A note on gyrotron traveling-wave amplifiers using rectangular waveguides

The coupling constant ε between a relativistic electron beam and the TE
10
rectangular waveguide mode is calculated. By reducing the height of the waveguide, this coupling constant may exceed the corresponding value for the operation with the TE
11
circular polarized mode, under identical conditions of the electron beam. The calculation is followed by a discussion on optimum mode choice in gyro-TWA's.