughness enhances the surface insulation ability. The O radicals in plasma and the carbonyl groups formed on the surface make the surface charge trap shallower, change the epoxy surface composition then increase the surface conductivity and accelerate surface charge dissipation. When the epoxy is treated for an appropriate time, the epoxy surface insulation performance will be enhanced obviously and the surface charge dissipation will be accelerated.

Bent paths of a positive streamer and a cathode-directed spark leader in diffuse discharges preionized by runaway electrons

Diffuse discharges preionized by runaway electrons can produce large-area homogeneous discharges at elevated pressures, which is an intriguing phenomenon in the physics of pulsed discharges. In this paper, runaway-electron-preionized diffuse discharge (REP DD) was obtained in a wide pressure range (0.05–0.25 MPa), and under certain conditions a positive streamer and a cathode-directed spark leader could be observed to propagate at some angles to the applied (background) electric field lines. For a 16-mm gap at an air pressure of 0.08–0.1 MPa, the percentage of pulses in which such propagation is observed is about 5%–50% of their total number, and in the other pulses such bent paths could not be observed because there is even no streamer or cathode-directed spark leader in diffuse discharges. In our opinion, such propagation of the positive streamer and the cathode-directed spark leader at some angle to the background electric field lines owes to different increase rates of the electron density in different regions of the discharge volume under REP DD conditions. Therefore, during the formation of a REP DD, the increase of the electron density is inhomogeneous and nonsimultaneous, resulting in an electron density gradient at the ionization wave front.

Effect of pulse polarity on the temporal and spatial emission of an atmospheric pressure helium plasma jet

A single needle-electrode plasma jet driven by a home-made microsecond pulse power supply is studied. The electrical characteristics and optical emissions of the plasma jets driven by positive- and negative-polarity pulses are compared. With the same magnitude of applied voltage, the plasma jet driven by positive pulses shows a higher discharge current, a higher optical emission intensity and travels to a longer distance. The temporal-spatially resolved He (706.5 nm), N2 (337.1 nm) and (391.4 nm) emissions behave differently in the plasma jets driven by different polarity pulses: They appear to be discrete emission packets in the positive plasma jet, but continuous emission in the negative plasma jet (under the time resolution in this study). The emission front propagates at a faster speed in the positive plasma jet than in the negative plasma jet. The different behavior of the plasma jets is attributed to the electric field distribution under different polarity pulses.

Dynamics of Plasma Bullets in a Microsecond-Pulse-Driven Atmospheric-Pressure He Plasma Jet

In this paper, an atmospheric-pressure helium plasma jet driven by a home-made microsecond pulse generator was studied. The discharge was characterized by monitoring the voltage-current waveforms and head-on/side-on intensified charge coupled device (ICCD) images. The effects of gas flow rate and pulse frequency on the plasma bullet propagation were studied. The plasma bullets showed a donut shape in the beginning of discharge and then became solid. This was due to the curling up of the plasma plume. A smaller gas flow rate was beneficial for a homogeneous discharge and a higher bullet peak velocity. The pulse frequency had no influence on the bullet peak velocity, but a higher frequency induced a faster bullet speed decay.

Electrical Characteristics in Surface Dielectric Barrier Discharge Driven by Microsecond Pulses

In recent years, surface dielectric barrier discharge (SDBD) sustained by pulse power supply has been widely investigated due to its high efficiency and fast response. In this paper, in order to obtain the effect of the electrode length, dielectric thickness, and electrode width on the characteristics of SDBD driven by microsecond-pulse generator, experimental investigation on the SDBD actuator using a dielectric barrier of epoxy glass cloth is performed. The experimental results showed that both the current amplitude at the rising edge of the applied voltage and the consumption energy increased with the electrode length. Moreover, dielectric thickness slightly affected the displacement current, while the consumption energy decreased when the dielectric thickness increased. Furthermore, when the electrode width increased, the current at the rising edge of the applied voltage increased, but it would reach the saturation when the electrode width exceeded 3 mm. Both the instantaneous power and the consumption energy increased with the electrode width. The experimental results can provide reference for the optimal design of the SDBD actuator driven by microsecond pulses.

Spatial–Temporal Evolution of a Radial Plasma Jet Array and Its Interaction with Material

This paper investigates the behavior of a radial plasma jet array and its interaction with various cylindrical targets. The jet array comprises six individual plasma jets arranged around a circle at even spaces, directed towards a common central axis, where the cylindrical objects are to be treated. The interaction of the six jets without the target is studied. The effect of pulse rise time on the electrical and optical properties of the plasma jets is investigated. The spatial–temporal evolution of the plasma jet array and its interaction with (1) dielectric target, (2) floating metallic target and (3) grounded metallic target, are studied respectively. It is calculated that the velocity of the ionization front in these three cases, although differs amongst themselves, is in general greater than that in the no-target case. The luminosity and the profile of the ionization wave front are noticeably different in these three cases: comparing to the no-target case, the intensity of the ionization front reduces with the presence of a dielectric target, while the luminosity of the ionization wave front is enhanced with the presence of a metallic target (either floating or grounded). Optical emissions at the quartz nozzle exit and at the central axis are different with different targets. The different behavior of the jet array with the presence of different targets is attributed to the local electric field distribution, which will be discussed in this paper.