September 17, 2004  


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.

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. 




The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Wednesday September 15, 2004 by Emily Ball