## 1 Introduction
This is aimed at achieving emission at any value of the \(K\) parameter for each frequency from the assumed range. The dimensions of the magnet blocks cause difficulties in the serial processing of large NdFeB magnets, particularly concerning the accuracy and repeatability of magnetization strength and orientation. For this reason, mainly high-grade magnets are used. Due to large dimensions and strong magnetic interactions, magnet block keepers must provide a precise position adjustment for deviation compensation and the necessary rigidity at the same time. The main support should be designed to effectively address factors, such as weight, a center of mass, the operational range of the movable girder, and the ability to endure variable working conditions. The calibration and positioning of magnet blocks are essential due to the geometrical inaccuracies inherent in the fabrication tolerances of each magnet block, as well as potential magnetization errors. To ensure the required parameters for the electron beam (free from displacement and deflection), a certain distribution of the magnetic field between the undulator jaws must be provided. This field must be symmetrical with respect to the electron beam [18]. The high-grade material used for neodymium magnets in the Halbach array required an innovative solution for magnet keepers that could fulfil the project's requirements. In the design process of the magnet holder, the various existing solutions allowing the calibration and movement (within the fine range up to submicron) of the magnet block were explored and validated. There are various techniques for the calibration and positioning of magnet blocks. The most frequently used solution involves alignment screws that are accessible from both sides of the girder. This approach enables vertical movements and angular compensations of the magnet and generally does not require additional shims. The magnet is placed in a small magnet holder susceptible to movement of the adjustment screws. As a result, precise positioning of the magnet blocks is achieved through a set of adjusting screws and locking bolts. This approach was implemented in the LCLS-II hard x-ray undulator [19] and was also adopted in the European XFEL Undulator [20; 21], which incorporates two tuning screws and two locking screws. However, in the second application, additional grooves (for the screws) in the magnet blocks were necessary (see Figure 1a). This solution was also applied for positioning magnet blocks in phase shifters [22]. More advanced designs of the magnet holders were introduced in ELETTRA's Apple II undulator [23]. Here, the magnets can be positioned in both horizontal and vertical directions using several bolts and pins (see Figure 1b). The Apple II-type undulators have a twin, movable magnetic array on each girder, making it impossible to access the alignment screws from both sides. Each Halbach array is equipped with a side positioning system that can align the magnets in both the vertical and horizontal directions and also offer partial tilt compensation of the magnet relative to the beam axis [24; 25]. The vertical movement of the magnet block can also be achieved by means of a sliding wedge [26; 27]. The rotation of the side screw is converted into the sliding motion of the wedge, which exerts pressure on the susceptible holder, thus causing the magnet to move. The disadvantage of this system is that it enables only vertical movement of the magnet block. A modification of this concept was proposed for the undulators used at the Aramis and Athos beamlines at SwissFEL [28; 29]. In this case, the magnet positioning system employs an aluminum holder that is susceptible to deformation (Figure 1c). The holder frame is deformed (raised or lowered) by sliding in or out of a wedge (part no. 1 in Figure 1c) controlled by a screw (part no. 2 in Figure 1c). The main requirement in this solution is that the walls responsible for movement must be suitably thin.