Switched detector for beam position monitor

The NSLS (National Synchrotron Light Source) storage rings use sets of four button electrodes to determine the transverse position of the stored electron beam in the vacuum chamber. By means of GaAs switches, the 211 MHz component of the induced signals on each of the four buttons is measured in turn by a single amplifier-detector channel. These signals are then stored in four sample and hold circuits. The measurement cycle is repeated at a rate of 40 kHz. The required sums and differences of these signals are obtained by analog means. The results are normalized with respect to beam intensity by servoing the gain of the amplifier-detector channel such that the sum of the signals from the four buttons is maintained at a fixed value. The prototype receiver provides ±20 μm resolution within a 300 Hz bandwidth over a 30 dB dynamic range. The receiver is relatively insensitive to beam energy and to the number of bunches stored in the machine