August 11, 2000

 
 
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The Spallation Neutron Source under construction at Oak Ridge

 

BERKELEY, CA A major milestone in the development 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 -- has been achieved at Berkeley Lab.

A startup negative hydrogen ion source and low-energy beam transport (LEBT) system, the first two components of the SNS "front-end system," have been built and are now undergoing commissioning tests. Results so far could not have been better.

WHEN A HIGH-ENERGY PROTON BOMBARDS A HEAVY ATOMIC NUCLEUS, CAUSING IT TO BECOME EXCITED, MANY NEUTRONS ARE EXPELLED

The SNS project is a collaboration between Berkeley Lab, and Argonne, Brookhaven, Jefferson, Los Alamos, and Oak Ridge national laboratories. When construction at Oak Ridge in Tennessee is complete and it goes on-line (scheduled for the year 2006), the SNS will be capable of delivering an average of two 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.

The SNS generates its neutron beams through the combination of a linear accelerator (linac) and accumulator ring which result in the production of a pulsed beam of protons, energized to about one billion electrons volts (1 GeV), that gets smashed 60 times a second into a target of liquid mercury. This yields a pulsed beam of hot neutrons that is immediately cooled to room or even lower temperatures and directed into samples for non-destructive neutron scattering studies.

Each of the collaborating national labs has been delegated a specific responsibility. Berkeley Lab was made responsible for the front-end system based on the demonstrated expertise of the Ion Beam Technologies (IBT) program.

"Having recently consolidated this expertise into a single program, we were the natural choice to take on this challenge," says IBT program head, Rick Gough. "No one had ever demonstrated that the SNS front-end performance requirements could actually be met. A substantial level of research and development was required, and the results of these efforts are now paying off."

"We have met the time-schedule and are beating the requirements," says physicist Rod Keller, senior team leader of the SNS Front-End Group, which is a part of the IBT program under the Accelerator and Fusion Research Division.

On April 26, physicist Rainer Thomas and his team fired up their ion source/LEBT and successfully produced an intense beam of negative hydrogen ions (peak current of 46-milliamperes) at pulse lengths of 200 microseconds. The beam was measured in a Faraday cup at the end of the LEBT. Less than two months later, high current pulses of the required one millisecond length are being routinely achieved.

Says Keller, "Our ion source current exceeds the startup requirement for neutron production from the SNS facility and is a significant step towards fulfilling the conditions for full-power operation."

Ultimately, the SNS front-end system will consist of the ion source and LEBT, a radio-frequency quadrupole (RFQ) accelerator, and a medium-energy beam transport (MEBT) system. The goal is to accelerate a beam of negative hydrogen ions to 2.5 million electron volts (MeV) and deliver it to the linac. Negative hydrogen ions are utilized in the front-end system and the linac because they lend themselves to efficient injection into the accumulator ring. As the beam enters the ring, it passes through a foil that strips the negative hydrogen ions of their electrons -- converting them into positively-charged hydrogen ions or protons.

"This charge-exchange injection process is very efficient," says Keller, "but it can only be accomplished through the use of negative ions."

The negative ion source constructed by the SNS Front-End Group is based on the "volume production" technology first developed by Berkeley Lab physicist Ka-Ngo Leung as part of the magnetic fusion energy effort. With this technique, negative ions are produced through atomic processes taking place within a confined volume of plasma, rather than through a reaction with a metal surface. When tiny quantities of cesium are added to the plasma, such sources require less power to produce the needed density of negative ions and offer long-term, dependable service. However, because the LEBT is more positively charged than the ion source, it extracts both negative hydrogen ions and electrons from the plasma.

"A major challenge during the commissioning of the front-end ion source/LEBT is to find the most effective way to separate the electrons from the negative hydrogen ions so that only the ions continue to be transported," says Keller.

The Front-End Group appears to have solved this problem by using a magnetic field to "kick" the electrons out of the beam, and tilting the ion source at a small angle (about 3 degrees) relative to the LEBT in order to compensate for the slight deflection imparted to the ions.

In addition to accelerating and transporting the negative hydrogen ion beam, the LEBT is also required to chop the beam into mini-pulses of 645 nanoseconds duration with separations of 300 nanoseconds. This is necessary to allow the extraction-kicker magnet in the SNS accumulator ring to rise to its full strength. At that point, the accumulated beam can then be directed towards the spallation target without spilling the high energy ions on the vacuum chamber.

Learning how to best manage and operate the LEBT is a major task of the commissioning effort. Later this year, an RFQ accelerator, designed by physicist John Staples, will also be added to the system. A production front-end system, which will include the MEBT, is scheduled to be installed at Oak Ridge in 2002.

There have been more than 40 contributing members of the SNS Front-End Group. Other key persons on the project include project manager Ron Yourd, chief engineer Richard DiGennaro, and lead electrical systems engineer Alex Ratti, all with the Lab's Engineering Division.

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