Stable, bright X-ray beam provides better data

ARGONNE, Ill. (Sept. 30, 2005) — What a difference a decade makes. Ten years ago the Advanced Photon Source (APS) produced its first light. Since then the APS has focused on continuous improvement to provide the best beamlines for science research. Its accelerator physicists pioneered a technique called "top-up" to replenish the particle beam for optimal beam performance. Today, the APS provides the most brilliant X-ray beams in the Western Hemisphere for research, and the beams are extremely stable and reliable.

"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 (DOE), 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

The APS is a billion-dollar facility housed in a complex two-thirds of a mile in circumference – large enough to place a major baseball park inside – with hundreds of thousands of intricate working parts that operate within tight tolerances.

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 (GeV) 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"
• "Zinc Deficiency Linked to Esophageal Cancer"
• "X-ray Movie Reveals Insect Flight, Muscle Motion"

Top Up Mode

Argonne accelerator physicists in the APS Operations Division and their counterparts in the Accelerator Systems Division are not happy just keeping the intricate machinery in fine working order. Since the APS is one of only three hard X-ray sources in the world, physicists challenge themselves to develop new methods to improve the beams.

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 (ESRF) 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

With the stability from top-up operations, physicists moved to improve the beam in the storage ring. Now the APS is the brightest of the third-generation light sources and is 30 times brighter – or better – than its original specifications called for.

"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.

Banks measures beam reliability as mean time between faults – the number of hours between uncontrolled beam downtime. The mean time between faults record was set in fiscal year 2005 when the beam was available for research an average of 110.7 hours at a time. "In 2004 the beam ran for 15.5 days straight without an unplanned downtime," Banks said.

The stable beam attracts researchers to Argonne. During CAT Chats – a casual coffee break for users and operators to talk – Banks has heard regularly that the steady, stable beam keeps researchers coming back to the APS. — Evelyn Brown