Zhongmin Xiong

Atmospheric pressure ionization waves propagating through a flexible high aspect ratio capillary channel and impinging upon a target

Atmospheric pressure ionization waves (IWs) propagating in flexible capillary tubes are a unique way of transporting a plasma and its active species to remote sites for applications such as biomedical procedures, particularly in endoscopic procedures. The propagation mechanisms for such IWs in tubes having aspect ratios of hundreds to thousands are not clear. In this paper, results are discussed from a numerical investigation of the fundamental properties of ionization waves generated by nanosecond voltage pulses inside a 15cm long, 600 µm wide (aspect ratio 250), flexible dielectric channel. The channel, filled with a Ne/Xe = 99.9/0.1 gas mixture at 1atm, empties into a small chamber separated from a target substrate by 1cm. The IWs propagate through the entire length of the channel while maintaining similar strength and magnitude. Upon exiting the channel into the chamber, the IW induces a second streamer discharge at the channel-chamber junction. This streamer then propagates across the chamber and impinges upon the target. The average speeds of the capillary-bounded IW are about 5 × 10 7 cms −1 and 1 × 10 8 cms −1 for positive and negative polarities, respectively. The propagation speed is sensitive to the curvature of the channel. In both cases, the peak in ionization tends to be located along the channel walls and alternates from side-to-side depending on the direction of the local instantaneous electric field and curvature of the channel. The ionization region following the IW extends up to several centimeters inside the channel, as opposed to being highly localized at the ionization front in unconstrained, atmospheric pressure IWs. The maximum speed of the IW in the chamber is about twice that in the channel. (Some figures may appear in colour only in the online journal)

Experimental and modeling analysis of fast ionization wave discharge propagation in a rectangular geometry

Fast ionization wave (FIW), nanosecond pulse discharge propagation in nitrogen and helium in a rectangular geometry channel/waveguide is studied experimentally using calibrated capacitive probe measurements. The repetitive nanosecond pulse discharge in the channel was generated using a custom designed pulsed plasma generator (peak voltage 10–40 kV, pulse duration 30–100 ns, and voltage rise time ∼1 kV/ns), generating a sequence of alternating polarity high-voltage pulses at a pulse repetition rate of 20 Hz. Both negative polarity and positive polarity ionization waves have been studied. Ionization wave speed, as well as time-resolved potential distributions and axial electric field distributions in the propagating discharge are inferred from the capacitive probe data. ICCD images show that at the present conditions the FIW discharge in helium is diffuse and volume-filling, while in nitrogen the discharge propagates along the walls of the channel. FIW discharge propagation has been analyzed numerically usi...

Atmospheric-pressure plasma transfer across dielectric channels and tubes

Atmospheric-pressure plasma transfer refers to producing an ionization wave (IW) in a tube or channel by impingement of a separately produced IW onto its outer surface. In this paper, we report on numerical and experimental investigations of this plasma transfer phenomenon. The two tubes, source and transfer, are perpendicular to each other in ambient air with a 4 mm separation with both tubes being flushed with Ne or a Ne/Xe gas mixture at 1 atmosphere pressure. The primary IW is generated in the source tube by ns to µs pulses of ±25 kV, while the transfer tube is electrodeless, not electrically connected to the first and at a floating potential. The simulations are conducted using a two-dimensional (2D) plasma hydrodynamics model with radiation transport, where the three-dimensional tubes in the experiments are represented by 2D channels. Simulations and experiments show that the primary IW propagates across the inter-tube gap and upon impingement induces two secondary IWs propagating in opposite directions in the transfer tube. Depending on the polarity of the primary IW in the source tube, the secondary IW in the transfer tube can have polarities either the same or opposite to that of the primary IW. The speed and strength of both the primary and secondary IWs depend on the rate of rise of the voltage pulse in the source tube. The modelling results were found to agree well with the behaviour of plasma transfer observed using nanosecond intensified charge-coupled device imaging.

Surface corona-bar discharges for production of pre-ionizing UV light for pulsed high-pressure plasmas

Multi-atmospheric pressure, pulsed electric discharge excited lasers require pre-ionization to produce spatially uniform glows. Many such systems use corona bars to produce ultraviolet (UV) and vacuum ultraviolet (VUV) light as photo-ionization sources for this purpose. Corona bars are transient surface discharges, typically in a cylindrical geometry, that sustain high electron temperatures and so are efficient UV and VUV sources. In this paper, results from a numerical study of surface corona-bar discharges in a multi-atmosphere pressure Ne/Xe gas mixture are discussed. The discharge consists of a high-voltage electrode placed on the surface of a corona bar which is a dielectric tube surrounding a cylindrical metal electrode. After the initial breakdown an ionization front propagates along the circumference of the corona bar and produces a thin plasma sheet near the dielectric surface. The propagation speed of the ionization front ranges from 2 ×10 7 to 3.5 ×10 8 cms −1 , depending on the applied voltage and dielectric constant of the corona-bar insulator. As the discharge propagates around the circumference, the surface of the corona-bar is charged. The combined effects of surface curvature and charge deposition result in a non-monotonic variation of the electric field and electron temperature as the ionization front traverses the circumference. The UV fluxes collected on a surrounding circular surface correlate with the motion of the ionization front but with a time delay due to the relatively long lifetime of the precursor to the emitting species Ne ∗ .

