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Abstract
A stream of electrons has been focused solely by the fields of a slow (0pc) electromagnetic wave
An analysis was obtained by transforming to the wave frame in which the equations of motion of electrostatic periodic focusing are valid. Using the well-known results for the static case, and retransforming to the laboratory frame, focusing behaviour and design information (circuit size, frequency, power for a given stream) wore obtained. A second but more appropriate analysis was made by transforming to the average electron frame in order to predict both fast and slow-wave results; for the axially symmetric modes considered, the slow-wave focusing requires orders of magnitude less power than for fast waves and has a more favourable field shape
Several experiments were performed using a helix as the slow waveguide. The stream was launched into the helix and was collected both on the helix and by a collector at the opposite end. Typically 10 to 50% of the current would be transmitted with no wave, rising linearly with wave power to a best transmission of over 80%. The best results were obtained with the wave velocity in the same direction and greater than the average velocity of the stream; use of oppositely directed wave and stream velocities gave poorest results. At best, a I vv stream (roughly I mA at 1000 v) was focused by about 10 w of r.f. wave power at 1000 Mc. These results are in rough agreement with the theory. For a given injected stream there appears to be a maximum r.f. power beyond which focusing decreases; this effect may be due to either a strong localized lens effect at the point of stream injection or to r.f. voltages largo compared with the average steam voltage.
Notes
†Communicated by Dr. R. Kompfner. This research was supported in part by the Air Force Office of Scientific Research, under grant No. AFOSR-117-63