Propagation characteristics of premodulated electron beam

Abstract

The propagation properties of an electron beam whose energy and current are simultaneously premodulated at the cathode are investigated by computer simulation using the two- dimensional (2-D) particle-in-cell code, MAGIC. The simulation is carried out to investigate the influence of the initial energy modulation and the phase difference between current and energy modulation. With appropriate initial energy modulation, the amplitude of the downstream current modulation can be kept as high as the initial modulation level. Appreciable growth of high harmonic components is also observed as the electron beam debunches. The simulation data are compared with results obtained from a nonlinear theory of the current modulation in an electron beam propagating through a drift tube. It is shown that the simulation data agree remarkably well with the analytical theory. the beam energy at the beginning with some phase difference between current and energy modulation. The initial energy modulation plays an important role in current modulation as the beam propagates downstream. With an appropriate initial energy modulation, the amplitude of the downstream current modulation can be kept as high as the initial current- modulation level. This means that the modulated current can propagate a considerable distance without deterioration of modulation amplitude. One way of introducing the initial energy modulation is installation of an idle cavity at the beginning of the tube. The initial current modulation excites the idle cavity, which, in turn, generates an alternating voltage difference at the cavity opening. This alternating voltage provides the necessary initial energy modulation of the beam electrons passing through the opening. The phase difference between the initial current modulation and cavity voltage is de- termined (5) from the current-modulation and cavity-resonance frequencies, and the cavity -value. It is well known as an effect of penultimate cavity tuning in a conventional klystron. Tighter bunching is realized by tuning the penultimate cavity higher than the input frequency. One way of achieving a simultaneous modulation of current and energy is placing an FEA-based cathode in the cavity. By tuning the cavity, the phase difference between the energy modulation and current modulation can be introduced. The current modulation for long-range propagation is achieved through less energy loss in the cavity. In reality, energy of the beam electrons in the conventional klystron amplifiers is modulated at the injection point by the input cavity without the current modulation. The current signal of the beam in downstream is decomposed by Fourier transformation to observe the evolution of fundamental and high harmonic mode components. First, the bunching of the premodulated electron beam along axial distance is com- pared with that of a conventional velocity-modulated electron beam. Secondly, to see the effect of energy modulation in the premodulated electron beam, the simulation is carried out varying the strength and phase of energy modulation at the cathode. Next, the growth of high harmonic components is discussed. Finally, results of a computer simulation are compared with analytical thoery.