Design and operation of a button-probe, beam-position measurements

Beam position measurement systems have been installed on the Advanced Free Electron Laser (AFEL) facility at Los Alamos National Laboratory. The position measurement uses a capacitive- or button-style probe that differentiates the beam-bunch charge distribution induced on each of the four probe lobes. These induced signals are fed to amplitude-to-phase processing electronics that provide output signals proportional to the arc tangent of the probe's opposite-lobe, signal-voltage ratios. An associated computer system then digitizes and linearizes these processed signals based on theoretical models and measured responses. This paper will review the processing electronics and capacitive probe responses by deriving simple theoretical models and comparing these models to actual measured responses