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Investigations have been made of plasma oscillations by studying space-charge waves in velocitymodulated electron beams. The convection current in a velocity-modulated slowly-drifting electron beam was measured as a function of distance from the modulating cavity. The experiments indicate that in the presence of an appreciable velocity spread, the fast space-charge wave is damped while the slow space-charge wave remains unattenuated. The measurements were made under conditions where the velocity spread is dominated by lens effects in the electron-optical system. The results confirm qualitatively the theory of Berghammer and thus demonstrate the existence of the much disputed Landau damping.

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