## 2 Accelerator
### Undulator Line  
The Aramis undulator line of SwissFEL, shown in Figure 3, consists of 13 in-vacuum undulator modules with 4 m length and 265 periods of 15 mm each. High-performing NdFeB permanent magnets with diffused dysprosium (Hitachi metals; remanence \(B_{r}\) = 1.25 T, coercivity \(H_{cl}\) = 2400 kA/m) and poles with a trapezoidal geometry made out of Vacoflux 50, Vacuumschmelze to focus the field on the beam axis, provide a peak field of \(B=1.3\) T (corresponding to a \(K\) value of 1.8) at a minimum gap of 3 mm. Thanks to a reduced pole tip width of 15 mm the magnetic forces can be limited to 25 kN (corresponding to the weight of 2.7 metric tons), under which a gap adjustment precision better than 1 \(\upmu\)m is needed. The undulator module design was made by PSI in close collaboration with industry to include specific manufacturing know-how right from the beginning. The production of the modules was carried out by our industrial partners (Daetwyler Industries (Huntersville, NC, USA), RI, VDL, Comvat (Sennwald, Switzerland), Schaeffler Schweiz GmbH (Romanshorn, Switzerland), EPUCREET (Wangen, Germany), Agathon, Rollvis (Plan-les-Ouates, Switzerland)) but the optimization and characterization of the magnet structures were performed in the undulator laboratory inside the SwissFEL building at a rate of one undulator module per month. An optimization of quality and cost in the early conceptual phase resulted in the following main design principles: a closed O-shaped support structure with cast mineral material, a wedge-based drive system, a common, modular design for in-vacuum, standard or APPLE II configurations, and a magnet keeper design enabling fast field optimization. Further constraints were the requirement for transport without crane and the target beam height of 1.2 m.  
The closed support structure was chosen because of the superior stiffness compared to a standard C-structure when accessibility for magnetic measurements and pre-installations of vacuum chambers are not an issue. The high stiffness is transferred to the I-beam by a small angle wedge system, a novel concept in undulator design. With our industrial partners we produced a full-size prototype in 2013. With a 70% support by bearings the height of the I-beam can be reduced, and the wedge works as a gear reduction. The drive system only consists of a servomotor and a satellite roller screw with a small pitch of 1 mm per turn. Two wedges move against each other, synchronized by the [PERSON] motion control system. The I-Beam is fixed in the longitudinal direction by a central guiding rod (Agathon). The backlash-free system allows gap changes with a reproducibility of 0.4 \(\upmu\)m. The gap position is monitored by absolute linear encoders. Twenty columns (Comvat) with integrated differential screws provide for the connection to the magnet arrays through the vacuum vessel produced by VDL. The top columns are shifted with respect to the lower ones, which reduces the critical variation of the gap (with an exponential field dependence) but comes at the price of a (less harmful) non-straightness of the magnetic field axis following a hyperbolic cosine dependence. To meet the critical time schedule and to achieve optimum results in terms of magnetic field profile, a block keeper was designed and realized from extruded aluminum, which allows all magnet-pole pairs to be adjusted in height. An accuracy of better than 1 \(\upmu\)m can be achieved with the 3\({}^{\circ}\) wedge shown in Figure 4. The pole height can be tuned within \(\pm 30\) \(\upmu\)m by means of a flexor system, enough for the shimming of all 13 undulators. The magnet arrays on the in-vacuum I-beams have been assembled aligned and pre-measured by RI.