February 15, 2002
Berkeley Lab Science Beat Berkeley Lab Science Beat
The Front End of the Spallation Neutron Source: Berkeley Lab sets the pace
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Faced with a deadline less than four months away, scientists and engineers with the Lawrence Berkeley National Laboratory cleared a major hurdle for completing their part of the Spallation Neutron Source (SNS), the accelerator-based facility that will provide the most intense pulsed-beams of neutrons ever available for scientific research and industrial development. On Friday, January 25, at around 4:30 p.m., they drew the first ion beam out of the radio-frequency quadrupole (RFQ) accelerator, which is the third and perhaps most technically challenging of the four components in the SNS front-end system.

"The 2.5 million electron volts [MeV] beam came out at a current of 24 milliamps [mA] on the very first try!" said Rick Gough, the physicist who heads the Ion Beam Technology program for Berkeley Lab's Accelerator and Fusion Research Division (AFRD). "A full characterization of the beam with peak-currents up to 33 mA is now underway. This current is already sufficient to support operation of the SNS at 1.2 MW average power."

Leaders of the SNS Front-End Group include (from left) Rod Keller, John Staples, and Alex Ratti

Said Rod Keller, the physicist who serves as senior team leader of the SNS Front-End Group, "To save costs, we had to make a production model and test it without the advantage of first working with a prototype. This is risky business with an RFQ accelerator because you never know what you have until you test it. Either your accelerator works or it doesn't, and in this case it worked extraordinarily well!"

The design of the SNS RFQ was led by physicist John Staples, who also led the design of the four previous RFQs built at Berkeley Lab.

"At nearly four meters in length, this is the longest RFQ we've ever built, which presents a big challenge in terms of field stability and tuning," says Staples. "This RFQ also had to handle the most powerful beam at a higher current and operate at a duty factor (the fraction of system operational time in which a beam is actually produced) about 60 times greater than any we've built before."

"That all of these challenges were so successfully met is a credit to the team effort that has characterized the work of the Front-End Group throughout this project," says Gough.

With the successful test of the RFQ the 40 scientists, engineers and technicians who comprise the SNS Front-End Group successfully completed the third stage of their four-stage assignment. In the spring of 2001 the group commissioned the negative hydrogen ion source and low-energy beam transport system. The fourth and final component of the SNS front end, the medium-energy beam transport (MEBT) system, has already been built and will be attached to the RFQ for testing later in the spring of 2002. When testing is complete, the front end's components will again be separated for shipment to Oak Ridge National Laboratory (ORNL) which will be the host institute for the SNS.

The SNS project is a collaboration between Berkeley Lab, ORNL, and the Argonne, Brookhaven, Jefferson, and Los Alamos national laboratories. When construction at ORNL is complete and it goes online (scheduled for the year 2006), the SNS will be capable of delivering an average of 1.4 million watts of neutron beam power onto a target, more than 10 times the capacity of today's most powerful pulsed neutron sources. The scattering of these neutrons when they strike experimental samples will reveal to scientists and engineers the most intimate structural details of a wide range of materials.

With Oak Ridge National Laboratory, where the Spallation Neutron Source is located, Argonne, Berkeley, Brookhaven, Jefferson, and Los Alamos national laboratories are building the facility's major components.

The SNS generates its neutron beams through the combination of a linear accelerator (linac) and accumulator ring, which result in the production of a one-microsecond long pulsed beam of protons, energized to about one billion electrons volts. This yields a pulsed beam of hot neutrons that is immediately cooled to room or even lower temperatures and directed into samples for nondestructive neutron scattering studies.

Each of the partner labs collaborating on the SNS was delegated a specific responsibility. Berkeley Lab's responsibility, the front-end system, is required to create a beam of negative hydrogen ions and prepare it for delivery into the SNS linac. Negative hydrogen ions are accelerated in the front-end system and the linac because they lend themselves to efficient injection into the accumulator ring. As they are injected into the accumulator ring, the hydrogen ions are stripped of their electrons and converted into positively charged hydrogen ions (protons).

The first two components of the front-end system, the ion source and the low-energy beam transport, combine to create a continuous beam of negative hydrogen ions. It is the role of the RFQ to prepare these negative ions for injection into the SNS linac by breaking the beam into discrete bunches at radiowave-sized pulse lengths, then accelerating those pulses to 2.5 MeV while simultaneously keeping them focused in a narrow beam.

The SNS RFQ consists of an encased cavity containing four strategically modulated copper electrodes or vanes between which radio-frequency electric fields are applied. These fields act on the beam both longitudinally and transversely to first establish the bunch structure, and then (due to increased depth of the vane modulations) accelerate the bunched beam to the full energy of 2.5 MeV.

Water-cooled copper vanes of the SNS radio-frequency quadrupole focus and accelerate bunches of negative hydrogen ions.

Because of the quadrupole arrangement of the vanes, these fields also overcome the effects of space-charge forces that would otherwise cause the beam's negatively charged ions to fly apart.
Careful temperature control of the RFQ structure is required to keep things properly working as the radio frequency power used to manipulate the ions also heats up the inner surfaces of the RFQ cavity. Usually, thermal expansion effects are compensated by moveable mechanical "tuners," but Steve Virostek of Engineering Division (ED), the lead mechanical engineer for the RFQ system, working with ED designer, Matt Hoff, came up with a unique water-cooling scheme that is far more cost-effective and also boosts the system's reliability.

"The design decisions to achieve dimensional stability of the RFQ during startup conditions and normal operating conditions were based mainly on Steve's analyses of the transient heat loads and heat distribution," says ED's Dick DiGennaro, chief engineer for the front-end system.

A series of tiny water channels run through the RFQ's cavity wall and copper vanes. The water temperature in the wall is held at a constant 24 degrees Celsius, while the water temperature running through the vanes is varied as needed.

Explains Virostek, "This cooling scheme, along with 80 pre-machined, fixed slug tuners has proven to be a reliable and effective solution to tuning the SNS RFQ. The cost savings as compared to a moveable tuner system is as much as $200,000 which is a significant fraction of the total RFQ fabrication cost."

The beam generated during the RFQ testing was directed into various devices for diagnostic purposes. When beam testing and characterization are completed, the RFQ will be shut down for installation of the remaining MEBT parts. The role of the MEBT is to create short gaps in the beam by chopping it into mini-pulses of 645 nanoseconds duration with separations of 300 nanoseconds in order to facilitate the beam's ultimate extraction from the SNS accumulator ring.

"The completion and successful testing of the SNS RFQ has been a huge milestone for us and a terrific accomplishment for Berkeley's Front End team," says Gough. "As we approach the final countdown, I'm pleased to say that we're still within budget and on schedule to begin shipping to ORNL in June."

In addition to those already mentioned, other key members of the SNS Front-End Group include project manager Ron Yourd and lead electrical systems engineer Alex Ratti.

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