R.E. Pechacek

Production of large-area plasmas by electron beams

An analysis is presented for the production of weakly ionized plasmas by electron beams, with an emphasis on the production of broad, planar plasmas capable of reflecting X-band microwaves. Considered first in the analysis is the ability of weakly ionized plasmas to absorb, emit and reflect electromagnetic radiation. Following that is a determination of the electron beam parameters needed to produce plasmas, based on considerations of beam ionization, range, and stability. The results of the analysis are then compared with a series of experiments performed using a sheet electron beam to produce plasmas up to 0.6 m square by 2 cm thick. The electron beam in the experiments was generated using a long hollow-cathode discharge operating in an enhanced-glow mode. That mode has only recently been recognized, and a brief analysis of it is given for completeness. The conclusion of the study is that electron beams can produce large-area, planar plasmas with high efficiency, minimal gas heating, low electron temper...

Microwave emission from plasmas produced by magnetically confined-electron beams

Microwave emission, in the x-band frequency range (8.2-12.4 GHz), from a thin, large, rectangular sheet plasma has been measured. The plasma electron density was such that the plasma frequency was within or just above this frequency range. The plasma was immersed in an external magnetic field from a set of Helmholz coils. The magnetic field was oriented parallel to the electric field between the anode ground plane and a cylindrical, hollow cathode. The spectrum of the emitted noise was measured for both ordinary mode (P to B) and extraordinary mode (/spl perp/ to B) polarization in the x-band. The emission was strongest at high harmonics of the electron cyclotron frequency. Mechanisms that might produce this noise and its potential use as a diagnostic tool are discussed.

Plasma study of a moly-oxide-argon discharge bulb

A mercury-free molybdenum-oxide-argon (Mo-O-Ar) electrodeless discharge is described with potential application to plasma lighting. The low-pressure metallic plasma is a nonequilibrium discharge capable of producing visible light directly with an efficacy of /spl sim/40 lm/W. The Boltzmann equation is solved with a limited set of chemical kinetics to provide a zero-dimensional model of the discharge. Model results indicate a transition in the power transfer from Ar to Mo as the partial pressure of Mo is increased. This feature is qualitatively similar to an intense transition from weak to strong visible emission observed in the Mo-O-Ar discharge with increasing power. The Mo partial pressure at the transition is estimated from an actinometry approach including a collisional radiative model of the plasma. The calculated electron density is also compared with interferometric data.

Large-Area Plasma Processing System

Summary form only given. The Naval Research Laboratory has developed a new type of plasma processing reactor called the Large-Area Plasma Processing System (LAPPS). This device uses a magnetically confined, sheet electron beam to produce planar plasmas with densities up to 5/spl times/10/sup 12/ cm/sup -3/, area /spl sim/1 m/sup 2/ and thickness /spl sim/1 cm. Other LAPPS attributes include: high uniformity; operation over a wide range of gas type and pressure; independent control of the ionization rate; efficient production of ions and free radicals; low metastable density; modest gas heating; a low and partially adjustable electron temperature; and independent bias control. Disadvantages of LAPPS include: the need for an energetic electron beam (a few keV and 10s mA/cm/sup 2/); the need for a longitudinal magnetic field (/spl sim/200 G) to confine the beam; cross-field restrictions on the plasma flow; beam-plasma instabilities; and energy losses to the beam dump. This talk will present the theory underlying LAPPS, while the experimental results and future plans will be presented elsewhere.

Optical emissions from a planar plasma discharge

A planar sheet of plasma is generated by propagating a kilovolt beam of electrons produced by a hollow cathode along a magnetic field. The optical emissions from the plasma illustrate the different regions of a glow discharge.

REB propagation experiments in neutral gas

Summary form only given. Experiments have been performed on the SuperIBEX generator (5 MV, 50-100 kA, 40 ns FWHM (full width at half maximum) pulse) at the US Naval Research Laboratory on quieting the initial radial motion of a relativistic electron beam (REB) and controlling the time-dependent parameters of the beam in order to optimize propagation of the beam in full-density air. An effort was made to modify the beam parameters and their effect on beam propagation in full-density air

UV and visible emission processes in a moly-oxide discharge

Summary form only given, as follows. Due to the hazardous material designation of spent fluorescent lamps on board naval vessels, the Naval Research Laboratory has been investigating alternative lighting concepts which are free of mercury. A moly-oxide discharge driven by an inductively coupled RF coil at 13 MHz emits a broad spectrum in the visible range with strong line emission from the quintet spin levels of the Mo atom near the peak of the photopic curve. One limitation of such a discharge for direct white light applications is the near UV emission from the Mo septet resonance lines between 300 and 400 nm. Analysis of the excitation processes and channels for both the visible and UV emission will be presented based on a Boltzmann model for the electron energy distribution function and detailed atomic physics calculations for the cross sections. The results indicate that a Mo partial pressure of -10 mTorr or more will render the resonance lines optically thick and concurrently increase the visible emission. Calibrated spectroscopic measurements throughout the UV and visible region will be reported as a function of power and pressure and compared, through actinometry, with the theoretical simulations.

Plasma conditions in a moly-oxide electrodeless bulb discharge

A mercury-free, molybdenum-oxide, electrodeless discharge is describe with potential application to lighting. A spectrum is presented which displays emission in the near UV and throughout the visible region. Plasma diagnostics using spectroscopy and interferometry are presented to obtain the electron temperature and density. The Mo density is estimated from an actinometry approach including a collisional radiative model of the plasma. The low pressure metallic plasma is a non-equilibrium discharge with /spl sim/40 lumens per watt.

Production of a sheet plasma distribution for reflection of high frequency microwaves

Summary form only given. The Agile Mirror experiment at the Naval Research Laboratory has produced a planar 60/spl times/60 cm plasma mirror capable of reflecting 10 GHz microwaves. The plasma distribution is produced in a low pressure chamber in a 200 Gauss magnetic field. The mirror turns on or off in <10 /spl mu/sec and has been maintained for up to 10 msec. A microwave beam reflected 90 degrees has a very similar far field radiation pattern as one reflected by a metal plate and is comparable to the radiation pattern produced by the illuminating dish alone. The plasma distribution is initiated with a 3-5 kV voltage pulse which drives an abnormal glow discharge between a linear hollow cathode and a planar anode. The high density plasma in the negative glow region of the discharge is maintained by the high energy electrons emitted from the cathode. A detailed picture of the understanding of how the mirror is formed and its evolution is presented. Initial efforts at making the mirror agile by designating different cathode orientations and by modifying the magnetic field geometry are described.

Neutral gas filled transport cells for high current electron beams

Summary form only given. A series of experiments is being conducted with a passive electron beam transport and centering system consisting of a conducting tube filled with neutral gas. The object is to study the transport and centering properties of the cell as a function of input beam current, transport cell length, cell diameter, fill pressure, and fill gas type. The electron beam is generated by the US Naval Research Laboratorys SuperIBEX accelerator, which produces a 5.0-MeV, 50-kA, 40-ns FWHM (full width at half maximum) electron beam. The electron beam diode consists of a 6-mm-diameter cold cathode and a 0.5-mil-thick titanium foil anode. The injected beams are <2.0 cm in diameter and have low divergence (<50 mrad). The experiments cover a wide range of parameters. The beam current is varied from 10 to 40 kA, the pressure from 0 to 100 mtorr, the transport length from 40 to 120 cm, and the cell diameter from 5 to 10 cm, and several gases (Ar, He) are used to examine the effects of ionization potential. The results of the experiments have been compared to computer simulations of SuperIBEX beams in the gas-filled transport cells