Characterization of beam position monitors for measurement of second moment

A dual-axis beam position monitor (BPM) consists of four electrodes placed at 90° intervals around the probe aperture. The response signals of these lobes can be expressed as a sum of moments. The first order moment gives the centroid of the beam. The second order moment contains information about the rms size of the beam. It has been shown previously that the second order moment can be used to determine beam emittance [1]. To make this measurement, we must characterize the BPM appropriately. Our approach to this problem is to use a pulsed wire test fixture. By using the principle of superposition, we can build up a diffuse beam by taking the signals from different wire positions and summing them. This is done two ways: first by physically moving a wire about the aperture and building individual distributions, and, second, by taking a two dimensional grid of wire positions versus signal and using a computer to interpolate between the grid points to get arbitrary wire positions and, therefore, distributions. We present the current results of this effort.