The interaction between the third harmonic resonance and parasitic modes inside the TRIUMF cavity

The TRIUMF resonator system has been designed for possible `flat-top´ operation. This mode of operation implies simultaneous excitation of the fundamental and the third-harmonic resonances in the same RF cavity. A precise 1:10 scale all-metal model of the TRIUMF cyclotron has been used to study the feasibility of achieving flat-topped operation. The third-harmonic voltage has been shown to vary considerably along the resonator tip due to the higher capacitive loading in the central region. In addition, higher-order parasitic modes of the fundamental group have been observed to interfere with the TEM (transverse electromagnetic mode) third-harmonic resonance, denoted by TEM₀₃₀, resulting in voltage variations along the dee gap of as much as 10 dB. The observations, a theoretical treatment of the interference, and possible solutions are discussed