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The SNS breaks the world record by ten-fold. As the Spallation Neutron Source passes the one megawatt milestone, one of the world's largest scientific facilities is restoring America's leadership in the field of neutron science. This success is made all the more impressive by the fact that, while the design and start-up of the facility have drawn on the experiences of its predecessors, the path to one megawatt has been an exploration of uncharted scientific territory.
"The SNS story begins more than two decades ago when the Department of Energy gathered a variety of scientific groups to think about the research tools that would be needed in the future," says Stuart Henderson, who heads the project's accelerator program. "One of the conclusions they reached was that a megawatt class pulsed neutron source would be critical to closing the gap between neutron research capabilities in the U.S. and those in the rest of the world." Henderson says that reaching one megawatt is significant on several different levels. "For researchers, the one-megawatt mark means delivering on a decades-old dream," he says. "In a more formal sense, reaching this threshold will fulfill the commitment we made when DOE agreed to undertake the SNS project." One-megawatt also represents a significant milestone on the way to the goal of a near ten-fold increase in power over international facilities that were state of the art when the SNS was proposed. When the SNS came online, the world's most powerful neutron source was the ISIS facility in the United Kingdom—running at a beam power of about 150 kilowatts. The SNS is designed for peak performance at 1.4 megawatts. "A quantum leap in performance is the kind of improvement funding agencies want to see with a proposed new facility," Henderson says. "They understandably don't want something that already exists. Agencies prefer to be at the clear forefront of science, which often means performing at a factor of 10 better than your predecessors." Since the first neutrons were generated in 2006, SNS scientists have been involved in a long and complex process of slowly increasing the facility's beam power. "Our initial goal was a modest 60 kilowatts," Henderson says. "That sounds low, but at the time it was a third of the power of the world's biggest pulsed neutron source." By August of the following year, the SNS was producing a 180-kilowatt neutron beam, surpassing the power of ISIS and entering the Guinness Book of World Records as the world's most powerful neutron source. Breaking the world record was one of several occasions when the excitement generated by reaching a milestone was tempered by the size of the task ahead. SNS scientists who thought getting to 180 kilowatts had been hard wondered aloud about the challenge of going all the way to 1.4 megawatts. The anxiety was well-founded. As the SNS runs at increasingly higher beam power, scientists have encountered phenomena never before seen and not entirely understood. As a result, a graph of the SNS's beam power since 2006 trends upward, but the upward slope is punctuated by dozens of peaks and valleys. "In the valleys we would be flummoxed by problems that kept us from making any progress," Henderson says. "Sometimes we would increase power and decide something wasn't quite right, so we would have to take a step back to address the problem. Each jump in the graph represents a point at which we reached an understanding of a limitation we were encountering well enough to fix it and move forward." As an example of this process, Henderson recalls that, in early to mid-2008, beam power was limited because beam particles were being "lost" on the walls of the accelerator in the area where the beam is injected into the accumulator ring. After diagnosing the problem, new hardware was designed, built and installed to provide more space and enable better steering of the beam. After this change, beam power nearly doubled in the following four-month period. The accelerator team's goal for the near term of achieving one megawatt of beam power was reached in the fall of 2009. To achieve this jump in beam power—a jump equal to the previous world record—one of the tactics Henderson's group used was to increase the length of the accelerator's pulse—or burst of particles—from 600 microseconds to the design level of 1000 microseconds. The result would sent 40 percent more particles to the target with every pulse. The new strategy pushed the SNS past the megawatt level, within sight of the 1.4 megawatt beam the facility was designed to produce. As the SNS operates at increasingly high beam power, the accelerator complex must deliver reliable and stable beams for the user program. Henderson views the process as a balancing act. "We must deliver reliable beams for users while simultaneously exploring new ways to increase the beam power. When these two goals conflict, reliable operation takes precedence." Since its inception, the SNS has been defined by long-term plans. With approximately one-half of the instruments in place in the target building, planning is already under way for both a power upgrade and a second target station that would double the facility's potential research output. "For the power upgrade, we are proposing to double the SNS beam power," Henderson says. "The promise will be to deliver two megawatts of power. But we are designing a system that should be able to produce three. We hope to start the project next year." The success of the SNS in achieving its power goals has been a research windfall for scientists, enabling them to run more experiments on a broader variety of materials, gather larger volumes of data and explore more physical and statistical detail than ever before. Henderson is emphatic about the scale of the effort required to make the achievement possible. "A lot of people appear to view the SNS as a sort of microwave oven with three buttons, for low, medium and high neutrons," Henderson says. "They seem to think we can just set it to high and turn it on." The reality is far different. Thus far, the SNS has succeeded, not simply because it is well-designed but because it is operated by about 250 highly trained engineers, physicists and technicians who constantly monitor and tune one of the world's most complex scientific instruments.
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