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SPEAR3 ‘Breathes’ in Response to Temperature Changes
By Matthew Early Wright
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.
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SPEAR3 team
members in the control room. Shown back row, left to right: Clemens
Wermelskirchen, Jeff Corbett, Laurent Nadolski (Soleil Light Source,
France), James Safranek, Bob Hettel, Fernando Rafael, Helmut Wiedemann,
Harvey Rarback, Greg Portmann. Shown front row, left to right: Nadine
Kurita, Stephanie Allison, Piero Pianetta (all SSRL unless otherwise
noted). (Photo by Keith Hodgson) |
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/
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