History: Position

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 (IPM1H04A) and 2.378 m (IPM1H04B) 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. The standard difference-over-sum technique is then used to determine the relative position of the beam to within 100 microns for currents above 1 uA. The absolute position of the BPMs can be calibrated with respect to the scanners (superharps) which are located adjacent to each of the BPMs (IHA1H04A at 7.353 m and IHA1H04B at 2.215 m upstream of the target).

A calibration of BPM's results in a precise knowledge of the position of the raster spot. There is, hewever, a significant delay between the time of an event and the radouts of the BPM's. For this reason, the BPM's cannot be directly used for a precise determination of the vertex of each evnt. The readouts of the raster currents are much faster and are used for that purpose. The detailed description of method used to calibrate the BPM's as well as that used to extract refined position from the raster current together with calibration results presented on Brandon's web page: [http://www.jlab.org/~bcraver/html/beamline|BPM's calibration for GeN].