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UBC Theses and Dissertations

Design study for third harmonic flat-topping of the Triumf 520 MeV cyclotron Michelson, David George


The accelerating voltage in the 520 MeV/200 uA H-cyclotron at TRIUMF is developed in a resonant cavity and consequently has a sinusoidal time dependence. It is possible to "flat-top" the accelerating voltage waveform by superimposing a third harmonic waveform of the appropriate relative amplitude and phase on the fundamental waveform. This increases the phase acceptance of the cyclotron by reducing the spread in energy gain per turn normally associated with the spread in phase of the ion beam with respect to the peak of the RF waveform. Slight adjustments to the shape of the waveform can introduce longitudinal bunching mechanisms which tend to compensate for several second order effects that normally limit the cyclotron's performance during high current or high energy resolution modes of operation. The relative simplicity of TRIUMF's dee geometry suggests that both the fundamental and third harmonic modes could be excited in the same rather than in separate accelerator RF cavities. Implementing such a system will require the development of new cavity coupling and tuning mechanisms for the existing radio frequency cavity and a new radio frequency control system. Because access to the cyclotron is severely restricted by scheduled beam production and high residual radioactivity in the cyclotron vacuum tank, a test cavity built fromcomponents identical to those used in the cyclotron RF cavity was adapted for initial development and testing of third harmonic cavity tuning and control systems at operational dee voltages (100 kilovolts at 23 MHz, 11 kilovolts at 69 MHz) and under hard vacuum (10⁻⁷ Torr). This thesis describes: a. the scope and technical objectives of the third harmonic flat-topping project at TRIUMF including required improvements to the fundamental RF system; b. the design and initial development of cavity tuning and coupling mechanisms for the RF test cavity; c. the demonstration of a flat-topped accelerating voltage in the test cavity at operational voltage levels while under vacuum; and, d. the design and initial development of a prototype version of the new radio frequency control system. The results obtained show that under realistic conditions of vacuum and RF input power, it is possible to simultaneously excite the fundamental and third harmonic modes in an RF cavity with mechanical construction and operational characteristics similar to those of the cyclotron RF cavity.

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