SPEAR3 Injector Beefed Up for Frequent Injection Regimen
Work at the SPEAR3 storage ring during the annual downtime involved projects both large and small to support the continued use of frequent-injection mode. Frequent injection is necessary to support SPEAR3's goal of reaching its full design current of 500 milliamperes—and improved data collection for researchers using the Stanford Synchrotron Radiation Lightsource's bright X-rays. But frequent injection has challenges of its own.
The SPEAR3 injector, which injects electrons into the storage ring, was commissioned in 1990 and was designed to operate only a few hours per day with frequent operator intervention to maintain approximately constant performance. Frequent injection requires the injector systems to cycle on and off every few minutes with no operator intervention. Not only must the injector perform reliably, every aspect of its performance must also be reproducible.
"[The injector] has to turn on and off like clockwork," said John Schmerge, head of the SPEAR3 Accelerator Division at SSRL. "If you can't turn it on and off and get it back to the same spot, nothing works."
These twin requirements of injector reliability and reproducibility guided the goals for this year's shutdown activities, which targeted the SPEAR3 injector systems to help improve performance. Data collected during the final months of the 2010 user run, during which SPEAR3 operated in frequent-injection mode for the first time, guided improvements to the systems.
"In the two months of frequent injection, we learned some of our vulnerabilities, and we concentrated on them," Schmerge said.
According to Ben Scott, SPEAR3 mechanical systems supervisor, the biggest job was probably the linac-to-booster alignment. The LTB is a short section of transport line that connects the SPEAR3 linac, which provides the initial acceleration to the electrons used to generate X-rays, to the booster ring, which boosts the electrons up to their final energy before they're injected into the SPEAR3 ring. The LTB houses three dipole magnets and six quadrupole magnets to guide the electron beam, but during the past year it was discovered that the electron beam could not be centered on the LTB quadrupoles and the injector was unusually sensitive to the magnet strengths.
To address these problems, the magnets required positioning adjustments of up to three-eighths of an inch. Proper alignment will ensure that electrons can enter the booster on the correct trajectory and is expected to significantly improve the reproducibility of the injector.
Also, a temperature-controlled rack was installed to help controll small, temperature-dependent variations in linac performance that had been evident over the course of a day.
The SPEAR3 radio frequency cavity, in which the circling electrons are given a boost of energy, was also inspected for signs of arcing or debris that might explain beam instabilities observed at high currents. No problems with the cavity were evident but some nearby vacuum components were removed or replaced.
And don't forget the annual ring alignment and power supply improvements. "A lot of time was devoted to standard maintenance," Schmerge said. Another project included installation of a high voltage power supply from the PEP collider which was moved to SPEAR3 to act as a spare.
Although Schmerge cautions that work remains to be done during next year's shutdown, he said, "We believe we're on the path to 500mA" during this run.
—Lori Ann White