Direct density display with a resonance cone rf probe

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Abstract
An rf technique is described for a direct oscilloscope display of the electron density in a magnetized plasma. The transmission between a point rf probe and a ring rf probe exhibits a maximum when the ring intersects the resonance cone of the point source. The ring is in the plane perpendicular to the magnetic field and the point is coaxial with the ring. For a given probe geometry and magnetic field, the frequency of the transmission peak is a simple function of the plasma density. A swept frequency display of the transmission curve on an oscilloscope gives an instantaneous, direct measure of the plasma density.