Figure 2.

The different cross sections of Apple-type undulators in chronological order. In the first row one can find the standard Apple cross sections, starting with Apple I and II, which are designed for synchrotrons operated with elliptical vacuum chambers, followed by Apple III for linac-driven FELs. All three types are regularly equipped with a gap drive system which is changing the distance between the upper and lower arrays. In the second row there are the new cross sections called Delta LEPP-CHESS and Delta, which are designed for linac-driven FELs without a gap drive system. The last type is called Apple X (at the SwissFEL project) or Delta II (at the LCLS project), where a new gap drive system is moving an Apple III cross section in a way that both axis symmetries and 90° symmetry are guaranteed.