A mechanism was sought that was both strong enough to accommodate the large variations that might be expected from low-grade
unsorted magnets and yet was linear enough to facilitate a matrix algebra based adjustment procedure. Some early attempts at using
tuning slugs near the outside edge of the pole tips was unsatisfactory on both counts. The mechanism finally chosen involves
physically displacing individual pole tips within the undulator gap. This is possible based on the principle that a system of
permanent magnets, separated by ferromagnetic material, exhibits a minimum energy state when gaps are zero. This results in a
mildly attractive force 
,
equivalent to about 10 - 30 lbs, that allows the pole pieces to easily slide between magnets in a mechanically stable manner.
This is somewhat counter-intuitive in that the structure consists of such powerful magnets oriented in opposition to each other.
The adjustment mechanism consists of push-pull screws threaded into the backs of the pole pieces to provide the variable displacements.
[gif 253K][jpeg 60K]This photo of the 30 µm undulator, during assembly,
clearly shows the adjustment mechanism. Stability and repeatability were verified through several levels of prototyping.