Beam condition monitors

All the experiments carried out in Hall C used certain level of beam monitors, such as two or three BCMs and two sets of BPMs (3H00A and 3H00C) near the target. Since the energy stability and repeatability is crucial to this experiment, we require additional monitors (as HNSS did) that were not set at display or observation for most of the experiments. Figure 76 shows Hall-C beamline magnets and monitors.

1. BPMs on the arc
   This includes the BPMs at the entrance (3C07), middle (3C12) , and exit (3C17) of the arc. Continuous display of the those monitors for two hours history is required to observe the energy stability or possible shift online. The EPICS readouts will also be fed into data stream for possible offline correction. A selected set of BPMs will be read on an event-by-event basis.

2. Mid-arc special beam position device
   Though it is non standard equipment, ACC can install a better beam position measurement device, synchrotron light interferometer (SLI), in the middle arc (3C12) where the momentum dispersion is largest. It uses double slit interferometry of the synchrotron light to determine the beam size at this location. With known dispersion, the size represents the energy spread. It has better spatial resolution than the OTR, so it can measure smaller beam size/energy spread. It is completely non-invasive for the beam, thus produces no halo, while the OTR is not. It does not measure the absolute beam energy, but the instantaneous spread; ``instantaneous'' means here an average over a timescale of tens of seconds.

   A two hour history display together with the history of the arc BPMs which give the energy shift information will provide an overall beam energy condition monitoring.

3. History display of BPMs in front of the target
   We will put the BPMs in front of the target in a two hour history display to observe the beam position stability on the target. Their readouts are required to be fed into data stream for the offline correction.

4. ACC arc energy measurement
   ACC arc energy measurement information is available in the EPICS. We will make online history monitoring display in group with mid-arc BPM and special device display to observe the consistency of the energy stability control.

All these monitoring information are or will be standard and available from EPICS. Setting them up is straightforward.