Beam Position Monitors

To determine the position and the direction of the beam on the experimental target point, two Beam Position Monitors (BPMs) are located at distances 7.524 m (IPM1H03A) and 1.286 m (IPM1H03B) upstream of the target position. The BPMs consist of a 4-wire antenna array of open ended thin wire striplines tuned to the fundamental RF frequency of 1.497 GHz of the beam  [1]. The standard difference-over-sum technique is then used  [2] to determine the relative position of the beam to within 100 microns for currents above 1 $\mu$A. The absolute position of the BPMs can be calibrated with respect to the scanners (superharps) which are located adjacent to each of the BPMs (IHA1H03A at 7.353 m and IHA1H03B at 1.122 m upstream of the target). The schematic of the readout electronics is shown in Figure  2.4. The position information from the BPMs can be recorded in three different ways:

Figure 2.4: Schematic of the BPM readout electronics

1.

1. The averaged position over 0.3 seconds is logged into the EPICS database (1 Hz updating frequency) and injected into the datastream every 3-4 seconds, unsynchronized but with an orientative timestamp. From these values we can consider that we know the average position of the beam calculated in the EPICS coordinate system which is left handed.

2. Approximately once a shift (or more often if requested by the experimenters) a B-scope procedure  [3] can be performed using the same EPICS electronics which then gives the peak-to-peak variation of the beam.

3. Event-by-event information from the BPMs are recorded in the CODA datastream from each of the 8 BPM antennas (2x4) from which the position of the beam can be reconstructed. However, these raw values belong to a parallel electronics chain whose constants have to be retrieved by calibrations to the EPICS or scanner data.