LCLS UNDULATOR DESIGN DEVELOPMENT

The undulator segments of the 130.4-m-long undulator line for the Linac Coherent Light Source project (LCLS) [1] must have a deflection parameter Keff that matches the nominal value for that segment to within ∆Keff/Keff < 1.5 × 10-4. Mechanical shims were used to set the undulator gap to control K in the prototype, but this is too tedious a procedure to be used for all 33 undulator segments. Although the prototype undulator [2-4] met all of the LCLS specifications, development continued in order to simplify the system. Various other alternatives for adjusting the field were considered. A canted-pole geometry was chosen, allowing the K value to be changed by lateral translation of the entire undulator segment. This scheme also facilitates tapering the undulator line to accommodate electron beam energy loss. The prototype undulator was subsequently modified to test the canted-pole concept. Magnetic measurements demonstrated that the undulator with canted poles meets all LCLS specifications, and is more cost effective to implement