Studies on boundary conditions for gyrotron interaction modeling

In gyrotron interaction simulations, radiation boundary conditions are used for the electromagnetic field in the open-ended resonator. Usually, the boundaries are considered matched, i.e. no power is reflected back into the resonator. In the vast majority of cases, a boundary condition matched only at one single frequency and having low reflection in a narrow band around this frequency is used. This is done on the grounds that the gyrotron radiation is basically monochromatic. However, a broadband matched boundary condition has also been proposed
¹
, which should be employed in the special cases where the gyrotron output has a broader frequency spectrum. Such cases include dynamic after-cavity interaction
²
and the appearance of side-bands. Following the broadband concept
¹
, a boundary condition has been formulated in a new way, compatible with the code-package EURIDICE. Two alternatives for the numerical implementation are discussed and tested. In addition, the behavior of both the single-frequency and the broadband boundary condition is studied analytically and compared with the behavior of the numerical implementation. Finally, the broadband boundary condition is applied in cases where the gyrotron radiation exhibits a wider frequency range.