Noise and oscillations in hot cathode arc |
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| Authors: | JD Cobine CJ Gallagher |
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| Institution: | Radio Research Laboratory, Harvard University, USA |
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| Abstract: | Gas discharges generate oscillations superimposed on random noise of several megacycles bandwidth.Positive ions oscillate in two distinct regions of the discharge, i.e., the plasma and the potential minimum at the cathode.The disturbances produced by the ion motions appear as voltage variations between the electrodes.Oscillations generated in the plasma are usually below 400 kc./sec., while “cathode” oscillations are about 700 kc./sec.As the discharge current is increased, the amplitude of the plasma oscillation increases while its frequency decreases. The random noise also depends on current, increasing rapidly as the Townsend discharge changes into an are. After arc conditions are fully established, the noise changes slightly with current. Probe studies show that the noise voltage between probe and cathode increases rapidly as space charge conditions change to permit acceleration of electrons to the probe. When the probe collects positive ions, the noise as viewed on an oscilloscope is coarse, i.e., lacking in high frequency components.An explanation of the noise generation may be found in the random fluctuations of space charge at the electrodes. Calculations based on this theory give voltage fluctuations of the observed order of magnitude. |
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