Some are the size of a fist. A handful are bigger than a filing cabinet and weigh as much as 10 tons. Individually, they range in price from $500 to $50,000, with a collective value of $6 million. Nearly all have been in place for more than five years. But you won’t find many problems among the 2,200 magnets that steer and focus the JLab accelerator’s electron beam.

"The magnets are about as bulletproof as any system can be," says Leigh Harwood, senior staff scientist in the Accelerator Division. "They’ll do anything the power supply tells them to do. Magnets simply don’t require a lot of looking after. They get attention when something goes wrong, which is not often."

The dipole magnets force the electron beam around the arcs at each end of the accelerator. The electron beam travels around the accelerator up to five times - each time gaining more energy and requiring stronger magnets to bend the beam around the arc. The lower-energy beam is guided around the curve by the smaller dipole magnets, while the higher-energy beam is forced around the arc through the larger magnets at the bottom of the rack.

Magnet robustness owes much to their simple design: a copper coil wound around an iron core and connected to a power supply. They are classified by their two primary duties, that of steering and focusing. "Without the magnets, we don’t have a recirculating linac," Harwood points out. "Without magnets, we wouldn’t be able to deliver beam to more than one hall."

Despite variations in size, the magnets’ field strength is relatively modest, many are comparable to that emanating from old-style horseshoe or children’s magnets. There is little "fringe field" to speak of; the magnets concentrate their fields on the one-inch gap through which the electron beam travels, bending and consolidating the beam as required.

Upgrade Planning Continues

Accelerator magnets are included in the planning for the Lab’s upgrade in the coming decade to 12 billion electron volts, or 12 GeV. Although the overall number of magnets should remain roughly the same, field strength will be increased, requiring additional magnet iron.

As currently configured, the magnets have limited capacity to magnify the field generated by the coils. Adding iron will extend the range to which the field is directly proportional to current. "If the field isn’t proportional to current, then we waste power," Harwood explains. To further accommodate the upgrade, a number of the 1,800 power-supply units now in operation will have to be replaced, while others will be reused and reconfigured.

The quadrupole magnets are interspersed around the accelerator's beam line - keeping the electron beam focused or collimated.

The Accelerator Engineering Department has been performing computer modeling of magnets’ performance and validating the outcome of those studies with tests of prototypes. These tests have shown good correlation between calculation and reality, according to Harwood.

Magnet reinstallation shouldn’t be difficult. Technicians will simply unbolt the existing magnets from their brackets, install the new magnets and then reattach the mounts.

The next magnet milestone comes on April 15, when the next report on the upgrade will be released. This report will include details of magnet construction, performance, and cost that were not in the previous report.

"It’s a pretty straightforward task," Harwood says. "I don’t foresee any major problem. As with any upgrade, the schedule will be driven by the availability of funding."

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