Computational investigation of helical traveling wave tube transverse RF field forces

Electron beam focusing structures are typically designed using beam optics codes primarily based on balancing radial space charge forces from the electron beam, neglecting RF field forces from the slow-wave circuit. The RF field forces of a helical traveling wave tube (TWT) have been calculated using an accurate three-dimensional (3-D) coldest (no electron beam present) circuit model and compared to radial space charge forces to determine their relative significance on electron beam behavior. The forces due to radial, azimuthal and axial RF electric fields at the tube output reach 61, 26, and 132% of radial space charge forces, respectively, at 16 GHz for the device studied. This effect is expected to increase with increased frequency and beam diameter. These results indicate that the RF fields are comparable in magnitude to the focusing fields and have a significant effect on electron beam behavior; their implementation into the beam-focusing design could significantly improve beam transmission near the output of the tube.