Showing papers by "Kevin J. Bowers published in 1999"

Script and language identification for handwritten document images

Abstract: A system for automatically identifying the script used in a handwritten document image is described. The system was developed using a 496-document dataset representing six scripts, eight languages, and 279 writers. Documents were characterized by the mean, standard deviation, and skew of five connected component features. A linear discriminant analysis was used to classify new documents, and tested using writer-sensitive cross-validation. Classification accuracy averaged 88% across the six scripts. The same method, applied within the Roman subcorpus, discriminated English and German documents with 85% accuracy.

Resonant discharges: initiation and steady state; comparisons with theory, simulation and experiment

Abstract: Summary form only given, as follows Discharges driven at the series resonance frequency have many desirable properties The input resistance is small, and the voltage and current are in phase The voltage drive is small (/spl sim/Te) and the average plasma potential is low (/spl sim/10 Te) Such is observed experimentally and in our PIC-MCC simulations Scaling laws at fixed pressure show peak electron density proportional to the cube of the drive frequency (capacitive discharge is as the square), permitting the density to be controlled Simulation results show at low pressure, the ion energy distribution at the target has a sharp peak at the plasma potential with narrow angular spread about the normal The V-I phase angle versus I curve is measured in simulation and compared with experimental results and the theoretical scaling laws are compared with simulation results and the transition of a capacitively coupled plasma to a resonantly sustained plasma is discussed During this transition or "lock-on" (which occurs in a few RF cycles, a time scale much faster then ion related frequencies), the plasma changes dramatically: the electron kinetic energy and the plasma potential more than doubles; the circuit impedance of the discharge goes from capacitive to resistive; the motion of bulk plasma changes from nearly in phase to nearly out of phase with the voltage drive; and the characteristic heating pattern of these discharges takes shape In these discharges, the formation of high velocity electron bunches in the sheath regions is seen During an RF cycle, these bunches are alternately accelerated from each sheath into the bulk plasma We speculate these bunches provide the ionization in resonantly sustained discharges We also speculate that the lock-on process is similar to the mode-jumping seen in other resonantly and surface wave sustained discharges

Electrostatic and electromagnetic surface wave theories compared

Abstract: Summary form only given, as follows. The theory of resonance oscillations in a non-uniform thermal plasma and the electrostatic surface wave theory is extended. The new theory is fully electromagnetic. The theory is being used to investigate large area plasma sources and sheath phenomena. For a planar plasma loaded waveguide, the theory predicts two classes of waves. Modes with TE (transverse electric) like field components can be found by solving Maxwell's equations with a non-uniform dielectric given by the local cold plasma dielectric. These modes are unaffected by thermal effects in the limit of this theory. Modes with TM (transverse magnetic) like field components are more complicated-electron inertial and thermal effects allow new quasi-electrostatic (QES) modes to propagate below the peak electron plasma frequency similar to Tonk-Dattner resonances. The wave dispersion predicted by the electromagnetic theory differ from the electrostatic theories for long wavelength surface waves though. This discrepancy is explored. The results illuminate limitations of the electrostatic theories when used to predict phenomena in large area plasma sources.

Resonances and surface waves in bounded plasmas

Abstract: Summary form only given. Surface waves provide a promising means of creating large area plasmas. These waves can uniformly distribute the excitation energy and while presenting a small resistance and zero reactance to the driving source. Experimentally and in our simulations, the electron temperature is low (like 1-3 eV) as is the plasma potential (like 10 Te). The use of surface waves experimentally, and now industrially, to sustain large area plasma sources with device size is comparable to free space wavelength have motivated us to refine the theories of Parker et al. (1964) and Cooperberg, 1998) to be fully electromagnetic. The wave dispersion predicted by the electromagnetic theory differs from the predictions of the prior theories and the results illuminate limitations of the electrostatic model. The use of surface waves have also motivated us to explore the mechanisms by which surface waves heat the plasma. In our 1-D electrostatic simulations (which model a resonant discharge experiment performed by Godyak et al. in 1994), high velocity electron bunches are formed in the sheaths and are alternatively accelerated from each sheath into the bulk plasma each RF cycle. We speculate similar mechanisms provide the ionization in surface wave discharges. We also see in these simulations the plasma makes an abrupt transition from capacitively coupled to resistively coupled and the series resonance "locks" onto the drive frequency; these abrupt transitions resemble "mode-jumping" seen experimentally in large area sources. Furthermore, the density profile of the plasma tracks the drive frequency while in the resonant mode giving a new mechanism by which the plasma parameters can be controlled. We are currently investigating the effect the driving electrode shape has on these resonances and conducting 2-D simulations of a large area surface wave source to explore the ignition of surface wave devices and how the plasma fills in the device.

2D electromagnetic PIC-MCC simulation of the initiation of a large area surface wave sustained plasma source

Abstract: Summary form only given, as follows. A 2D electromagnetic PIC-MCC simulation of a large area plasma source is being performed. In this simulation, the plasma is contained in a metal bound cylindrical region. In one version, the plasma is excited by a ring slot on top of the chamber by oscillatory radial electric fields across the slot. The simulation demonstrates the initiation and fill-in process of a slot excited surface wave sustained plasma source. The steady state density is compared with the predictions of a simple model of the diffusion of the plasma from the source. In the diffusion model, plasma undergoes ambipolar diffusion and is only lost at the walls. Also, the role of electromagnetic effects, such as ponderomotive forces, in the initiation and fill-in are explored.