Cavity Beam Position Monitor System And Related Study

A new high brightness injector supported by 985 projects is planned to be installed at HLS of NSRL. To meet the requirements of high precision beam position monitor system for the injector, a project to develop novel beam position monitor based on resonant cavity has been worked on. Basic theory of cavity BPM is discussed and the key points that affect the position resolution of cavity BPM is showed. Computer simulation tools are used to study the eigen modes of resonate cavity, the characteristic of waveguide coupling method and mechanic tolerance. A prototype cavity is fabricated based on discussion about optimization problem of S-band re-entrant cavity BPM. According to cold test results of the prototype, cross-talk problem is analyzed. A new BPM concept that can suppress the cross-talk based on a racetrack cavity is then studied.Cavity BPM is a new method of beam position measurement. BPM couples out dipole modes of resonant cavity. In paraxial condition, signal of TM110 mode in the beam-pipe region has a linear dependence on the beam offset, while the phase of the signal indicates the sign of the offset. Optimization of external quality factor should be taken into consideration when designing a cavity BPM. The theoretical resolution of BPM is different from its ideal resolution.Computer simulation softwares such as MAFIA, Omega3p and Microwave Studio are used. According to results of MAFIA simulation, waveguide coupling structure suppress the leakage of TM010 mode very well while there will be match up problem between feedthroughs and waveguides. A design worked on 2448 MHz with re-entrant cavity and cut-through waveguide coupling structure is then chosen. Farther optimization is processed at mainframe computer named Bassi with Omega3p software. The ideal resolution of this design is 7 nm, while the theoretical resolution is 31 nm. Based on the results above the mechanic error and tolerance is discussed.An off-line test is processed on the prototype and the basic theory of cavity BPM is confirmed. Position resolution of prototype reaches 2μm while there is cross-talk of 5%-10%. Methods to improve the precision of off-line test are discussed. To get beam position information from BPM signal and send it to local network and central computer of accelerator, signal processing system is required. Signal processing systems based on Libera or chip AD8302 are both practical. A new BPM design with racetrack cavity is given after discussion about reason of cross-talk and how to suppress it. Microwave Studio is used to get the optimal scheme. The racetrack BPM outputs x offset signal at 2448MHz with a resolution of 15 nm, while it outputs y offset signal at 2652MHz with a resolution of 13 nm.Investigation of this subject is supported by National Natural Science Foundation project 10875117.