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Maintaining
Outstanding Linac Performance
By Shawne Neeper
After its six-week shut down, the linear accelerator (linac) is back on
line and ready for another year of record-pushing luminosity. The
linac’s electrical, cooling and control systems stay in top form thanks
to a hard-working cadre of unsung heroes in Site Engineering and
Maintenance (SEM). This August, while physicists around the world
celebrated new discoveries in beta particle decays, the SEM utilities
and operations groups worked around the clock to complete a flurry of
upgrades and repairs during the all-too-brief downtime.
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Michael Strittmatter (SEM),
working on a control panel in Sector 20. (Photo by Diana Rogers) |
The downtime provides a rare window of opportunity to maintain systems
that are inaccessible during the linac’s near-continuous operation overthe rest of the year, explained Utility Support Group leader Forrest
Brown. His team is responsible for all high-voltage operations on site,
and they run the heavy digging equipment and dump trucks that service
campus utilities, from sewers to natural gas lines. Since the linac shut
down on August 1, the utilities group has worked in tandem with the
operations team, putting in long hours to complete a carefully-planned
schedule of maintenance and upgrades.
As part of preventive maintenance, utility support technicians combed
the length of the Klystron Gallery for potential electrical problems,
and made repairs to prevent unpleasant surprises. For example, by
pulling out the linac’s circuit breakers—each weighing several hundred
pounds—Brown’s team discovered a burned-up breaker serving linac sectors
zero, one and two, threatening power to the injector gun and damping
rings. Left alone, the breaker could eventually melt or catch fire. The
team replaced the breaker, and averted a potential shut down of one week
or more.
It is impossible to complete 100 percent of the desired work during each
downtime—even with the utilities group putting in long days and
weekends, and the operations group running three shifts per day, seven
days a week. “But we know what it takes to get the machine up and
running,” said Mechanical Operations Group leader Bernie Romero. His
team maintains the gas, air pressure and water cooling systems that help
keep the linac and research yard in business.
This year’s downtime projects included upgrades to Sector 20’s cooling
system pumps, valves, heat exchangers and instrumentation to boost
performance beyond previous standards. To update cooling systems for the
beam stopper—served by the massive, corrugated steel cooling tower that
dominates the research yard—they replaced original equipment with custom
designed pneumatic controls. They also updated the computer system that
monitors the accelerator, to help control the water temperatures for the
coming long run.
Keeping Cool
A jungle of pipes and valves fills a large gallery inside the linac
building at Sector 29. Nearby, several yellow, scuba-style tanks process
water through ion exchange resins to produce ultra-pure low conductivity
water (LCW). Green-tinted copper pipes labeled ‘LCW’ use the purified
water to carry heat away from the linac’s klystrons, wave guides and
accelerator structure. A 20 foot length of 18-inch steel pipe houses a
‘tube and shell’ heat exchanger that brings the hot LCW alongside water
from a nearby cooling tower. The system’s precisely-tuned mixing valve
adjusts continuously to ensure that the LCW returns to the linac at 113
degrees Fahrenheit. The constant temperature helps ensure a stable
accelerator beam.
As part of August’s downtime maintenance, Romero’s group flushed the
water systems in every sector of the linac and around the PEP ring with
a mild detergent to remove copper oxide and other build-up that could
hamper performance. Then they disassembled, cleaned and reassembled the
heat exchangers. As final preparation for linac re-launch, they
calibrated and tested the controls that will maintain temperature within
0.2 degrees Fahrenheit of optimal throughout the year.
It’s Under Control
In a small office within Building 35, instrumentation technicians Craig
Butler and Mike Strittmatter demonstrate the Distributed Control System
(DCS), which they and others use to monitor the accelerator and storage
ring systems. The DCS is the group’s first and best source of input,
according to Butler.
Strittmatter launched the DCS, and a map of the linac and PEP-II
appeared on his screen. With a click, he zoomed in to view temperature,
valve settings and other operations in one of the sectors. Another click
brought up a chart of cooling system function over time. A plot of
temperature over the spring months is smooth and steady, with only small
fluctuations. “[Linac scientists] ask for temperature control within
plus or minus a half degree,” Strittmatter said. “We aim for better.”
With linac systems newly checked, repaired and upgraded, Strittmatter
and his colleagues are ready to meet that aim.
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