RF Cavity Tuning Based on Reflected Power Measurements

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RF cavity tuning
Extremum seeking
Gradient estimation
Sliding mode extremum seeking

RF cavities are resonators and the key structures in particle accelerators. An electromagnetic field within the cavities provides the acceleration field. Within linear particle accelerators, particle bunches travel along a beam pipe and through the aligned cavities. A positive electric field along the beam line results in an acceleration kick of the particle bunch. The speed of the traveling particle bunches is synchronized with the RF field withinthe cavities. Hence, tuning of the natural resonance frequency of an RF cavity is essential for accelerator structures to achieve efficient beam acceleration and to reduce power requirements. Operational cavities are typically tuned using phase comparison techniques. Phase measurement is subject to temperature drifts which renders this technique labor and time intensive. In this thesis, we developed two novel resonance frequency tuning schemes solely depending on the reflected power component of a cavity. The base for control scheme development is a mathematical model of the cavity in terms of the steady state signals. The first control scheme was derived through a Lyapunov analysis and incorporates a gradient estimator of the performance function. The second control scheme is based on sliding mode extremum seeking. Both systems are analyzed in terms of the stability; conditions are provided which guarantee stable system behavior up to twice the cavity bandwidth. A simulation study verifies the derived stability conditions. An experimental test bench, including a room temperature quarter wave cavity, was built to test the control schemes under various conditions. Although both control schemes show similar tuning results, a higher tuning accuracy is obtained by the sliding mode based control scheme. Hence, the latter was chosen to be implemented on two resonators, DTL tanks, of TRIUMF’s ISAC I facility. The system was fully commissioned on both DTL tanks and has been in operation since April 2016. Reflected power, forward power, and tuning position are monitored and analyzed. Long term measurements showed the influence of environmental temperature variations. As the influence of environmental temperaturevariations can be neglected for reflected power measurements, the reflected power based tuning system provides a higher tuning accuracy compared to the traditional phase based tuning system. The start-up time and the need for human oversight are reduced significantly.

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This thesis may be printed or downloaded for non-commercial research and scholarly purposes. Copyright remains with the author.
Mehrdad Moallem
Applied Sciences: School of Mechatronic Systems Engineering
Thesis type: 
(Thesis) Ph.D.