The PETRA accelerator on the DESY site in Hamburg is now operating as the most brilliant storage ring based X-ray source worldwide.
Figure 1 shows an aerial view of the almost 300 m long experimental hall that runs along one octant of the 2304 m long PETRA storage ring on the DESY site. The construction phase started in July 2007 and ended in June 2008. This event was celebrated by handing over the key for the experimental hall on the same day (Fig. 2).
The construction of the PETRA III hall posed several challenging tasks, including the fabrication of the world’s longest single-slab concrete plate and the production of 99 sleeved concrete piles that reach 20 m deep into the earth to carry the outer shell of the experimental hall. These measures ensure a nearly vibration free experimental environment on the 7000 m2 large experimental floor that provides space for 14 beamlines and 30 experimental stations. Here, some of the most brilliant beams of hard X-rays world wide are provided with a brilliance exceeding 1021 ph/(s mm2 mrad2 0.1% BW).
The PETRA III project is the third reincarnation for the PETRA storage ring, which was built as an electron-positron collider in the 1980s and later became a pre-accelerator for the proton-electron collider HERA. The overall budget of the PETRA III project was €225 million, shared between the German Federal Government (90%) and the City of Hamburg (10%).
The particle energy of the storage ring was chosen to be 6 GeV which is a compromise between achieving a small horizontal emittance and providing high photon flux in the energy range of 50eV–150keV. The beam current is initially limited to 100 mA, however, all components handling heat load or dealing with radiation safety have been designed for a current of at least 200 mA in order to leave room for further upgrades.
The conversion of the PETRA machine into PETRA III comprised the complete rebuilding of one octant of the 2304 m long storage ring and the modernisation and refurbishment of the remaining seven octants. In this process all the accelerator components of the storage ring had to be removed from the tunnel, an operation that was achieved in only three months. Within one year more than 600 dipole, quadrupole and sextupole magnets were refurbished, most of them received new coils and were magnetically characterized. They were then reinstalled together with a new accelerator vacuum system, the last dipole having been back in place in May 2008.
PETRA III operates in top-up mode, i.e. the storage ring current is kept constant to within 1% or less via frequent injections of particles. Since the time between top-up injections can be as short as 60 s for a 40 bunch filling pattern, a very high availability of the injector and pre-accelerator systems is mandatory which required significant refurbishments of these systems, too. The new light source has a horizontal emittance of 1.0 nm rad being 3 – 4 times smaller than that of other high-energy (above 6 GeV) storage rings world-wide. This was achieved by installing 20 damping wigglers, 4 m long each, in two of the long straight sections of the ring (West and North). The radiative power loss of the beam in these wigglers damps the horizontal motion of the stored particles and thus reduces the horizontal momentum spread of the photon beams.
The vertical beam parameters are close to the diffraction limit and hence are very similar to other high-energy 3rd generation sources. The major improvement provided by PETRA III therefore concerns the horizontal emittance. Figure 3 shows one of the straight sections in the tunnel that is equipped with damping wigglers which were developed and produced by the Budker Institute in Novosibirsk, Russia.
Except for the additional damping wigglers, the magnetic lattice, i.e. the arrangement of quadrupole and dipole magnets, in the original seven octants remained unchanged. In the new octant, however, the magnets are arranged in a so-called Chasman-Green lattice which is optimized for synchrotron radiation sources. In this section, the magnet structures are mounted on separate girders and have been precisely pre-aligned before installation in the tunnel.
The magnetic structure in the arc of the new experimental hall comprises nine straight sections allowing the installation of either one 5 m or two 2 m long insertion devices (ID). In order to split the beam of two 2 m IDs into independent beamlines, they are inclined relative to each other by 5 mrad. Five straight sections accommodate two 2m long undulators and three a 5 m long undulator resulting in 13 independent beamlines. The ninth straight section is considerably longer and allows the installation of an ID up to a length of
20 m. Currently two 5 m long modules of the 20 m undulator are installed in this section. Similar to the ESRF there is the option to choose between a low or a high value of the β-function in a straight section of the new octant. Changing the β-function in a straight section is possible in a short shut down time (service week) of the machine.
The beamline frontends, i.e. the beamline components located in the ring tunnel, serve to condition the photon beam for the downstream experiments. Here the beam cross section is defined by high-power slit systems capable of withstanding the considerable heatload of the direct undulator beams. Moreover, the frontend comprises elements like collimators and primary beamshutters to fulfill radiation safety requirements. All frontend devices were designed to be accommodated in a very compact arrangement on granite girders (Fig. 4).
The arrangement of the beamlines and hutches on the experimental floor is schematically shown here. Due to the relatively large bending radius of the storage ring, the angular separation of the beams is small and hence the space between neighbouring beamlines is rather confined leaving only narrow aisles between hutches. As a consequence, there is no space for additional bending magnet beamlines in-between.
The space limitation is even more stringent for the two beams coming from a pair of canted undulators. In these cases a larger geometric separation is achieved by specially developed large offset monochromators or horizontal mirrors..
First installations on the experimental floor began right after it was officially handed over to DESY in April 2008, starting with the placement of the concrete shielding blocks for the ring tunnel, followed by the installation of optics enclosures for the beamlines in mid-July and setting up of the first lead hutches for end stations in August 2008.
The beamlines are organized in nine sectors which correspond to the straight sections in the new octant of the storage ring. Beamlines in sectors 3, 5, 7 receive radiation from a 5 m long undulator (10 m device for sector 1), the other sectors host two independent beamlines each attached to a pair of 2 m long canted undulators. The techniques and instruments were selected by an international advisory board based on proposals as part of the technical design report and make specifically use of the high brilliance of the PETRA III beam.