Currently, there are few devices delivering output in the sub-millimetre range which exploit the electron cyclotron maser (ECM) instability, due to the expense and technical limitations associated with the current level of magnet technology. Devices operating within this THz regime could fulfill a variety of potential applications, ranging from plasma diagnostics to emerging biochemical diagnostic spectrometry. Therefore, operation at a harmonic, s, of the electron cyclotron frequency, ωc, is an attractive alternative of attaining such frequencies. However, given that the starting current, Ist, of a high harmonic will usually satisfy the starting criteria of a lower harmonic, this can prove problematic.
To that end, a novel resonant cavity has been designed, which realizes co-harmonic generation of second and fourth harmonic resonances. This is achieved through an azimuthal corrugation of the walls of the interaction region, which in turn, allows the frequencies of the two resonances to have an exact integer ratio. Through the use of a specially designed output aperture, the lower harmonic should be effectively trapped within the cavity, facilitating a pure output of the upper harmonic.
The 3-D PIC code MAGIC-3D has been used to examine the proposed geometry. Simulations conducted thus far have displayed the intended co-harmonic generation; however, the second harmonic is seen to undergo mode conversion at the output aperture, which in turn, dominates the emitted radiation. The focus of the current research is therefore to address this problem.