SPEAR3 'breathes' in response to temperature changes
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As the sun rises each day, warming the grounds and buildings of SLAC, the entire SPEAR3 facility expands in response. The change is minuscule, on the scale of a few microns—far too slight to observe with the naked eye. This expansion doesn’t escape the watchful gaze of the SPEAR3 feedback regulation system. In fact, the system responds by ‘breathing’ in time with daily fluctuations in temperature.
As the infield area and the shielding tunnel of SPEAR3 heat up and expand radially, the lattice of magnets that keeps the beam focused expands with it. Due to the megawatt RF system the beam stays put, becoming slightly displaced in relation to the magnets. In order to stay centered within the magnets, the beam also has to expand in circumference.
An array of sensitive beam position monitors (BPMs) keeps an eye on the displacement of the beam. As they move outward, they relay information to the feedback system.
“The BPMs signal that the beam is not where it’s supposed to be,” explained Jeff Corbett (ACP). “The beam circumference is set by radiofrequency, so the feedback system adjusts the radiofrequency to keep the beam centered in the BPMs.”
The feedback signal from the BPMs cycles every six seconds. While the sun is rising and SPEAR3 is expanding, the radiofrequency drops by about half a hertz per cycle. Then, as temperatures begin to cool off in the early afternoon, the radiofrequency rises again as the building contracts.
Recent plots of the daily frequency shift confirm the pattern. Corbett expects to see a similar effect in response to the change in seasons over an annual time scale.
“We suspected this was happening with SPEAR2, but couldn’t see it,” Corbett said. SPEAR2 experienced more irregular temperature fluctuations than SPEAR3, largely due to gaps in the thick concrete shielding. This made it difficult to discern a pattern in radiofrequency fluctuations.
“The effect is roughly proportional to the circumference of the machine,” Corbett explained. With a bigger machine, a bigger shift in frequency is generally observed. The LEP ring at CERN and the APS in Chicago have even been observed to breathe in circumference due to lunar gravitational effects, according to Corbett.
SPEAR3 recently celebrated its first year of operation. “Relative to SPEAR2, it is performing orders of magnitude better in terms of reproducibility, stability and small spot size,” Corbett said. “It has run like a champ.”
For more information, see: http://www-ssrl.slac.stanford.edu/spear3/