Design of a Multi-Bunch BPM for the Next Linear Collider

The Next Linear Collider (NLC) will collide 180‐bunch trains of electrons and positrons with bunch spacing of 1.4 ns. The small spot size (σy < 3 nm) at the interaction point requires precise control of emittance, which in turn requires the alignment of individual bunches in the train to within a fraction of a micron. Multi‐bunch beam position monitors (BPMs) are to determine the bunch‐to‐bunch misalignment on each machine pulse. High bandwidth kickers will then be programmed to bring the train into better alignment on the next machine cycle. A prototype multi‐bunch BPM system with bandwidth (350 MHz) sufficient to distinguish adjacent bunches has been built at SLAC. It is based on 5 G sample/s digitization of analog sum and difference channels. Calibration tone injection and logging of the single bunch impulse response provide the kernel for deconvolution of bunch‐by‐bunch position from the sum and difference waveforms. These multi‐bunch BPMs have been tested in the Accelerator Test Facility at KEK and i...