Mark A. Cappelli

An Investigation of Diamond Film Deposition in a Premixed Oxyacetylene Flame

Diamond film has been deposited in a single‐nozzle premixed oxyacetylene flame. Results of runs of varying duration suggest that diamond is deposited via the transport of hydrocarbon fragments produced at the secondary flame front. Planar laser‐induced photodissociation fluorescence suggests that this region is rich in C2H species. Emission studies also suggest that the post‐primary flame zone presents a source of C2 radicals that may account for the observed graphite and diamondlike carbon deposited on the substrate exposed to this region of the flame. The results on the premixed flame suggest that it would be possible and more convenient to attempt large area deposition using a multinozzle diffusion flame.

Plasma-Discharge Stabilization of Jet Diffusion Flames

The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge is found to be higher than that for the DBD. Furthermore, this pulsed discharge causes the most significant improvement in the flame stability. The optimal placement of the discharge electrodes is investigated, and it is found that there is a close relation between this placement and the emission spectra, suggesting use of the emission spectra as a possible indicator of fuel/air mixture fraction. The optimal placement is mapped into mixture-fraction space by use of a fully premixed flame experiment of known mixture fraction. The result shows that the mixture fraction, which corresponds to the optimal placement, is much leaner than that of a conventional lifted jet flame

A 3‐dimensional model for low‐pressure chemical‐vapor‐deposition step coverage in trenches and circular vias

A general analytical model has been developed to calculate particle transport and spatial step coverage evolution within 2‐dimensional and 3‐dimensional microelectronic device structures during low‐pressure chemical vapor deposition. The model can account for spatially dependent nonunity reactive ‘‘sticking probabilities,’’ anisotropic source fluxes, and trench ‘‘shadowing’’ effects. There is no restriction on the initial and evolving shape of the structure. Model results are compared to direct Monte Carlo simulations for step coverage on rectangular trenches, and are found to more accurately describe the observed experimental step coverages during phosphorous‐doped silicon dioxide glass film deposition. We present here, for the first time, detailed calculations of step coverage in circular vias for a wide range of reactive sticking probabilities.

Laser-induced fluorescence diagnostic for temperature and velocity measurements in a hydrogen arcjet plume

A diagnostic has been developed to measure velocity and translational temperature in the plume of a 1-kW-class arcjet thruster operating on hydrogen. Laser-induced fluorescence with a narrow-band cw laser is used to probe the Balmer α transition of excited atomic hydrogen. The velocity is determined from the Doppler shift of the fluorescence excitation spectrum, whereas the temperature is inferred from the lineshape. Analysis shows that although Doppler broadening is the only significant broadening mechanism, the fine structure of the transition must be taken into account. Near the exit plane, axial velocities vary from 4 to 14 km/s, radial velocities vary from 0 to 4 km/s, and swirl velocities are shown to be relatively small. Temperatures from 1000 to 5000 K indicate high dissociation fractions.

A characterization of plasma fluctuations within a Hall discharge

Experimental results are presented for studies of low (2-20 kHz) and intermediate-frequency (20-100 kHz) oscillations in crossed-field closed electron-drift Hall discharges. Conditional sampling using two electrostatic probes is used to identify and extract properties of coherent structures associated with the propagation of azimuthal and longitudinal instabilities within the discharge channel. The azimuthal component phase velocities are determined for a wide range of wave frequencies and over characteristic regimes of operation of these devices. A variety of propagation modes are observed and analyzed, including the appearance of an induced mode due to the presence of the probes themselves. This later result is believed to be the first direct evidence of how fluctuations can be influenced in these Hall discharges using relatively simple actuation methods.

On the role of oxygen in dielectric barrier discharge actuation of aerodynamic flows

Phase-locked particle image velocimetry is used to study the mechanism of induced flow in the near field of a rf dielectric barrier discharge actuator mounted in the separated flow region of a bluff body. Flow actuation is found to be asymmetric, with suction toward the buried downstream electrode when it is biased positively relative to the upstream exposed electrode. Lesser flow is seen on the reverse voltage swing, where the buried electrode should attract positive ions. This phenomenon is enhanced when oxygen is added to the flow, suggesting that oxygen negative ions, possibly O2−, play a dominant role in plasma actuation.

Comparison of hybrid Hall thruster model to experimental measurements

** A two -dimensional hybrid particle -in -cell numerical model has been constructed in the radial -axial plane with the intent of examining the physics governing Hall thruster operation. The electrons are treated as a magnetized quasi -one -dimensi onal fluid and the ions are treated as collisionless, nonmagnetized discrete particles. The anomalously high electron conductivity experimentally observed in Hall thrusters is accounted for using experimental measurements in the Stanford Hall thruster. An evaluation is made of differing treatments of electron mobility, background gas, neutral wall interactions, and charge exchange collisions. The results are compared to experimental measurements of ion and neutral number densities and velocities, electron t emperature, and electric potential.

A low-power, linear-geometry Hall plasma source with an open electron-drift

The operating characteristics of a linear geometry Hall plasma source scaled to operate in the 50 to 100 Watt power range are described. Two thruster acceleration channels are implemented-one of alumina and one of boron nitride. Differences in operation with the two channel materials are attributable to differences in the secondary electron emission properties. In either case, however, operation is achieved despite the lack of a closed electron current drift in the Hall direction, suggesting that there is an anomalous axial electron mobility, due to either plasma fluctuations or collisions with the channel wall. Strong low frequency oscillations in the discharge current, associated with the depletion of propellant within the discharge, are seen to appear and vary with changes in the applied magnetic field strength. The frequency of this oscillatory mode is higher than that seen in larger (and higher power) discharges, due to the decreased residence time of the propellant within the channel. Linear geometry Hall thrusters permit simpler magnetic circuit configurations and enable stacking of multiple thrusters to provide modular arrays.

Dielectric barrier discharge control of a turbulent boundary layer in a supersonic flow

We demonstrate effective manipulation of a turbulent boundary layer at Mach 4.7 conditions using a surface dielectric barrier discharge (DBD) actuator. The freestream conditions of low static pressure (1 kPa) and temperature (60 K) are conducive to the visualization of flow features using Rayleigh scattering from condensed CO2 particles. The boundary layer thinning is observed when spanwise momentum is induced by the low power (6.8 W), low frequency (28 kHz) single actuator pair oriented parallel to the freestream flow.

Jet Flame Ignition in a Supersonic Crossflow Using a Pulsed Nonequilibrium Plasma Discharge

A short-pulse repetitive discharge is used to ignite hydrogen jet flames in supersonic crossflows. Nonequilibrium plasma is produced by repetitive pulses of 7-kV peak voltage, 20-ns pulsewidth, and 50-kHz repetition rate. Sonic or subsonic hydrogen jets are injected into a pure-oxygen supersonic free-stream flow of Mach numbers M = 1.7-2.3. The fuel injection nozzles and electrodes are mounted flush with the surface of a flat plate that is oriented to be parallel to the flow to minimize stagnation pressure losses associated with generated shock waves. A configuration combining an upstream subsonic oblique jet and a downstream sonic transverse jet serves to provide an adequate flow condition for jet flame ignition. The flow pattern and shock waves induced by the dual hydrogen jets are characterized by Schlieren imaging. Planar-laser-induced fluorescence and emission spectroscopy are employed for imaging the distribution of OH radicals. The OH fluorescence image of the region in the vicinity of the discharge confirms jet flame ignition by the plasma.