Reduction of multipactoring effects in the TRIUMF cyclotron

The reduction of rf leakage into the accelerating chamber has been one of the main development tasks at TRIUMF for the past few years. Rf leakage seriously limits the use and operation of diagnostic devices and is the driving force behind a resonant multipactoring discharge that occurs in the beam space. This in turn causes many problems in performance and reliability of the whole rf system. Two major problems are: the excessive heating of the mechanical dee-support structures that warps individual resonator segments; and instabilities in resonant frequency caused by erratic behaviour of the multipactoring discharge. By improving the rf gap alignment, and by adjusting the distances between dees and ground panels to compensate for electrical asymmetries, a leakage reduction of 20 dB was achieved, bringing the maximum operating temperature on the resonator strongbacks down from ≤150 degrees C to ≤80 degrees C. Also the signal-to-noise ratio on various probes was greatly improved, and multipactoring frequency shifts were practically eliminated