Optimization of four-button BPM configuration for small-gap beam chambers

The configuration of four-button beam position monitors (BPMs) employed in small-gap beam chambers is optimized from 2-D electrostatic calculation of induced charges on the button electrodes. The calculation shows that for a narrow chamber of width/height (w/h)>1, over 90% of the induced charges are distributed within a distance of 2h from the charged beam position in the direction of the chamber width. The most efficient configuration for a four-button BPM is to have a button diameter of (2 - 2.5)h with no button offset from the beam. The button sensitivities in this case are maximized and have good linearity with respect to the beam positions in the horizontal and vertical directions. The button sensitivities and beam coefficients are also calculated for the 8 mm and 5 mm chambers used in the insertion device straight sections of the 7 GeV Advanced Photon Source. copyright 1998 American Institute of Physics