A primary objective for the field of vacuum electronics is the development of
devices capable of generating coherent, high power radiation within the sub-mm
regime. The electron cyclotron maser instability offers an attractive method of
fulfilling such requirements, being a well established mechanism for obtaining
coherent, high power radiation within the mm-band. However, operation at high
harmonics of the electron cyclotron frequency, ωc, proves challenging, given the
sensitivity of parasitic modes to the large beam currents required.

Previously, particle-in-cell simulations have been presented on the
simultaneous operation of a gyrotron cavity at the 2nd and 4th harmonics of the
electron cyclotron frequency[1]. Such a co-harmonic scheme is used to obtain high frequency radiation through the direct excitation of a low harmonic signal. By then trapping the low harmonic within the interaction region, through the use of a cut-off aperture, pure output of the high harmonic signal can be realised. However, the intended output radiation is dominated by a mode converted 2nd harmonic signal.

Analysis of the setup suggests that such mode conversion occurs due to the cut-off aperture, and while the magnitude of the converted signal can be reduced, it cannot be eliminated entirely.

As a result, the current focus is to confirm the presence of this mode
conversion experimentally. To that end, the design and fabrication of several
additional components has been required. A brief overview of these components will be given, along with some initial experimental results