R.H. Davis

Preliminary Investigation of Prebreakdown Phenomena and Chemical Reactions Using a Pulsed High-Voltage Discharge in Water

Prebreakdown phenomena in water were investigated for point-plane geometries using high-voltage pulses. Spot discharges, filamentary magenta streamers, isolated microdischarges, and microbubbles were observed and photographed. Emission spectra were obtained using a prism spectrograph. Maximum streamer lengths were determined as a function of applied voltage, pulsewidth (decay constant), and water conductivity. The bubbling of gas through the underwater discharge resulted in the disintegration of the gas bubbles, and also caused gas-phase discharges to occur near the nozzle electrode. The production of 03, accomplished by bubbling O2 gas through a discharge in deionized water, was investigated using a colorimetric indigo dye test that measured the concentration of 03 in the water. Chemical reactions occurring when 02 or N2 gas was bubbled through a discharge in an anthraquinone dye solution were studied by photometrically measuring the decolorization of the dye.

A Method for the Removal of Sulfur Dioxide from Exhaust Gas Utilizing Pulsed Streamer Corona for Electron Energization

The performance of a new method using pulsed streamer corona for the removal of SO2 from humid air has been evaluated. The pulsed streamer corona produces energetic free electrons that excite, dissociate, and ionize gas molecules, forming radicals that enhance the gas-phase chemical reactions that convert SO2 to acid mist and/or particulate aerosols. The aerosols are then collected by conventional means, i.e., by electrostatic precipitator or bag filter. The SO2 removal efficiency was compared with that of the electron-beam and dc corona flue-gas treatment processes. The comparison demonstrates the advantage of the novel method.

Combined removal of SO/sub 2/, NO/sub x/, and fly ash from simulated flue gas using pulsed streamer corona

A pulse-energized electron reactor utilizing pulsed streamer corona has been developed for the combined removal of SO/sub 2/, NO/sub x/, and particles from effluent gases. In the pulse-energized electron reactor process, fast-rising narrow high voltage pulses are superimposed on a DC bias voltage and applied to a nonuniform electric field geometry to generate pulsed streamer corona. The pulsed streamer corona produces energetic free electrons, which dissociate gas molecules, forming radicals. These radicals cause chemical reactions that convert SO/sub 2/ and NO/sub x/ into acid mists and/or solids, which can be removed from the gas stream by conventional means. In pulse-energized electron reactor performance tests on a humid air stream with an initial SO/sub 2/ concentration of 1000 p.p.m., more than 90% of the SO/sub 2/ was removed with an advantageously small power requirement. >

Aerosol particle charging by free electrons

Abstract The strong dependence of electrostatic precipitator efficiency on particle charge has initiated a number of investigations of charge enhancement techniques. Measurements of free electron charging of aerosol particles reported here for nitrogen and argon confirm the earlier evidence (DuBard et al., J. Aerosol Sci. 14 , 5, 1983) that free electron charging in nitrogen produced a substantially higher particle charge than charging by ions. The measured particle charge was 2–10 times larger for free electron charging than for ionic charging depending upon the experimental conditions. The dependence of free electron charging upon the gas, electric field, particle radius, and the charging parameter N 0 t has been measured. Previously available charging theories do not predict the observed dependence of free electron charging on the various parameters. The importance of electric-field-enhanced electron temperatures is recognized for diffusion charging, and a new high electron temperature continuum-regime field-diffusion charging model is presented which utilizes the Boltzmann relation as an approximate solution for the electron densities near the particle. The predictions of this model are dependent upon the boundary conditions utilized at the particle surface, but are in satisfactory agreement with the data (considering the possible sensitivity of electron charging to gas impurities).

