USE OF MOVABLE BEAM POSITION MONITORS FOR BEAM SIZE MEASUREMENTS

The use of beam position monitors (BPMs) as nonintercepting emittance monitors has been proposed in 1983 by Miller et al. The emittance measurement relies on the beam size dependency of the BPM signals. It is shown that the original proposal can be improved by using movable BPMs. Changing the BPM position with a stepping motor allows accurately calibrating the beam size measurement. The absolute scale on the beam size measurement is given by the scale of the stepping motor and can be determined in the laboratory and measured in situ. Uncontrolled changes of the beam position can be monitored through the use of a BPM triplet. It has been proposed in [1] that the quadrupole term in the signal of standard four button beam position monitors (BPM’s) can be used for a non-intercepting measurements of beam size. Recent publications have shown that the method can indeed be used successfully for measuring beam size [2,3]. Here, we propose an improvement of the method. It is shown that the use of BPM step movers greatly simplifies the method. It is explained how movers can be used for a precise calibration of the absolute scale of beam size. 2 THEORY We consider BPMs that consist of four pickup buttons with an angle of /2 between neighbouring buttons. The pickup buttons shall be located at upper left, top, bottom, left, and right position, as illustrated in Figure 1. The distance of a button to the BPM centre is given by a and its azimuthal angle by . The image current in a four-polar beam position monitor has been calculated by Miller et al [1] in 1983. We consider an infinitely long line current I(r, ) at radial location r and azimuthal angle . The image current density Jimage (r, ,a, ) on a conducting cylinder of radius a at azimuthal angle is then: