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 localized demagnetizing field experienced by different parts of the magnet blocks, over the
full range of intended gaps. A normal goal of the design process is to maximize the on-axis field
of the undulator segment at the smallest gap while ensuring that the demagnetizing field seen by
the magnets is not excessive at small gaps, nor at any larger gaps – after all, the undulator
segment must be assembled one jaw at a time (corresponding to a very large gap) before the jaws
are brought close together. The maximum allowable demagnetizing field is determined by the
grade of the magnet material being used and what that grade of material can tolerate without
permanent demagnetization. Additional margin in the maximum allowable field should be
included for temperature effects, because permanent demagnetization will occur at a weaker
demagnetizing field if the magnet is at a higher temperature, and neither an air-conditioning
failure nor transport in an enclosed truck on a hot day should put undulator segments at risk.
Temperature dependence information is available from the magnet manufacturer in the form of B-
H curves at different temperatures. Similarly, additional margin should be allowed for radiation
exposure because exposure is more likely to result in demagnetization if the demagnetization
field seen by the magnet is closer to the field at which permanent demagnetization occurs in the
absence of radiation.
One way of reducing the demagnetization risk of the magnets is in the choice of the grade of
magnet material. Different grades of NdFeB magnet material were considered and material
(N39SH) meeting LCLS needs is commercially available. The remanence of this material is 1.23
to 1.29 Tesla and the intrinsic coercivity iHc is a minimum of 21 kOe. This grade of magnet has a
high remanent field but it was mainly chosen because of its particularly high intrinsic coercivity.
The high coercivity correlates with a better resistance of the magnet material to radiation-induced
demagnetization, and with this grade of magnet the high coercivity can be obtained without
sacrificing magnetic remanence as compared to the older N38H grade of NdFeB that has been
used in many insertion devices, including most of those at the APS.
Table 8.3 Predicted values for the undulator magnetic model.
Parameter Value
Period length 30 mm
Gap 6 mm
Peak field on axis 1 1.390 Tesla
Effective field on axis 2 1.348 Tesla
Effective K 3.776
Force per pole 258 N
1 The peak field is the maximum measured field amplitude.
2 The measured field amplitude is not exactly sinusoidal. The effective field is the amplitude of an equivalent sinusoidal field that
would produce the same first harmonic energy as the measured field.
U N D U L A T O R ♦ 8-21