The S-DALINAC was operated in a twice recirculating scheme since 1991. Both recirculations are equipped with so called path length adjustment systems. These systems enable the movement of dedicated magnets to change the length of a recirculation by some degree of the RF phase. These systems are used to perfectly adjust the phase of the beam re-entering the main LINAC for an optimized acceleration. In 2016 the S-DALINAC was upgraded by an additional recirculation. The path length adjustment system of this new recirculation is capable to change the phase of the beam up to 360° of the RF phase. With this upgrade the S-DALINAC can be operated as an Energy Recovery LINAC (ERL) (phase shift of 180°) which is capable of recovering the kinetic energy of the previously accelerated particles after usage by decelerating them again. An ERL is more energyefficient and can sustain higher beam currents then conventional linear accelerators. After finishing a complex modification (for details see below) the commissioning of the modified S-DALINAC as an ERL has started. At the moment it is the first German ERL under commissioning. The following pictures will illustrate the modification from a twice to a thrice recirculating set-up. As one of the first steps major parts of the existing beam line had to be disassembled (see Figure 1).
During the construction phase some dipole magnets had to be taken apart for the installation of new vacuum chambers (see Figure 2 as an example). Main parts of the installation were done under very limited spatial conditions (see Figure 3 as an example). At the end a lasertracker based alignment was done to ensure the positioning of all magnets (see Figure 4).
Figure 2: Installation of a vacuum chamber through several dipole magnets.
Figure 3: Installation of elements under very limited spatial conditions
Figure 4: Lasertracker based alignment of lattice.