Sirius, the new synchrotron light source from the Brazilian Center for Research in Energy and Materials (CNPEM), is one of the three 4th generation storage-ring-based light sources currently in operation worldwide.

Sirius is based on a 3-GeV electron storage ring with 518 m circumference and bare lattice emittance of 250 pm.rad. The magnet lattice is composed of 20 five-bend achromat (5BA) arcs connected by 20 straight sections, of which 17 are for insertion devices. In the 5BA arcs, the center dipole (BC) is a permanent magnet superbend with a short high-field insert in its middle part, that deflects the beam by 1.1 degree, and reaches a peak magnetic field of 3.2 T. The optics configuration has been optimized so that this high-field insert helps to reduce the emittance while providing for 20 hard X-ray sources with critical photon energy of 19.2 keV. The hard X-ray radiation is produced only at the beamline exit and the needed RF power increases by a relatively modest amount. The remaining 4 dipoles in the arc are electromagnetic (0.6 T) and are of two types, B1 and B2, with the same magnetic field and transverse gradient, but with different lengths, in a way that helps to optimize the emittance. The facility can house up to 40 beamlines based on insertion devices or low field and high-field bending magnets of the lattice covering an energy range from infrared to hard x-rays.

Currently, Sirius is operating with a current of 100 mA in decay mode, with 2 injections per day. There are 5 commissioning planar APU undulators installed and 6 beamlines in operation in science commissioning mode, 5 are under different assembling stages and 3 in component fabrication phase. The stored beam current is presently limited by the provisory installed RF system, consisting of a Petra7-cell cavity with 120 kW of transmitter power available.

The critical path to achieving the nominal current of 350 mA are the installation of the final superconducting 500 MHz RF cavities, the production of solid-state RF transmitters to reach the needed power of 480 kW, the purchase of a superconducting third harmonic passive cavity (3HC) and the installation of the cryogenic system.

For several beamlines, the acquisition of undulators is in the critical path. The in-house development of Delta undulators has been delayed, and a new undulator baseline plan has been established including commercial undulator options. A study to define the best alternatives for each beamline is in progress. In parallel, the in-house development of Delta undulators continues.

By the end of Phase I, the project will deliver a facility comprising the 4^th generation storage ring, with 350 mA in top-up mode, 14 beamlines, support labs, and high-performance computing infrastructure. All facility instruments were optimized for cutting-edge agriculture, environmental science, health, and energy experiments, spanning diverse and strategic scientific programs.

Operation Statistics

Since March 2021, after the alignment of the accelerators, Sirius is operating regularly, with beamtime divided between machine study and beamline shifts.

In 2021, the time allocated to beamlines increased progressively over the year. In the beginning of the year, the machine studies required more time due to the new commissioning after the alignment of all accelerator girders. Over the year, a good reliability, defined as the delivered beam time to the experiments within programmed time, has been achieved. Except for March 2021 and March 2022, which are periods just after long machine shutdowns, the machine reliability was always higher than 92%. During this period, the mean time between failures was about 38 hours and the mean time duration of each failure, about 2 hours. There were 23 beam interruptions over this period, with a concentration in the beginning of 2021.

The presently measured lifetime at 100 mA is 17h, limited by the Touschek scattering effect. Experiments trying to measure the contribution of each effect to the total beam lifetime are underway.  For the Phase-I operation at 350 mA, the beam lifetime is expected to be kept >10h with the bunch lengthening promoted by the 3HC.

Beam Orbit Stability

The requirements for beam orbit stability for Sirius Phase-I has been initially set as better than 10% of the rms beam size in both transverse and longitudinal coordinates. This is, of course, subject to revision as specific stability requirements may depend on the sensitivity of particular experiments, and one of the major goals during the components design phase was to reach the highest reasonably possible stability of the electron and photon beams. This goal oriented the integrated design of girders, magnets, concrete plinths, accelerators tunnel floor slab, beamlines floor slab, building, feedback systems, BPM electronics, and so on. It is expected that tighter beam stability requirements will be possible to be met in the future, as perturbation sources are identified, and feedback systems are implemented and perfected.

The currently measured beam stability parameters show horizontal stability slightly above the goal of 10% beam size, and vertical stability of about 40% beam size. Since Sirius is operating with 3% emittance ratio, the beam size in the vertical direction is much smaller, and stability requirements are consequently tighter to achieve. The staff is currently working to identify the main sources of beam position instability. Key stabilizing mechanisms, such as the fast orbit feedback system and the top-up operation mode, which will guarantee a constant electron beam current and thus a constant heat load at the beamlines, are being implemented. Both mechanisms are project milestones to be reached in 2022.

Top-up Operation

Sirius operation in top-up mode at 100 mA has been established as a milestone to be achieved by the end of 2022. In preparation for a safe and reliable top-up operation, a series of activities were performed, and others are underway.

A major booster realignment was performed in the beginning of 2022 to match its circumference to the storage ring RF frequency, thus reducing the off-energy orbit of the injected beam from the Linac in the first turns in the booster at low energy. The beam survival rate increased from about 15% to about 70% after the booster circumference matching.

To improve the efficiency of booster to storage ring injection, ef_BTS, a study to implement emittance exchange in the booster is in progress with promising results. Other studies to improve ef_BTS are under study, such as the optimization of the nonlinear beam dynamics in the storage ring. The presently optimized value for ef_BTS is already at a satisfactory level of >95%, but the repeatability is still poor, constantly dropping the efficiency to <75%. The main cause was found to be related to the variation of the injection septa temperature. A larger dynamic aperture will also help in reducing the sensitivity of injection efficiency on the injected beam position and angle.

RF System

Sirius is presently operating regularly for users with initial current of 100 mA in uniform filling and decay mode, with two injections per day. The nominal operation mode consists of 350 mA in top-up mode. To increase the current, the RF system installation must be finished.

The current RF system consists of a temporary 7-cell room temperature PETRA cavity, built in the 1980s for the PETRA collider at DESY, Germany, that is driven by a 120 kW RF plant comprised of two 60 kW solid state amplifiers (SSA). This cavity has no HOM dampers and a careful optimization combining cavity operation temperature tunning and bunch-by-bunch feedback system parameters was performed.

In its final design configuration, the RF system for the storage ring will employ two CESR-B type 500 MHz superconducting (SC) cavities with 480 kW power available. The installation of the two SC cavities is expected to take place in the beginning of 2024, soon after the installation of the cryogenic plant.

The final RF design also contemplates the installation of a SC passive third harmonic cavity to be developed in collaboration with the Shanghai Synchrotron Radiation Facility (SSRF) and planned to be installed by the beginning of 2024.