Hi All,
 
The first 4D102.36 dipole (4573 is its barcode #) has been measured for integrated field strength and the results show that it is weaker than it should be.   
In a previous email from Mark Woodley, sent on 4/12, he explained that these 4D102.36 dipoles should ideally give an integrated field strength of 4.9485 kG-m at 216.886 amps, 
since that is the current that the BYD magnets will run at 17 GeV (the maximum beam current).   Using the stretched wire measurements made on 4573
http://www-group.slac.stanford.edu/met/MagMeas/MAGDATA/LCLS-II/Dipole/4573/  one can see that at 217.0283 amps  the integrated field strength is 4.75176  kG-m , 
which is about ~4% lower than the needed 4.9485 kG-m.  There are trim coils on the magnet which give have a +/-4.5% range.  
If the trim is set to 6 amps, their max current, with the main coils at 216.900 amps (run 3) the integrated field strength is 
4.96631 kG-m which is larger than the 4.9485 kG-m needed, but with only 0.3% overhead.  This is seems too small a margin.  
To determine what gap would give the correct integrated field one can use the integrated field vs current data (run 1).  
If one fits a 4th order polynomial to the increasing current data of run 1, the current needed to reach 4.9485 kG-m is 226.0092 Amps.  
The gap was measured for the fiducialization as 0.707”.  Calculating the correct gap is as follows:
 
Correct gap = 0.707” /(Current for 4.9485 from Run 1 polynomial / 216.886) = 0.707/ (226.0092 Amps / 216.886 Amps) = 0.707”/1.04206 = 0.6785”.   
One can also use Jack Tanabe’s magnet efficiency formula to make the gap change calculation. Using this formula I calculate a gap of 0.6786”, which is very close to the other result.  
 
If 0.707” is the correct gap (this should be double checked) then to have the correct gap, the gap should be reduced by ~4.21%  to 0.679”.    This should give much more head room on the trims.   
 
Of course this is my analysis and someone should check these numbers before metal is cut.  
 
Regards,
 
Scott Anderson
Magnetic Measurements
sda@slac.stanford.edu
x4590