A simple method of forming a matched input in helical undulators

A helical undulator was used in the initial experiment to realize a free el ectron laser [D.A.G. Deacon et al., Phys. Rev. Lett. 38 (1977) 892]. Since then it has been widely used in FELs with pulsed electron beams. To inject a beam, an adiabatic input, 5-10 periods long, is generally used. Such a se ction is also often used at the undulator output. In all, this lengthens th e undulator by 30-50%. An alternative method of forming a matched fringe fi eld with a short winding no more than a period long, in accordance with a s pecial law, is effective but difficult to realize. A new, simpler method of matching the undulator input, buttressed by calculations and experimentall y realized, is proposed. In this case, the fringe field is formed by means of conductor segments in the form of straight pieces and circumference arcs . This enables one to accurately meet the geometric tolerances. The axial l ength of this section is about 1/6 of an undulator period. Experimental res ults obtained on a 4.8 cm-period undulator gave good agreement with calcula tions. (C) 1999 Elsevier Science B.V. All rights reserved.