Exo-electrons are released from the abraded cathode surface of a positive point-to-plane gap in room air, when stimulated by visible light from an irradiating incandescent lamp. The emitted electrons become attached to electronegative gas components in air and form negative ions which drift towards the positive point. Near the point attachment of electrons initiates the burst pulses. Thus exo-electron emission causes indirectly the onset of a corona. The effective negative ion transit time, the burst pulse delay time and the gap voltage has been observed and calculated as a function of the electrode separation. Oscillograms of the burst pulses indicate their frequency rises with the irradiation intensities; changes of the latter effect the I-V characteristics. If extraneous irradiation providing initial electrons is excluded, the exo-electron effect has a marked influence on the initiation of a positive corona in atmospheric air.
The electrode system used for determination of negative ion mobilities consists of three electrodes: two parallel plane electrodes (C1 and C2) and a point electrode (A) perpendicular to C1 and C2. The central part of C1 is a wire mesh. The gap between C1 and C2 is an ion drift space and, that between C1 and A is used as a negative ion detector (detecting gap). C1 is earthed, C2 is kept at negative and A at positive potential. The potential across the detecting gap is kept at that corresponding to its Geiger counter region. When pulsed UV irradiation is directed at the mesh, perpendicular to it, photoelectrons are released simultaneously from the mesh and C2 and cause two burst pulses to appear in the detecting gap. As the time interval between the two pulses is equal to the time of flight of negative ions in the drift space, their mobilities can easily be determined. Advantages of this technique are: the determination can be done easily and quickly, and the device is of very simple construction and may be used even for high pressures. It is not capable of the determination of positive ion mobilities. Results obtained for negative ion mobilities for zero field at atmospheric pressure for dry air, humid air, O2 and SF6 are: 2.25, 1.9, 2.3 and 0.57 cm2 V-1 s-1 respectively.
Photoelectrons are emitted from the quartz surface of an ultraviolet lamp under atmospheric air conditions, and the lamp behaves like an electron emitter. The lamp used in the present experiment is 4 W and has light emission peaks at 185 nm and 254 nm. With this lamp, the emission current of the order of 10-9 A is produced in atmospheric air. This photoemission phenomenon is of great importance to charge elimination of charged matter.
When the potential across a positive-point-plane gap in air is increased, discharges proceed in order as follows: intermittent streamer corona, glow corona and spark. The streamer is markedly enhanced by adding a small amount of an electronegative gas e.g. Cl2, NO2 NO etc. This is because the streamer can be initiated at a higher potential than is corona onset potential due to extinction of the glow corona. This effect becomes more marked with increased concentration of the additive. For a gap system mounted in an enclosed chamber (air), the streamer formation is enhanced by successive discharges and a pronounced fall of the sparking potential is expected to occur. This is due to NO2 formation resulting from successive discharges.
With a positive-point-plane gap having an anode radius of curvature of 0.5 mm at atmospheric pressure, when the voltage across the gap is gradually raised, a streamer corona is triggered by pulsed uv irradiation. In these conditions, two types of triggering are clearly found to exist. For non-electronegative gases, the streamer corona is triggered directly by photo-electrons released from the cathode and has a short delay time (type 1). Whereas for the electronegative gases such as O2 and SF6, it is triggered by electrons detached from negative ions arriving at the anode and has a long delay time (type 2). For air, the triggering of the streamer is type 1 for the gap-length range d < 10 mm and type 2 for d > 11 mm. The critical gap length at which the transition occurs from type 1 to type 2 depends on the anode radius.
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