LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
The magnet installed in the bottom part of the fixture remains in place and only the top
magnet is changed. A 2-axis Hall probe and a moving coil were used to measure the field
integral. Hall probe measurements of the vertical field integral for each of the magnets, are shown
in Figure 8.4. The direction of the vertical component of the magnet moment makes a systematic
difference for nearly all of the magnet blocks. The magnet in the bottom of the fixture has
positive My. When magnets of opposite sign of My are paired, the integrated field is smaller, and,
in fact, is very close to the earth’s field contribution of -15 G-cm. This difference can also be seen
in Figure 8.5, where the field integral measurements were repeated with the vertical moment of
the top magnet reversed for some of the magnet blocks. The vertical moment of the magnet block
makes a difference in the field integral that is nearly systematic, but a few of the magnet blocks
give a different result. It probably results from a different distribution of magnet moment within
the block, and cannot be determined by the Helmholtz coil measurements of the total moment.
First Vertical Field Integral(G-cm)
100
80
60
40
20
0
-20
-40
-60
-80
-100
50
100
150
200
J1p Aver=27.1
Jin Aver= -10.6
250
Magnet
Figure 8.4 Integral of the vertical field through the half-period fixture, for different top magnets.
The red triangles are for magnets with positive M y (the same as the bottom magnet); the
blue circles are for negative M y (opposite from the bottom magnet). The lines show the
average value for the similarly shaped points
8-18 ♦ U N D U L A T O R
300
350
400
450
500