A long electromagnetic wiggler for the PALADIN free-electron laser experiments

A report is presented of the design, fabrication, and testing of a 25.6-m-long wiggler for a free-electron-laser (FEL) experiment. It is a DC iron-core electromagnetic wiggler that incorporates a number of important and unique features: permanent magnets are used to suppress saturation in the iron and extend the linear operating range; steering-free excitation allows real-time adjustment of the field taper without causing beam steering; and wiggle-plane focusing is produced by curved pole tips. The magnitude of random pole-to-pole field errors is minimized by a mechanical design concept that reduces tolerance stackup in critical locations. Three five-meter sections of this wiggler design were tested individually, and when connected together the measurements show exceptionally low levels of random errors. Detailed analysis of the error distribution has shown that the errors are not truly random, and the net effect on steering is much less than would be expected from random errors of the same magnitude. Analysis is still in progress to identify the cause of this beneficial effect.