Determination of the RF Leakage Field in the Vacuum Tank of the TRIUMF Cyclotron

In the TRIUMF cyclotron the dees are partially defined by the two halves of the quasi-circular vacuum chamber, which completely contains the 1 MW resonating cavity. A stray electric field or "RF leakage" exists inside the dees and has occasionally caused problems to probes or other tank equipment. In order to understand the nature of this field a precise 1:10 scale metal model of the entire tank and resonator system has been built and is being investigated. Various resonances have been identified: the push-pull mode at 226 MHz and the push-push mode at 233 MHz; cross modes along the dee gap resonate at 243 MHz and 253 MHz. In the quasicircular tank region outside of the main resonating cavity the Tm310 and Tm410 modes are mainly responsible for the configuration of the leakage field since they are closer to the operating frequency. The measurements are in reasonable agreement with the results of numerical relaxation calculation and with measurements in the cyclotron tank.