Parasitic Mode Losses versus Signal Sensitivity in Beam Position Monitors

A beam position monitor (BPM) for a storage or damping ring may be subject to heating problems due to the parasitic mode (PM) losses, beam interception and synchrotron radiation interception. In addition, high PM losses can cause beam instabilities under some conditions. Recessing and/or masking the BPM may increase the PM losses in the process of solving the latter two problems. This paper presents three complementary methods for estimating the PM losses and for improving the design of a stripline directional coupler type of BPM: bench measurements, computer modeling (TBCI) and an equivalent circuit representation. These methods lead to a decrease in PM losses without significant reduction in output signal for the north SLC damping ring BPMs.