To control the electron beam emitted from the carbon nanotube （CNT） cathode, four different electron chunnels are designed. A common basic structure used in the simulation is an insulating chunnel. When primary electrons hit the surface of the chunnel, secondary electrons are generated, which make the electron distribution at the exit hole of the chunnel more uniform. By analyzing and comparing the state of electrons emitted from the exit of chunnel among the four structures, an optimal structure is obtained. In the optimized structure, the electron distribution at the exit hole of the chunnel is more uniform and the electron beam is rather slim. Furthermore, by adding a magnetic field along the slow wave line, the electron beam can be constrained. In the optimized structure, a very small magnetic field is needed to make most of electrons pass through the slow wave line.
