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    User name horn

    Log entry time 21:50:18 on March 14, 2005

    Entry number 539

    keyword=Horizontal Beam Position

    In order to determine the absolute horizontal position of the beam in the target-beam system two effects have to be taken into account: The horizontal position of the target itself and the horizontal position of the beam relative to the target system.

    Due to the circular geometry of the target cells used a horizontal offset of the target (>2mm -> 1% difference in length, Fig 1) will result in a significant reduction of the target length relative to the nominal cell diameter. Combining the inital target survey result with the measured values for target vacuum motion (1.20 mm left) and target cooldown motion (3.22mm left) results in a horizontal target offset of 3.42mm beam left. This value agrees well with the observed burn marks on the target cell, which are located ~3-4mm beam left of the center of the target.

    To determine the relative horizontal beam position in the target system a combination of the horizontal BPM information projected to the target and the Ytar reconstruction from optics data (sieve measurement, carbon z=0 target) is used. From the reconstructed optics data the beam location in the target x-z plane can be depicted as the intersection of two lines parallel to the line emanating from the origin of the x-z coordinate system with a slope given by the central spectrometer angle. The parallel distances are determined by the value of the reconstructed Ytar offset corrected for the spectrometer mispointing . Further discussion and a visual representation of this procedure by D. Gaskell for the HMS specific case is available here. Although it is possible to use the information from one spectrometer only, this analysis will use the information from both HMS and SOS due to the limited angle range of the available data.

    The following two tables summarize the sieve data used in determining the horizontal beam offset:


    Distances of BPM to the target: 
      BPM A=327.15cm
  BPM B=231.47cm
  BPM C=135.38cm



    
HMS:
---
    
theta   Run     Ytar(mm)  HMisptg  Ycor(mm)  BPMAx  BPMBx   xtar(proj)

    
12.00   46996    0.16      0.19d    0.35      0.93   0.81    0.52
12.00   47012    0.13      0.19d    0.32      0.97   0.86    0.59
19.99   47037    1.04      0.0      1.04      0.81   0.70    0.43
12.00   47336    0.31      0.19d    0.50      0.98   0.81    0.40
12.00   47525   -1.38      0.19d   -1.19      1.05   1.29    1.87
12.00   47526   -1.42      0.19d   -1.23      1.04   1.27    1.83
10.55   47556   -1.72      0.14d   -1.58      1.06   1.30    1.88
12.55   47619   -1.64      0.19d   -1.45      1.06   1.47    2.46
15.00   47631   -0.55      0.25d   -0.30      1.49   1.46    1.39
15.00   47644   -0.45      0.25d   -0.20      1.56   1.53    1.46
15.00   47645   -0.36      0.25d   -0.11      1.36   1.33    1.26


    SOS
---
    
theta   Run      Ytar(mm)  HMisptg  Ycor(mm)
    
30.0    46996     -0.71     0.95d    -1.66
30.0    47012     -0.81     0.95d    -1.76
30.0    47037     -0.59     0.95d    -1.54
30.0    47336     -0.90     0.95d    -1.85
25.0    47525     -1.43     1.08d    -2.51
25.0    47526     -1.51     1.08d    -2.59
25.0    47556     -1.52     1.08d    -2.61
27.0    47619     -2.14     1.12d    -3.26
30.     47631     -1.90     0.95d    -2.85
23.0    47644     -1.21     1.05d    -2.26
23.0    47645     -1.02     1.05d    -2.07
    Note that horizontal mispointing downstream implies a different sign of the correction for HMS/SOS due to the definition of +y in the spectrometer coordinate system.

    Depicted in Figure 2 are the intersection points in the target x-z plane for each beam energy considered. In this coordinate system the vertical axis represents the target z-position (along the beam) and the horizontal the horizontal beam position. The green lines indicate the values of the initial and final survey values for the target position along the beam.

    The data suggests that it is not possible to use a single value for the horizontal beam position throughout the entire run. However, given the stability of the beam positions within one kinematic setting (defined here by the beam energy) one value can be assigned to each such setting. 

    
E_e  x(mm)  z(mm)  

    
4.2  +0.25  +3.15
5.2  +0.15  +3.15
3.8  +2.00  +3.51
4.7  +1.00  +3.15




    Fig 1

    target_len.eps

    Fig 2



    Fig 3



    Fig 4

    bpm_epics_ytar_test.eps