The PETRA III extension

The 3rd generation low-emittance 6 GeV PETRA III facility has been substantially modified to add new beamlines. This extension project involved the complete removal and reconstruction of part of the storage ring with a double-bend achromat lattice as well as the construction of two new experimental halls. Making use of two long straight sections and a canted undulator scheme in the rebuilt arcs, up to ten new insertion device beamlines can be accommodated. In the current phase of the project, seven insertion device and one bending magnet beamline are being implemented.


OVERVIEW
PETRA III is a low-emittance 6 GeV storage ring having evolved from the conversion of the 2.3 km PETRA accelerator into a 3 rd generation light source since 2007 [1]. Today, a total of 14 undulator beamlines with 15 experimental stations operating in parallel for high-brilliance techniques are in user operation in the Max-von-Laue experimental hall which covers 1/8 of the storage ring. In order to extend the experimental capabilities with more beamlines, a major reconstruction started in February 2014 adding two smaller halls in the north and the east of the present Max-von-Laue hall by making use of two long straight sections and part of the adjacent arcs ( Fig. 1) [2].
The northern straight section already accommodates a series of damping wigglers producing an extremely hard Xray beam while the eastern straight is available for additional insertion devices (IDs). For the implementation of ID beamlines also in the original arcs, the machine lattice had to be modified to allow for suitable straight sections. Due to the large machine radius, the use of the arc sections for X-ray bending magnet beamlines is not feasible. During the reconstruction, the existing ring tunnel at the site of the new buildings had been completely removed and rebuilt as part of the experimental halls. In the east, the tunnel lies below ground level resulting in a complex reconstruction scenario. After eleven months of shutdown, the rebuilt machine had been re-commissioned for operation of the existing beamlines while in parallel the completion of the new halls and installation of new beamlines continued [3]. The initial project phase comprising the implementation of seven new X-ray ID beamlines and one VUV bending magnet beamline will be completed in 2018.

STORAGE RING RECONSTRUCTION
In order to allow for the use of insertion devices in the northern and eastern arc sections, the existing cell structure (with 5.3 m long dipoles) has been partly replaced by double-bend achromat (DBA) cells using 1 m and 0.5 m dipoles. Similar to beamlines in the Max-von-Laue hall, the straight sections between the DBA cells are shared by two independent 2m long IDs by using a canting dipole in the middle of each cell. For the PETRA III Extension, a canting angle of 20 mrad was chosen to allow for sufficient space downstream for photon science instrumentation. The new lattice configuration with two DBA cells is shown in Fig. 2 for the northern arc. A high-β optics was chosen for all ID sources. The lattice modification implies a change in the synchrotron integrals I 2 and I 5 resulting in a 20% emittance increase. The measured horizontal emittance after storage ring re-commissioning was 1.2 nmrad, the coupling being 1%. The main machine parameters are summarized in Tab. 1. Two 60m long arc sections of the storage ring, adjacent to the long straights, were completely reconstructed. In order to accommodate the beamline front ends, 80m long segments of the concrete tunnel were demolished and later rebuilt as part of the new experimental halls on a common thick concrete slab together with the canted beamlines. Both straight section IDs, however, remain on the original, separate concrete foundation.
The damping wiggler section was left unchanged, only the end piece of the high-power photon absorber will be replaced to extract a 3x2 mm 2 cross section of the on-axis photon beam. The downstream part of the eastern long straight section (total length 108m) will be modified to realize two straights of 5m length with β hor = 20m and β vert = 3.5m for use of a 4m long in-vacuum undulator and a canted (1mrad) 2m long side station ID. The new ID vacuum chambers (extruded NEG-coated aluminum profiles with 7 mm aperture) allow for a minimum magnetic gap of 9.5 mm.
The impedance increase is about 40%, which is expected to be compatible with 40 bunch mode operation at 100 mA total current.

INSERTION DEVICES AND BEAMLINE FRONT ENDS
In the northern straight section, beamline P61 will use the existing damping wiggler (DW) structure as a source consisting of ten 4 m long fixed-gap wigglers with K max = 28.4. The eastern long straight section will be equipped with a short period (λ U =21.2 mm) in-vacuum undulator (4m magnetic length) optimized for high energy applications at beamline P21. It will be used mainly above its 5 th harmonic of 41.5 keV. In addition, the side station at this beamline will be fed from a 2 m long U29 undulator in the canted straight.
Spectroscopy beamlines P22 and P64 will be equipped with 2m long U33 undulators with period lengths of 32.8 mm. With K max = 2.7 these devices will reach a minimum photon energy of 2.3 keV in the 1 st harmonic and are fully tunable. Spectroscopy beamline P65 will initially use a compact, water-cooled double-crystal monochromator (DCM), which had been very successfully used at DORIS III bending magnet beamlines. This device may not be exposed to the heatload of a long undulator, instead a short U33 with 10 periods will be used initially. Since its magnetic design and mechanical support structure are identical to the respective 2m device, a later upgrade is easily accomplished. The main characteristics of all IDs used at the PETRA III extension beamlines are summarized in Table 2 [4,5]. For comparison, the power of the damping wiggler section has been calculated through a 1x1 mm 2 pinhole at 40 m behind the last wiggler.
The design of the new beamlines is based on the PETRA III generic beamline concept [6,7]. All frontend components have been pre-assembled and aligned on girders for precise and timely installation during the reconstruction (Fig. 3). The canted beamlines are crossing the tunnel concrete shielding at 50 m from the source

BEAMLINES
In the current project phase, seven insertion device and one bending magnet beamline are being implemented, whereof three ID beamlines are realized in collaboration with international partners. Four ID beamlines for hard Xray techniques are being built in hall east (P21-P24) and three in hall north (P61, P64-65) while three beamports (P62-63, P25) are left open for future options (beyond 2018). A bending magnet beamline (P66) for VUV spectroscopy is located in a separate enclosure outside hall north. Beamlines P22-23, 64 will be equipped with an improved version of the PETRA III LN 2 -cooled DCM [8]. Beamline P64 will receive an additional cryo-cooled fast scanning channel-cut monochromator for time-resolved X-ray absorption spectroscopy. Table 3 summarizes the main characteristics of the new beamlines, which will be implemented in three stages. Stage 1 beamlines P64-65 for XAFS applications are planned to start commissioning in Sept. 2015.