Propagation of negative electrical discharges through 2-dimensional packed bed reactors

Plasma-based pollutant remediation and value-added gas production have recently gained increased attention as possible alternatives to the currently-deployed chemical reactor systems. Electrical discharges in packed bed reactors (PBRs) are of interest, due to their ability to synergistically combine catalytic and plasma chemical processes. In principle, these systems could be tuned to produce specific products, based on their application by combinations of power formats, materials, geometries and working gases. Negative voltage, atmospheric-pressure plasma discharges sustained in humid air in a PBR-like geometry were experimentally characterized using ICCD imaging and simulated in 2-dimensions (2D) to provide insights into possible routes to this tunability. Surface ionization waves (SIWs) and positive restrikes through the lattice of dielectric rods were shown to be the principal means of producing reactive species. The number and intensity of SIWs and restrikes are sensitive functions of the alignment of the lattice of dielectric beads (or rods in 2D) with respect to the applied electric field. Decreased spacing between the dielectric elements leads to an increased electric field enhancement in the gas, and therefore locally higher plasma densities, but does not necessarily impact the types of discharges that occur through the lattice.

Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modelling

Nanosecond electrical discharges in the form of ionization waves are of interest for rapidly ionizing and exciting complex gas mixtures to initiate chemical reactions. Operating with a small discharge tube diameter can significantly increase the specific energy deposition and so enable optimization of the initiation process. Analysis of the uniformity of energy release in small diameter capillary tubes will aid in this optimization. In this paper, results for the experimentally derived characteristics of nanosecond capillary discharges in air at moderate pressure are presented and compared with results from a two-dimensional model. The quartz capillary tube, having inner and outer diameters of 1.5 and 3.4 mm, is about 80 mm long and filled with synthetic dry air at 27 mbar. The capillary tube with two electrodes at the ends is inserted into a break of the central wire of a long coaxial cable. A metal screen around the tube is connected to the cable ground shield. The discharge is driven by a 19 kV 35 ns voltage pulse applied to the powered electrode. The experimental measurements are conducted primarily by using a calibrated capacitive probe and back current shunts. The numerical modelling focuses on the fast ionization wave (FIW) and the plasma properties in the immediate afterglow after the conductive plasma channel has been established between the two electrodes. The FIW produces a highly focused region of electric field on the tube axis that sustains the ionization wave that eventually bridges the electrode gap. Results from the model predict FIW propagation speed and current rise time that agree with the experiment.

Ionization wave propagation on a micro cavity plasma array

The simulation was performed using the computer modeling platform nonPDPSIM, described in detail in Refs. 10–12 and briefly discussed here. Poisson’s equation for the electrostatic potential is self-consistently coupled with driftdiffusion equations for the transport of charged species and the surface charge balance equation. The set of equations is simultaneously integrated in time using an implicit Newton iteration technique. This integration step is followed by an implicit update of the electron temperature by solving the electron energy equation. To capture the non-Maxwellian behavior of the electrons, the electron transport coefficients and rate coefficients are obtained by solving the zerodimensional Boltzmann’s equation for the electron energy distribution. A Monte Carlo simulation is used to track the trajectories of sheath accelerated secondary electrons. The transport of photons is treated by means of a Green’s function propagator. The discharge is sustained in argon at atmospheric pressure. The species in the model are electrons, Ar(3s), Ar(4s), Ar(4p), Ar þ ,A r �, and Ar þ . The photon transport we tracked in the model is dimer radiation from Ar � .I n

Branching and path-deviation of positive streamers resulting from statistical photon transport

The branching and change in direction of propagation (path-deviation) of positive streamers in molecular gases such as air likely require a statistical process which perturbs the head of the streamer and produces an asymmetry in its space charge density. In this paper, the mechanisms for path-deviation and branching of atmospheric pressure positive streamer discharges in dry air are numerically investigated from the viewpoint of statistical photon transport and photoionization. A statistical photon transport model, based on randomly selected emitting angles and mean-free-path for absorption, was developed and embedded into a fluid-based plasma transport model. The hybrid model was applied to simulations of positive streamer coaxial discharges in dry air at atmospheric pressure. The results show that secondary streamers, often spatially isolated, are triggered by the random photoionization and interact with the thin space charge layer (SCL) of the primary streamer. This interaction may be partly responsible for path-deviation and streamer branching. The general process consists of random remote photo-electron production which initiates a back-traveling electron avalanche, collision of this secondary avalanche with the primary streamer and the subsequent perturbation to its SCL. When the SCL is deformed from a symmetric to an asymmetric shape, the streamer can experience an abrupt change in the direction of propagation. If the SCL is sufficiently perturbed and essentially broken, local maxima in the SCL can develop into new streamers, leading to streamer branching. During the propagation of positive streamers, this mechanism can take place repetitively in time and space, thus producing multi-level branching and more than two branches within one level.

Experimental and numerical study of fast gas heating and O atom production in a capillary nanosecond discharge

A nanosecond repetitively pulsed discharge in a quartz capillary filled with flowing synthetic air was investigated as a benchmark to address the mechanism of fast gas heating for conditions of up to complete dissociation of O2 and heating of a few thousand K occurring during the near afterglow phase. Electric current, electric field, gas temperature, energy deposition, and O atom concentrations were measured with respect to time. A 2-dimensional model was used to simulate discharge initiation and early breakdown, while a detailed 0D kinetic model was used to address the afterglow phase and fast gas heating. The high oxygen dissociation degree enables investigation of the key role played by O atoms in the fast gas heating chemistry.

Ionization Wave Splitting at the T-Junction of a Dielectric Channel

Fast ionization waves generated by nanosecond pulses are capable of delivering intense electric field, UV light, and charged and excited species to spatially distant locations. In this paper, the splitting of a fast ionization wave in a N2 plasma in a T-shaped dielectric channel is numerically investigated. Images of electron impact source function and density are presented.