Radial and axial RF current and velocity distributions in large-signal velocity-modulated electron beams

Measurements of the amplitude and the phase of the current density in a Brillouin beam modulated at large signal levels were made at 1940 MHz and at harmonics of 1940 MHz as a function of the amplitude of the modulating signal. In addition to a surface current similar to that reported for small-signal Brillouin beams, current existed along the axis of the beam that was very large and often 180° out of phase with the surface current. Thus, at some axial positions, a cavity surrounding the beam would detect practically none of the current in the beam. At some drive levels above the point where electron overtaking occurred, it is shown that the RF current in the beam disappeared completely in both the radial and axial directions after the beam drifted approximately one plasma wavelength from the modulating cavity. Explanations based on measurements of the velocity distribution of the current are given for the unusual behavior of the current in the Brillouin beam. The distribution of the second and third harmonic currents in the beam were similar to the fundamental current distribution. One of the implications of this result is that it is probably not possible to use the second harmonic current in klystrons utilizing Brillouin beams for efficiency enhancement.