Use of an electron beam for particle charging

The charging performance of an electron beam precharger has been evaluated by measuring the charge acquired by particles with diameters of 1.1 and 3.0 mu m. In the experiment, the beam energy is smaller than the electrode geometry. This avoids spurious ionization by scattered electrons and allows the establishment of a monopolar charging region (by separating the bipolar ions with an electric field). Particle charges greater than the theoretical ionic charging value were observed using the bi-electrode electron beam precharger. The increased charge may be caused by free-electron charging. >

Microampere negative helium ions using potassium vapour exchange

Abstract Modification of the ion source of a model EN tandem Van de Graaff accelerator to utilise potassium vapour charge exchange has increased the negative helium ion currents to 2.0μA. Alpha particle beam currents used in experiments depend on the transmission through the machine and vary from 0.3 to 0.8 μA. The modified source has been used with potassium vapour exchange for continuous periods of up to three weeks. No upper time limit has been established. time limit has been established.

Stable Secondary Ionization in a Test Geometry for Electron Beam Precipitators

Stable secondary ionization which produces an ion current density as much as a factor of four higher than that produced by primary ionization alone has been observed in a test electron beam precipitator geometry. Full utilization of secondary ionization in precipitator design considerations critically depends upon the stability of the secondary ionization processes which are ordinarily the immediate precursor phenomena to breakdown. Measurements are reviewed in which the ion current saturation is followed by the onset of secondary ionization as the electric field is increased. Secondary ionization currents are large and override saturation effects at high electron beam currents or large electrode spacings. Recent measurements of primary and secondary ionization, ion current saturation, and secondary ionization stability are presented. High electron beam currents are used to produce large ion current densities, and the resistance of the ionized gas is determined. The effects of the electron beam on the stability of secondary ionization and on the onset electric field for secondary ionization are discussed.

Gas target cell for a precision scattering chamber

Abstract A gas target cell with demountable cylindrical windows has been developed for use with the Florida State University 45.7 cm (18″) scattering chamber. Designed for maximum experimental flexibility, this cell allows observation throughout an angular range from 20° to 170°. Several interchangeable gas handling plugs, one of which incorporates a Ti getter pump within the cell interior, are described. A 16 detector array permits the simultaneous acquisition of 16 excitation functions or the efficient measurement of detailed angular distributions by angular displacement of the detector ring. This cell has been used at a gas pressure of 35 Torr for measurements of cross sections for elastic scattering of alpha particles by 20 Ne and 36 Ar in the energy range from 10 to 14 MeV.

Modification of an Ion Source for the Production of Microampere 4He- Beams

Several modifications of the ion source of a model EN Tandem Van de Graaff Accelerator have increased the production of 4He- beams by a factor of 100 over that first obtained with the source (~10 nA). While using H2 gas for charge exchange, the replacement of the source drift tube by a screen and the installation of a system for precise adjustment of the button alignment with respect to the extractor electrode increased the output to 150 nA at the ion source. Recently potassium vapor has been used for charge exchange. Since the introduction of this technique, 4He- beam currents of 1. 2 ?A have been regularly produced. The 4He+ ions from a duoplasmatron ion source are accelerated to 20 keV and passed through the standard exchange canal which is modified by the addition of a potassium vapor chamber. Analyzed 4He++ beams of 0.3 to 0.8 ?A have been obtained with terminal voltage between 1.8 and 6 MV. The modified source has been used with K-vapor exchange for continuous periods of up to 120 hours without deterioration of source performance. Normal operation with H2 gas exchange for other beams is possible without the interruption of the source operation.

Ion current densities produced by energetic electrons in electrostatic precipitator geometries

A new laboratory test system for electron beam ionization in electrostatic precipitator geometries has been constructed to measure ion current density as a function of voltage difference for clean plate (no dust contamination) conditions. The new system incorporates improved electrodes which withstand an applied voltage of ± 55 kV, a factor of 5 increase over the previous test system. A 3 MeV Van de Graaff accelerator produced ionizing electron beams of 1.2 and 2 MeV energy with beam currents of 10.5 and 21 μA in place of corona wire ionization. Ion current densities of up to 130 mA/m2 were measured before breakdown between the plates, and no ion current saturation was observed. A comparison of I-V curves and sparkover voltages for various beam energies, currents, and types of collimation is discussed and the need for measurements with good beam geometry is addressed.