The depth-dose curves of a high-intensity relativistic pulsed electron beam at various distances from a tube face in helium of various pressures were determined by using a stack of aluminum absorbers and blue cellophane sheets. The curves indicate that the beam is self-focused at two pressure regions, i.e., near 10 Torr and above 600 Torr, and that the beam is defocused considerably between these two pinches. The extrapolated ranges decreased rapidly with distance from the tube face at 0.01, 50 and 655 Torr but not at 10 Torr. This decrease is explained in terms of some beam-plasma interactions-especially, in terms of an induced backward electric field and weak focusing of lower-energy electrons.

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