Some Measurements of Steady and Fluctuating Plasma Number Densities

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Abstract
Observations of dc and ac components of electron number density have been made along the positive columns of hot-cathode, mercury-vapor discharges, at a pressure of about 1 μ Hg. Direct measurements were made by a microwave method involving a dipole resonance of the column, while indirect measurements were made from the accompanying light output and, in the case of fluctuating components, by capacitive pickups sliding on the outside of the tube. The frequency spectra of number density, light output, and potential fluctuations generated by the tube itself were found to be similar, and their variations in amplitude along the column were found to be closely related. For injected ac signals of variable frequency, phase and amplitude variations in the number density are shown to agree closely with the theoretical predictions of Crawford. Unsuccessful attempts were made to excite low-frequency fluctuations by injecting electrons into the positive column from an auxiliary cathode.

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