Angular-dependent modes in circular corrugated waveguide

The paper gives an account of an investigation, both theoretical and experimental, of angular-dependent EH11 modes in a circular corrugated waveguide. The investigation was undertaken because oscillation in a mode of this type has been shown to be the cause of pulse-shortening or beam blow-up troubles occurring in electron linear accelerators under certain high-beam-current conditions. It is shown that the dispersion characteristics can represent either forward waves or backward waves dependent on the dimensional parameters, and under certain conditions there can be a changeover from one to the other within the pass-band. There are marked similarities between these results and those found theoretically by other workers, for hybrid modes in a circular waveguide containing a coaxial dielectric rod. The measurements also include the distribution of magnetic and electric fields in the axial region, using perturbation techniques. Series impedance values (relating fundamental electric field intensity to power flow) are derived, and are comparable with the theoretical results. They confirm that in a linear accelerator an EH11 oscillation need only build up to a power level of the order of kilowatts to account for the disruptive effect on the beam observed during pulse shortening.