## 2 Bypass Line Design Requirements  
The synchrotron SIS100 is divided into six sectors (see Figure 2), each composed of an arc section containing a number of superconducting magnets and a straight section containing three superconducting quadrupole magnets (QDMs). Between the QDMs of the straight sections, warm accelerating components are located. Thus, electric current and liquid helium need to be bypassed at the warm components. The single bypass line consists of five process pipes and four pairs of superconducting busbars. The SIS100 ring contains six bypass lines with a length of about 50 m each, connected hydraulically and electrically to the magnets. It is designed to bypass warm gaps between magnets on the straight sections of SIS100 and to provide electric current and liquid helium to arcs sections and quadrupole magnets. The total length of the bypass lines is equal to about 300 m.  
Figure 1: The layout of the Facility for Antiproton and Ion Research (FAIR; **left**) and a bird’s eye view of the construction place (**right**). Electric power and liquid helium are provided to sections of the bypass line by short feed-in lines from three feed boxes, one per two sectors. The electric power is transferred from feed boxes by the NbTi superconducting busbars, cooled by a two-phase liquid-vapor mixture of helium. The required helium mass flow adjustment is done by isenthalpic throttling in a capillary [8]. A cross-section of the NbTi superconductor is shown in Figure 3. The outer superconducting wires are cooled by the helium flowing through the central CuNi tube. Details of the cable structure are given in [9]. The main innovative feature of the cryogenic bypass line is transferring the electrical current and helium in a single vacuum insulation vessel. In existing transfer systems, those functions are usually separated. The standard transfer line contains only hydraulic process pipes [11]. The coexistence of busbars and helium process pipes in one vacuum vessel allows the reduction of the number and overall size of the required vacuum vessels, which is very important in the limited space of the SIS100  
Figure 3: Cross section of the NbTi superconducting cable for the busbars of the SIS100 synchrotron [10]. Figure 2: Schematic representation of the SIS100 accelerator showing the six bypass line (BPL) sections. The cold arc sections and BPL sections are for clarity and are not shown in proportion to each other. FB1, FB3, FB5—feed boxes. tunnel. The space restrictions are more pronounced in the second accelerator, SIS300, which is planned to be located in the same tunnel, above SIS100 [12]. The magnets of SIS100 will be ramped up with an electrical current of up to 13.2 kA and a frequency of up to 1 Hz. Four busbar pairs supply the electrical current independently to the different magnet circuits, all of which need to be integrated in the BPL. Due to the significant total BPL length of 300 m and usage in SIS100 of the independently controlled fast-ramping magnets, the presence of the cross-talk between busbars pairs and parasitic capacities can affect the quality of the magnetic fields [13; 14]. The layout of the BPL cross-section was determined by maximizing the distance between the busbars pairs. One of the requirements for the magnetic system is the extremely high precision of the magnet alignment on the whole 1100-m-long ring of the SIS100. The high precision of the magnet's cold structure position must not be affected by pressure forces or by thermal contraction stresses from the BPL. Therefore, the next significant feature of the BPL is to provide helium at a pressure of up to 18 bar(a) to the magnets without exerting any significant forces on the magnet's cold structure. Simultaneously, as the magnets will be installed and positioned after BPL installation, the BPL design should allow for the adjustment of the magnet cryostats within a range of \(\pm\) 15 mm in any direction. The next feature of the BPL is the partitioning of the SIS100 vacuum space.