20 Years of Microplasma Research: A Status Report

The field of microplasmas gained recognition as a well-defined area of research and application within the larger field of plasma science and technology about 20 years ago. Since then, the activity in microplasma research and applications has continuously increased. A survey of peer reviewed papers on microplasmas published annually shows a steady increase from fewer than 20 papers in 1995 to about 75 in 2005 and more than 150 in 2014. This count excludes papers that deal exclusively with technological applications where the microplasma is used solely as a tool. This topical review aims to provide a snap shot of the current state of microplasma research and applications. Given the rapid proliferation of microplasma applications, the topical review will focus primarily on the status of microplasma science and our understanding of the physics principles that enable microplasma operation. Where appropriate, we will also address microplasma applications, however, we will limit the discussion of microplasma applications to examples where the application is closely tied to the plasma science. No attempt is made to provide a comprehensive and in-depth review of the diverse range of all microplasma applications, except for the inclusion of a few key references to recent reviews of microplasma applications

Dispersion of Water for Impulse Propagation

This paper calculates the dispersive loss of water when a Gaussian impulse, or a step function impulse travels over a distance. The minimum risetime tmr was calculated

Direct Current Glow Discharges in Atmospheric Air

Direct current glow discharges have been operated in atmospheric air by using 100 μm microhollow cathode discharges as plasma cathodes. The glow discharges were operated at currents of up to 22 mA, corresponding to current densities of 3.8 A/cm2 and at average electric fields of 1.2 kV/cm. Electron densities in the glow are in the range from 1012 to 1013  cm−3. Varying the current of the microhollow cathode discharge allows us to control the current in the atmospheric pressure glow discharge. Large volume atmospheric pressure air plasmas can be generated by operating microhollow cathode discharges in parallel

Excimer Emission From Cathode Boundary Layer Discharges

The excimer emission from direct current glow discharges between a planar cathode and a ring-shaped anode of 0.75 and 1.5 mm diameter, respectively, separated by a gap of 250 μm, was studied in xenon and argon in a pressure range from 75 to 760 Torr. The thickness of the “cathode boundary layer” plasma, in the 100 μm range, and a discharge sustaining voltage of approximately 200 V, indicates that the discharge is restricted to the cathode fall and the negative glow. The radiant excimer emittance at 172 nm increases with pressure and reaches a value of 4 W/cm2 for atmospheric pressure operation in xenon. The maximum internal efficiency, however, decreases with pressure having highest values of 5% for 75 Torr operation. When the discharge current is reduced below a critical value, the discharge in xenon changes from an abnormal glow into a mode showing self-organization of the plasma. Also, the excimer spectrum changes from one with about equal contributions from the first and second continuum to one that is dominated by the second continuum emission. The xenon excimer emission intensity peaks at this discharge mode transition. In the case of argon, self-organization of the plasma was not seen, but the emission of the excimer radiation (128 nm) again shows a maximum at the transition from abnormal to normal glow. As was observed with xenon, the radiant emittance of argon increases with pressure, and the efficiency decreases. The maximum radiant emittance is 1.6 W/cm2 for argon at 600 Torr. The maximum internal efficiency is 2.5% at 200 Torr. The positive slope of the current–voltage characteristics at maximum excimer emission in both cases indicates the possibility of generating intense, large area, flat excimer lamps

Direct Current High-Pressure Glow Discharges

Stabilization and control of a high-pressure glow discharge by means of a microhollow cathode discharge has been demonstrated. The microhollow cathode discharge, which is sustained between two closely spaced electrodes with openings of approximately 100 μm diam, serves as plasma cathode for the high-pressure glow. Small variations in the microhollow cathode discharge voltage generate large variations in the microhollow cathode discharge current and consequently in the glow discharge current. In this mode of operation the electrical characteristic of this system of coupled discharges resembles that of a vacuum triode. Using the microhollow cathode discharge as plasma cathode it was possible to generate stable, direct current discharges in argon up to atmospheric pressure, with estimated electron densities in the range from 1011 to 1012  cm −3. The recently demonstrated parallel operation of these discharges indicates the potential of this technique for the generation of large volume plasmas at high gas pressure through superposition of individual glow discharges

Electron Heating in Atmospheric Pressure Glow Discharges

The application of nanosecond voltage pulses to weakly ionized atmospheric pressure plasmas allows heating the electrons without considerably increasing the gas temperature, provided that the duration of the pulses is less than the critical time for the development of glow-to-arc transitions. The shift in the electron energy distribution towards higher energies causes a temporary increase in the ionization rate, and consequently a strong rise in electron density. This increase in electron density is reflected in an increased decay time of the plasma after the pulse application. Experiments in atmospheric pressure air glow discharges with gas temperatures of approximately 2000 K have been performed to explore the electron heating effect. Measurements of the temporal development of the voltage across the discharge and the optical emission in the visible after applying a 10 ns high voltage pulse to a weakly ionized steady state plasma demonstrated increasing plasma decay times from tens of nanoseconds to microseconds when the pulsed electric field was raised from 10 to 40 kV/cm. Temporally resolved photographs of the discharge have shown that the plasma column expands during this process. The nonlinear electron heating effect can be used to reduce the power consumption in a repetitively operated air plasma considerably compared to a dc plasma operation. Besides allowing power reduction, pulsed electron heating also has the potential to enhance plasma processes, which require elevated electron energies, such as excimer generation for ultraviolet lamps

