Design of the button beam position monitor for PEP-II

The beam position monitor (BPM) was designed to provide a robust UHV feedthru and a reliable electromagnetic sensor. Stringent resolution requirements at low beam currents, bunch parameters, along with mechanical and chamber requirements produced challenges in the electrical, thermal, and structural design of the BPM´s. Numerical modeling and experimental analyses were used to optimize the design. The higher order modes (HOM´s) and beam impedance were modeled using MAFIA. Measurements agreed with the calculated 1 Ω transfer impedance at the 952 MHz signal processing frequency, and the first two HOM´s found in MAFIA. Tests and analysis both showed the button signal power approaching 40 W. Temperature and stress distributions were analyzed using this power loading with ANSYS. An electronic grade CuNi was selected for the BPM to reliably weld into the copper chambers. Pin seal and compressive joints were considered for the insulator vacuum seals. Both glassy ceramic-to-metal and ceramic-to-metal seals were evaluated