BPMs and fast feedback locks

The BPMs are used to measure the center position of the beam alone the line. The BPMs located at C07, C12, C17, and H00 are able to be read at a frequency of 1kHZ in order to install the fast feedback lock systems. They provide important information about stability of the central beam energy and position on target.

Under a specific tune of the arc, an optics for the given beam energy is defined. A position shift indicated by the BPM at C12 represents a central energy shift, if positions at C07 and C17 are relatively stable. For experiments that require high precision beam energy, monitoring the stability of these positions are important. The readout of these BPMs can also be added into data stream using a suitable readout frequency. Energy shift correction can be made offline using arc optics matrix.

With the same principle, energy fast feedback lock system reads these positions at a frequency of 1kHZ and calculates the needed energy correction. Then correction is added to the last cavity module in the south linac. Such system is installed on both Hall A and C arcs. Only one of these two can be used to control the beam stability. Practically, the hall with the lock on its arc has the best energy stability and $\sigma < 2x10^{-5}$ was achieved in the past. For other halls the energy shift is larger but less than $10^{-4}$.

The BPMs, H00A and H00B, are in front of the target. They are 3.455 and 1.637 meters upstreams from the target, respectively, and are used to indicate the beam position on target and the incident angles. Checking and monitoring these positions allow experiments maintain their desired beam position on target and the stability.

A position fast feedback lock is also installed in order to stablize the beam position on target. This lock is independent for each hall. Since the position stability is coupled with the beam energy stability, whichever the hall having the enegry lock on will also have the best position on target stability ( $< 200 \mu{\rm m}$.