A Novel Pulsed Corona Discharge Reactor Based on Surface Streamers for NO Conversion from N2-O2 Mixture Gases

A novel pulsed corona discharge reactor is described which utilizes surface-plasma along insulating surfaces. The electrodes are comprised of a stainless steel wire anode of 150 µm diameter stretched along the surface of a glass sheet and two parallel aluminum strips as cathodes. An eight-stage Marx bank, which provides 60 ns, 40-45 kV monopolar pulses, was used to produce the surface streamers in nitrogen-oxygen mixtures at atmospheric pressure. With increasing oxygen content, the energy efficiency for NO2 and O3 synthesis was found to increase. The energy efficiency is almost the same for the surface-plasma and volume-plasma. However, the surface-plasma was found to be significantly more energy efficient than the volume-plasma for conversion of dilute NO in a feed gas containing 0-15% oxygen and with the balance being nitrogen. It is explained on the basis of surface-mediated reactions, the electric wind effect, and the diffusivity of the plasma which covers a larger fraction of the volume of the discharge gap as compared to volume-plasma. The surface-plasma reactor will be used to explore the treatment of NOx and hydrocarbons in diesel engine exhaust

Nanosecond Pulsed Electric Fields: A New Stimulus to Activate Intracellular Signaling

When new technologies are introduced into the sci-entific community, controversy is expected and both ex-citement and disappointment enrich the lives of those who initiate the new ideas. It becomes the mission of the “inventors ” to embrace the burden of proof to estab-lish their ideas and convince the skeptics and disbeliev-ers who will undoubtedly temper their enthusiasm and test their patience. While open mindedness is generally a scientific motto, those who review patents, manuscripts, and grants do not always readily practice it, even when the evidence is convincingly presented; old ideas and concepts often die hard. So it has been and still is in many instances as engineers, physicists, biologists, and physicians pursue innovative ideas and novel technolo-gies. So what is “Bioelectrics”? It is the application of ultra-short pulsed electric fields to biological cells, tissues, and organs. More specifically, it is the analysis of how these bi-ological systems respond to high electric fields (10–100 s of kV/cm) when applied with nanosecond (1–300) dura-tions. Compressing electrical energy by means of pulsed power techniques allows the generation of ultrashort (bil-lionth of a second) electrical pulses [1]. Because the pulses are so short the energy density is quite low and there-fore nonthermal. However, the power is extremely high generating billions of watts. This can be compared to a coal power plant, which generates less than billion watts, but does it continuously. For example, for a 10 ns, 40 kV, 10Ω pulse generator, the power provided by the pulse is 160MW, however, the energy is only 1.6 J. Depositing thi

Generation of Intense Excimer Radiation From High-Pressure Hollow Cathode Discharges

By reducing the diameter of the cathode opening in a hollow cathode discharge geometry to values on the order of 100 μm, we were able to operate these discharges in noble gases in a direct current mode up to atmospheric pressure. High-pressure discharges in xenon were found to be strong sources of excimer radiation. Highest intensities at a wavelength of 172 nm were obtained at a pressure of 400 Torr. At this pressure, the vacuum ultraviolet (VUV) radiant power of a single discharge operating at a forward voltage of 220 V and currents exceeding 2 mA reaches values between 6% and 9% of the input electrical power. The possibility to form arrays of these discharges allows the generation of flat panel VUV lamps with radiant emittances exceeding 50 W/cm2

Emission of Excimer Radiation From Direct Current, High-Pressure Hollow Cathode Discharge

A novel, nonequilibrium, high-pressure, direct current discharge, the microhollow cathode discharge, has been found to be an intense source of xenon and argon excimer radiation peaking at wavelengths of 170 and 130 nm, respectively. In argon discharges with a 100 μm diam hollow cathode, the intensity of the excimer radiation increased by a factor of 5 over the pressure range from 100 to 800 mbar. In xenon discharges, the intensity at 170 nm increased by two orders of magnitude when the pressure was raised from 250 mbar to 1 bar. Sustaining voltages were 200 V for argon and 400 V for xenon discharges, at current levels on the order of mA. The resistive current–voltage characteristics of the microdischarges indicate the possibility to form arrays for direct current, flat panel excimer lamps