Nonlinear theory of a folded waveguide gyrotron traveling-wave tube amplifier.

We have developed a three-dimensional nonlinear theory for broadband gyrotron traveling-wave amplification in an H-plane-bend folded waveguide. The H-plane-bend configuration is formed by folding a rectangular waveguide so that the orientation of the magnetic field changes along the bend in contrast to the conventional E-plane bend, where the orientation of the electric field changes. Calculations show the feasibility of amplifier operation with large instantaneous bandwidth and moderate efficiency both in the slow wave region and the reduced dispersion fast wave region. Numerical results for a cold beam show an efficiency of 26% with 11% bandwidth in the slow wave region and an efficiency of 47% with 10% bandwidth in the fast wave region. For an electron beam with 2% velocity spread and 2% guiding center spread, the slow wave efficiency drops to 6% but the bandwidth increases to 16% whereas the efficiency and bandwidth of the fast waves decrease by a small amount to 42% and 9.5%, respectively. The efficiency is more sensitive to the axial beam velocity spread in the slow wave region due to larger Doppler shift. (c) 1995 The American Physical Society