Hybrid modelling of low temperature plasmas for fundamental investigations and equipment design

The modelling of low temperature plasmas for fundamental investigations and equipment design is challenged by conflicting goals having detailed, specialized algorithms which address sometimes subtle physical phenomena while also being flexible enough to address a wide range of process conditions. Hybrid modelling (HM) is a technique which provides many opportunities to address both fundamental physics and practical matters of equipment design. HM is a hierarchical approach in which modules addressing different physical processes on vastly disparate timescales are iteratively combined using time-slicing techniques. By compartmentalizing the physics in each module to accept given inputs and produce required outputs, different algorithms can be used to represent the same physical processes. In this manner, the algorithms best suited for the conditions of interest can be used without affecting other modules. In this paper, the basis and implementation of HM are discussed using examples from simulations of inductively coupled plasmas.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65081/2/d9_19_194013.pd

Modeling of surface flashover on spacecraft

A model for predicting the onset of surface flashover discharges (SFDs) in the context of high voltage pulse power modulators was developed and used to investigate mechanisms leading to the onset of SFDs. We demonstrated that it is possible to analyze surface discharges in a manner similar to gas phase discharges using transport coefficients such as the first Townsend coefficient. Our parameterization of various methods to prevent, or at least delay, the onset of SFDs was not particularly successful in that many of the strategies that we investigated do not yield significantly improved performance. The only safe strategy to reduce the occurrence of SFDs is to prevent the dielectric from being charged in the first place. This leads one to consider passive or active schemes which employ the low pressure of attaching gases which flood the surface prior or coincident to pulsing the high voltage apparatus. Our calculations indicate that only small amounts gas (10s Torr effective pressure at substrate) would be sufficient for many of the anticipated applications. If the surface is flooded only when high voltage is applied across the dielectric, the gas consumption would be nominal

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 15 cm long, 600 µm wide (aspect ratio 250), flexible dielectric channel. The channel, filled with a Ne/Xe = 99.9/0.1 gas mixture at 1 atm, empties into a small chamber separated from a target substrate by 1 cm. 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 cm s −1 and 1 × 10 8 cm s −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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98612/1/0963-0252_21_3_034001.pd

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 × 107 to 3.5 × 108 cm s−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 {\rm Ne}_2^\ast .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85395/1/d10_50_505204.pd

Photo-triggering and secondary electron produced ionization in electric discharge ArF* excimer lasers

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98721/1/JApplPhys_110_083304.pd

Mechanisms for sealing of porous low-k SiOCH by combined He and NH[sub 3] plasma treatment

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98727/1/JVA051305.pd

Modeling of implantation and mixing damage during etching of SiO[sub 2] over Si in fluorocarbon plasmas

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98728/1/JVA051306.pd

Ion energy and angular distributions onto polymer surfaces delivered by dielectric barrier discharge filaments in air: I. Flat surfaces

In atmospheric pressure discharges, ion energies are typically thought to be thermal with values dominantly <1 eV. In the heads of filaments in dielectric barrier discharges (DBDs), electric fields can exceed 200 kV cm _1 when the filament is far from a surface. As the filament approaches and intersects a dielectric surface, much of the applied potential is compressed into the voltage drop across the head of the filament due to the high conductivity of the trailing plasma channel. When the filament strikes the surface, this voltage is transferred to the resulting sheath and into the material of the surface. The degree of electric field compression depends on the dielectric constant _/_ 0 of the surface. Upon intersection of the filament with the surface, the electric fields in the resulting sheath can exceed 400–800 kV cm _1 , with larger values corresponding to larger _/_ 0 . When accelerated in these fields, ions can gain energies across their mean free path (0.5–1 µm) up to 20 eV for dielectrics with low _/_ 0 and up to 150 eV for dielectrics with high _/_ 0 , albeit only for the duration of the intersection of the streamer with the surface of a few ns. In this paper we report on results from a computational investigation of the ion energy and angular distributions (IEADs) incident on dielectric flat surfaces resulting from the intersection of DBD filaments sustained in atmospheric pressure air. We describe the transient and spatially dependent IEADs as the filament spreads across the polymer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90807/1/0963-0252_20_3_035017.pd

Impact of Electron Collision Mixing on the delay times of an electron beam excited Atomic Xenon laser

The atomic xenon (5d¿6p) infrared laser has been experimentally and theoretically investigated using a short-pulse (30-ns), high-power (1-10-MW/cm3) coaxial electron beam excitation source. In most cases, laser oscillation is not observed during the e-beam current pulse. Laser pulses of hundreds of nanoseconds duration are subsequently obtained, however, with oscillation beginning 60-800 ns after the current pulse terminates. Results from a computer model for the xenon laser reproduce the experimental values and show that oscillation begins when the fractional electron density decays below a critical value of &ap;0.2-0.8×10 6. These results lend credence to the proposal that electron collision mixing of the laser levels limits the maximum value of specific power deposition that can be used to excite the atomic xenon laser efficiently on a quasi-CW basi

Ion energy and angular distributions onto polymer surfaces delivered by dielectric barrier discharge filaments in air: II. Particles

Atmospheric pressure streamers intersecting particles are of interest in the context of plasma aided combustion, where the particle may be a fuel aerosol droplet, or in sterilization of air, where the particle may be a bacterium. The ion energy and angular distributions (IEADs) incident on the particles, small curved dielectric surfaces, then in part determine the propensity for activating chemical reactions or, in the case of bacteria, the plasma's sterilization capability. In this paper, we discuss results from a computational investigation of IEADs on small particles (45 µm radius) produced by atmospheric pressure discharge. Streamers intersecting a particle momentarily generate a large sheath potential as the streamer passes by as the particle charges towards the plasma floating potential. During that time, ions of energies up to 3–10 eV can strike the particle. The permittivity of the particle and the streamer polarity in part determine the character of the IEAD.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90808/1/0963-0252_20_3_035018.pd