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STABLE, BRIGHT X-RAY BEAM PROVIDES BETTER DATA
"From a managerial standpoint, stability is absolutely the most critical factor that I worry about," said John Quintana, associate director of the APS Operations Division. "The time that an experimenter spends at the APS is extremely valuable, and they are dealing with difficult scientific problems. If the storage ring and the optics are not stable, then the experimenters are worrying about the beam in addition to the experiment. "If they see a 'unique' result or a problem," he said, "then they need to know that it is due to their experiment and not the X-ray beam. A stable beam means that the conditions are controlled and therefore repeatable and reliable." The APS is a Department of Energy, Office of Basic Energy Sciences facility that in the last year supplied more than 5,000 hours of X-ray research time to more than 3,000 researchers from across the world. Researchers may come from universities, industry or other national laboratories, but they all seek to understand the molecular structure of matter for basic and applied research in materials science, biology, physics, chemistry, geophysics, or environmental science. The APS was originally built with a strong focus on materials science research, but with funding to users from the DOE Office of Biological and Environmental Research, the National Institutes of Health, and pharmaceutical companies, structural biology is an ever-growing research area. Scientists from a variety of fields are finding creative new ways to use the APS to expand their knowledge. The APS To create the world's most brilliant X-ray beams, the APS begins with the linear accelerator. An electron gun much like a cathode-ray tube in a television emits electrons that run through a series of electromagnetic accelerators until they reach 450 million electron volts. The electrons are boosted in energy 200,000 times to 7 billion electron volts in the booster synchrotron and injected into the storage ring. They orbit through this 1,104-meter-circumference racetrack more the 271,000 times each second. (View large aerial photo of the APS with labels.) The electron beam is steered and focused by 1,097 powerful electromagnets as it travels within a closed system of 240 aluminum alloy vacuum chambers running through the magnet centers. As the electrons pass through special magnets called insertion devices, they emit powerful beams of X-rays, which are transported down the beamlines to illuminate experimenters' samples. At these beamlines, researchers take data leading to such headlines as: "Cheaper Silicon Found Effective for Solar Cells" During its first five operating years, accelerator operators refilled the storage ring with electrons twice a day because eventually the beam decayed, or lost some "steam." At this time, the beamline-scientists had to stop their experiment and close a series of shutters on each X-ray beamline to protect their research equipment. This new load of electron beams "shocked" the system, and researchers at the beamline had to wait until the heat load evened out before restarting experiments. Senior Physicist Glenn Decker explained how the shock affected all of the equipment: "It was the equivalent of turning on 5,000 100-watt light bulbs of X-ray power inside the storage ring." All of the equipment is so precisely aligned that even a 1 degree change of temperature could move sensitive components around by 10 micrometers; the system took at least an hour to stabilize. Researchers questioned tradition and performed computer simulations that lead to the top-up procedure being implemented in 2000. "Now," Decker said, "we regularly replenish the particle beam, by injecting a small amount of charge every minute." The technique is somewhat like eating small snacks at short intervals so your body never experiences hunger and the side effects. Researchers do not notice the change, nor do they have to stop their experiments and close the shutters on their beamlines. "The European Synchrotron Radiation Facility has evaluated top-up operation, Japan's SPring-8 ring uses it routinely, and virtually every other third-generation light source, including those existing and under construction are considering it," Decker said. Quintana explained that using top-up allows "all of the optics to reach an equilibrium temperature, and this alone has contributed greatly to the stability of the final beam on experiments." Quintana was a beamline scientist at one of the research beamlines before accepting his current position, and he appreciates the importance of providing a steady, stable beam to the researchers. Better, brighter beam "We regularly operate with the lowest emittance of any third-generation light source, 2.5 nm-rad," Decker said. "Lower emittance means that the electron beams are 'smaller' and more pencil-like." SPring-8 operates at about 6 nm-rad and the ESRF at about 4-nm-rad. Brightness is a specific term in the X-ray world that translates into having an intense and small beam. Quintana explained: "A light bulb and a laser pointer are two different light sources. A light bulb is actually more intense than a laser beam, but a laser beam is 'brighter' because all the light is heavily collimated in a single direction. "For users," he said, "this directly translates into the amount of light, photons, that can be put on to a sample. This translates not only into how fast experiments can be done, but generally it also improves the experiment's resolution." The brighter beam allows faster and clearer data. Researchers can "see" patterns at the APS that may be completely invisible at another laboratory. This allows researchers to explore areas of science that are not possible at other laboratories. A stable beam is another important APS goal. "With the laser beam analogy," Quintana said, "imagine trying to point it at something far away and then all of a sudden have the beam move in unpredictable ways, either its intensity or its angle changes. Since the input signal is varying in unpredictable ways, it is difficult to determine the true nature of the data coming from the experiment." To keep the beam stable, the APS employs a tight control system. More than 643 steering magnets and 260 beam-position monitors spread along the storage beam communicate constantly with two computer networks. More than 80,000 variables are monitored and regulated, including voltages, power levels and timing controls. The beam is readjusted minutely every 0.6 milliseconds. Five teams manage beam operations around the clock. "The equipment stability just keeps getting better," explains Operations Group Leader Greg Banks. The facility went through growing pains during the first years of operation. Now the thousands of beam acceleration and storage components work together well, and each week one or two days are set aside for minor repairs.
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