Ultra-short pulsed non-equilibrium atmospheric pressure gas discharges

This thesis presents experimental studies of various non-thermal atmospheric pressure gas discharges generated using short pulsed excitation as an alternative to widely used sinusoidal excitation. Several pulse generators are detailed that provide high voltage pulses ranging from hundreds of microseconds to less than ten nanoseconds in duration. A key enabler to the generation of a stable discharge is a suitably high repetition rate; this prerequisite precludes many conventional pulsed power technologies. Fortunately, recent advances in semiconductor technology have made it possible to construct solid state switches capable of producing high voltage pulses with repetition rates of many kilohertz. Pulsed excitation introduces many opportunities to tailor the applied voltage and consequently enhance the discharge which are not possible with sinusoidal excitation sources. Through detailed electrical and optical analysis it is shown that pulsed excitation is not only more energy efficient than a comparable sinusoidal source but produces a higher flux of excited species that are essential in many applications. When pulse widths are reduced to a sub-microsecond timescale a novel barrier-free mode of operation is observed. It is shown that diffuse large area plasmas are easily produced at kilohertz repetition rates without the usually indispensable dielectric barriers. Experimental results show that a short pulse width prevents the onset of the undesirable glow-to-arc transition thus introducing an added degree of stability. A further benefit of pulsed excitation is the ability to produce gas discharges with a high instantaneous peak power yet low average power consumption, resulting in a high density plasma that exhibits roomtemperature characteristics. Finally, as an acid test to highlight the many benefits of pulsed excitation several real-world applications are considered. It is shown that in all cases pulsed gas discharges provide real benefits compared to their sinusoidal counterparts

Room-temperature atmospheric argon plasma jet sustained with submicrosecond high-voltage pulses

In this letter, an experimental study is presented to characterize a room-temperature plasma jet in atmospheric argon generated with submicrosecond voltage pulses at 4 kHz. Distinct from sinusoidally produced argon discharges that are prone to thermal runaway instabilities, the pulsed atmospheric argon plasma jet is stable and cold with an electron density 3.9 times greater than that in a comparable sinusoidal jet. Its optical emission is also much stronger. Electrical measurement suggests that the discharge event is preceded with a prebreakdown phase and its plasma stability is facilitated by the short voltage pulses

Sharp bursts of high-flux reactive species in submicrosecond atmospheric pressure glow discharges

In this letter, the authors present an experimental study of the temporal characteristics of submicrosecond pulsed atmospheric glow discharges. Using electrical measurements and nanosecond-resolved optical emission spectroscopy, they show that a long initial period of each voltage pulse is spent building up space charges and is then followed by a large current pulse in the voltage-falling phase. Reactive plasma species such as oxygen atoms and OH radicals are produced in a train of sharp and independent pulses of 50–100 ns wide. Finally, their production is shown to increase significantly as the voltage pulse width reduces or the repetition frequency increases

10 ns pulsed atmospheric air plasma for uniform treatment of polymeric surfaces

This letter reports an experimental study of a 10 ns pulsed dielectric barrier discharge in atmospheric air, excited with a train of 65 ns voltage pulses at a repetition frequency of 5 kHz. It is shown that these ultrashort pulses produce a homogenous discharge with very high electron density in excess of 1013 cm−3 and low gas temperature, which are particularly desirable for uniform treatment of thermally sensitive polymer films. Their treatment of polypropylene films is found to introduce microscale surface patterns as well as various carbon-oxygen bonds, both useful for improving the hydrophilic properties of polymeric materials

Contrasting characteristics of linear-field and cross-field atmospheric plasma jets

This letter reports an experimental study of two types of atmospheric pressure plasma jets in terms of their fundamental properties and their efficiency in etching polymeric materials. The first plasma jet has a cross-field configuration with its electric field perpendicular to its gas flow field, whereas the second is a linear-field device having parallel electric and flow fields. The linear-field jet is shown to drive electron transportation to the downstream application region, thus facilitating more active plasma chemistry there. This is responsible for its etching rate of polyamide films being 13-fold that of its cross-field counterpart

Frequency effects of plasma bullets in atmospheric glow discharges

Pointlike plasma bullets have been reported in atmospheric plasma jets below 50 kHz. This paper presents 10-ns images of both pointlike and striplike plasma bullets generated at frequencies between 80 and 394 kHz. The striplike plasma bullets are always present, whereas the pointlike ones are suppressed at sufficiently high frequencies. Abel-converted images show that the pointlike plasma bullets have a long tail of 4.5 cm extended to the anode region

Atmospheric glow discharges from the high-frequency to very high-frequency bands

This letter reports an experimental investigation of an atmospheric glow discharge in both the high-frequency (HF) band of 3–30 MHz and the very high frequency band of 30–300 MHz. At constant input power, increased frequency is found to change little the electron density and to reduce slightly the electron excitation temperature. Significantly, an eightfold frequency increase from 20 to 80 MHz leads to a 20-fold increase in the maximum plasma power without plasma constriction. The maximum power density of 355 W/cm3 achieved at 80 MHz is far greater than those reported in the HF band

From submicrosecond-to nanosecond-pulsed atmospheric-pressure plasmas

We have developed a time-hybrid computational model to study pulsed atmospheric-pressure discharges and compared simulation results with experimental data. Experimental and computational results indicate that increasing the applied voltage results in faster ignition of the discharge and an increase in the mean electron energy, opening the door to tunable plasma chemistry by means of pulse shaping. Above a critical electric field of ~2 kV/mmfor ~1-mm discharges, pulsed plasmas ignite right after the application of an externally applied voltage pulse. Despite the large pd value (30–300 torr · cm) and the high applied electric field, the discharges are found to be streamer free in a desirable glowlike mode. The comparison of the time evolution of the mean electron kinetic energy as a function of the pulse rise time suggests that a fast rise time is not necessarily the best way of achieving high mean electron energy

Submicrosecond pulsed atmospheric glow discharges sustained without dielectric barriers at kilohertz frequencies

In this letter, the authors report the experimental observation of a large-volume atmospheric glow discharge sustained without dielectric barriers at 1 kHz. This barrier-free mode of operation is made possible with a submicrosecond pulsed excitation instead of the usual sinusoidal excitation. Its current-voltage characteristics are shown to be very different from conventional atmospheric dielectric barrier discharges, and its generation mechanism is studied with nanosecond resolved optical emission spectroscopy. The pulsed barrier-free atmospheric plasma is shown to produce very intense atomic oxygen emission line at 777 nm, up to one magnitude more intensive than that of a comparable atmospheric dielectric barrier discharge

Contrasting characteristics of pulsed and sinusoidal cold atmospheric plasma jets

Pulsed excitation of cold atmospheric plasmas is commonly believed to offer valuable benefits compared to the mainstream sinusoidal excitation. However, direct comparison of pulsed and sinusoidal atmospheric plasmas remains few, if any, thus casting an uncertainty of whether pulsed excitation facilitates any significant advantage. In this letter, we report a comparison study of pulsed and sinusoidal cold atmospheric plasma jets through electrical characterization, gas temperature measurement, and optical detection of reactive plasma species. An example of pulsed excitation is shown to reduce the electrical energy consumption by a factor of 12 for producing the same amount of oxygen atoms