On 12 October the National Synchrotron Radiation Research Center (NSRRC), Taiwan, became the fourth synchrotron facility in the world to operate fully in top-up mode, joining the Swiss Light Source (SLS), the Advanced Photon Source (APS) in the US, and SPring-8 in Japan. While the SLS and APS were originally designed to operate in top-up mode, the NSRRC is an example of how a third-generation synchrotron accelerator that previously operated in decay mode can successfully advance to full top-up operation.

In top-up mode, the storage ring is kept full by frequent injections of beam, in contrast with decay mode, where the stored beam is allowed to decay to some level before refilling occurs. Top-up operation has the advantage for light-source users that the photon intensity produced is essentially stable. This provides valuable gains in usable beamtime for experiments, and significantly shortens the time for optical components in beamlines to achieve thermal equilibrium.

The upgrade to top-up mode at the NSRRC, which started in 2003, included improvement to kickers, the addition of various diagnostic instruments, a redesign of radiation safety shielding, modification of control software, and a revised operation strategy for the injector and booster. In parallel a more powerful superconducting radio-frequency cavity was installed and commissioned in November 2004, as part of a five-year programme. This has prepared the NSRRC to serve its users in biology and genomic medicine.

The injection chain at the NSRRC consists of a 140 keV electron gun, a 50 MeV linac and a 1.5 GeV booster that sends the beam into the storage ring at a rate of 10 Hz. With the upgrade, the time interval between two injections is now set to 2 min, while previously, in decay mode, it was every 6 h. The stored beam current has initially been maintained at 200 mA with approximately 0.6 mA per current bin and photon stability in the range of 10-3 to 10-4. As experience is gained, the current will gradually be increased up to the 400 mA maximum allowed by the new superconducting RF in the storage ring.

As a user-driven facility, NSRRC chose the fixed time interval injection mode rather than fixed current bin to reduce interference with data-acquisition processes. Since early 2005, the operational division has informed beamline managers of the new characteristics of the beam's time cycle, injection perturbations and top-up status. Users thus have access to enough information to conduct their experiments successfully.

During the transition period, special attention was paid to finding a reproducible filling pattern through optimizing and fine-tuning a variety of parameters. Other tasks included mastering the timing jitters of injection components, launching position and angle, as well as understanding the horizontal acceptance of the ring. These are some of the key determinants of injection efficiency.

The overall programme, led by NSRRC director Chien-Te Chen, now allows students from more than 60 universities access to beamtime allocated on one of 27 beamlines. These include two at SPring-8 Japan that are owned by the NSRRC. The NSRRC itself supported more than 3000 user-runs in 2005, 20% more than in 2004.

Further reading

G H Luo et al. The Status of Top-Up Injection at NSRRC APAC'04 Proceedings Gyeongju, Korea.