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Top Off Injection

Over the course of a workday, synchrotrons—much like people—lose energy. When the current within the SPEAR3 storage ring falls below a certain level, accelerator operators must replenish that energy to keep beamline users happy and experiments running smoothly.

As it happens now, with SPEAR3 still operating at 100 mA, replenishing the current in the storage ring is a process that takes several minutes. Every 8 hours, when the current falls to around 85 mA, a loss of about 15 percent, users are required to shut their beamlines while current is pumped back into the storage ring. Each time these beamlines are closed, powerful x-rays are removed from the mirrors and other optical equipment used to focus and shape the x-ray beam. When the beamlines are reopened, these devices are walloped with the full current intensity, and the strain of going from zero to full power puts stress on these delicate instruments that upsets the precise beam stability needed by the users.

That stress is not much of a problem as long as SPEAR3 is operating at 100 mA. But when SPEAR3 begins operating at the full 500 mA it was designed for, opening and closing the beamlines will cause a much higher level of beam instability as they are zapped each time with five times the power they receive now. Complicating matters, at 500 mA the drop in current over 8 hours will be much steeper, falling as much as 40 percent to 350 mA.

To solve these problems, synchrotron engineers and physicists are developing protocols to replenish the storage ring at much shorter intervals with a process called "top-off injection." The first step in implementing this technique will be replenishing the beam with the beamlines open, which will relieve much of the on–off strain on the optics. Engineers and physicists are currently assessing the safety of this approach in order to be completely certain there is no danger of the electron beam from the storage ring traveling down the x-ray beamlines as new beam is delivered.

"Once we have the go-ahead to inject new beam without having to close the beamlines, it opens up the world to inject more frequently," said Bob Hettel, head of the SSRL Accelerator Systems Department. His group hopes to complete this phase within a year.

Eventually SPEAR3 will be able to replenish the beam as frequently as once a minute, keeping the current close to constant all the time. This too will present new challenges to users, who will have to synchronize their data acquisition to cancel out the effects of beam injection. But the payoff will be the improvement to the beam's ability to hold current, which will fluctuate less than one percent once top-off injection is fully implemented. According to Hettel, the last phase of preparations should be complete in about 3 years.

—Brad Plummer
   SLAC Today, July 7, 2006