A simple method of forming a matched input in helical undulators

A helical undulator was used in the initial experiment to realize a free electron 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 section is also often used at the undulator output. In all, this lengthens the undulator by 30–50%. An alternative method of forming a matched fringe field with a short winding no more than a period long, in accordance with a special law, is effective but difficult to realize. A new, simpler method of matching the undulator input, buttressed by calculations and experimentally 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 length of this section is about 16 of an undulator period. Experimental results obtained on a 4.8 cm-period undulator gave good agreement with calculations.