Cutting-edge accelerator design gets results 60 years later

January 20, 2012 | 11:33 am

EMMA particle accelerator. Image: STFC

At Daresbury Laboratory in England, a team of scientists recently completed the first successful runs on a new prototype that may change the way accelerators speed up particles. Its novel design, scientists said, is capable of energies beyond the reach of current cyclotrons, with acceleration rates exceeding those of the most powerful synchrotrons – all within a compact, cost-effective and operationally simple package.

Daresbury’s accelerator, called EMMA, gains its technological edge through a high-intensity accelerator concept nearly abandoned a half century ago.

The accelerator’s magnet configuration incorporates a fixed field, rather than a pulsed field like a cyclotron, but includes alternating gradients like a synchrotron. The first versions of these fixed-field alternating-gradient machines were invented independently in Japan, Russia and the U.S. in the early 1950s.

The U.S. FFAG came about through the Midwest Universities Research Association, a group of 15 universities dedicated to developing a machine capable of accelerating particles beyond a billion electron volts, or 1 GeV. This unique institution soon built the first FFAG prototypes, albeit low-energy models.

“It was an exceedingly clever group of people. They had lots of ideas,” said Alvin Tollestrup, a long-time Fermilab physicist who attended MURA’s FFAG workshop.

For nearly 10 years, the MURA team pushed for a high-energy, high-intensity FFAG accelerator. One of several designs the group developed included colliding beams that essentially doubled the collision energy. They submitted a number of their proposals for machines in the 10-20 GeV range to the Atomic Energy Commission. None were approved.

Meanwhile, the invention of the storage ring and cascaded synchrotrons significantly reduced the cost of high-energy accelerators. As a consequence, a 1963 report, chaired by Norman Ramsey, to the Atomic Energy Commission ranked the FFAG project lower in priority than the competing proposals for high-energy particle beams.

Under rising excitement over a new strong-focusing synchrotron design that could achieve 300 GeV, and in the heat of an increasingly contentious political climate, the new president, Lyndon B. Johnson, rejected the MURA proposal on the basis of the report. MURA then disbanded, with the scientists shifting to accelerator laboratories like Fermilab, Berkeley and Brookhaven.

Although the technological advances and heavy influence of the MURA team did in part lead to the construction of Fermilab in the Midwest, the FFAG fell into obscurity.

Decades later, in a 1997 UCLA workshop on muon colliders, Fermilab physicist Carol Johnstone, while collaborating with one of the original MURA physicists, Fred Mills, proposed a new type of FFAG, termed non-scaling.

The FFAGs of the ‘50s followed a strict scaling method for the magnetic field to confine and accelerate beam. In the quest for a rapid acceleration scheme for unstable particles began a project that would for the first time ever combine FFAG with a new non-scaling method of beam confinement that resulted in smaller, simpler magnets.

EMMA at Daresbury Laboratory. Image: STFC

The U.K.’s drive for an affordable collider that would accelerate and collide heavy elementary particles called muons set the stage for the prototype to be built at Daresbury Laboratory by a volunteer team of international scientists. Applied to a muon collider, the new non-scaling FFAG design would rapidly accelerate and inject the short-lived muon particles into storage or a collision before they decayed away.

“Emma was a bandwagon effect. It attracted accelerator physicists from across the world,” Johnstone said. “It’s the new technology that attracts the best.”

This FFAG format has the potential to quickly reach energies higher than 1 GeV, though the EMMA electron prototype tested at a moderate 20 million electron volts. The success of the preliminary runs put to rest decades of skepticism over FFAG technology. At a time when laboratories are pushing for more high-intensity experiments, the new approach is piquing interest among investors.

“Now that EMMA works, it’s considered a breakthrough,” Johnstone said. “And it’s a proof of principle for industry, too.”

Eventually EMMA’s method could be applied to a broad spectrum of accelerators. In medicine, the lower cost, versatile design and higher performance would enhance and expand proton and ion cancer therapy. For nuclear power, this accelerator-based approach could generate energy more safely while reusing old waste stockpiles. Along with muon colliders, a more cost-efficient generation of accelerators would greatly enhance experiments requiring high-energy neutrino beams.

“It’s interesting, I think, that something that was cooked up and kicked around 50 years ago all of a sudden could become quite interesting,” said Alvin Tollestrup. “There’s a number of interesting ideas for accelerators that haven’t been explored yet that could really change the way we accelerate particles.”

The journal Nature Physics recently covered the science behind EMMA in detail. For more history on FFAG, MURA and Fermilab, read “Fermilab: Physics, the Frontier, and Megascience,” by Lillian Hoddeson, Adrienne Kolb and Catherine Westfall.

Brad Hooker

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The making and tending of heavy ion beams for the LHC

November 15, 2011 | 1:41 pm

Heavy-ion expert Detlef Kuchler holds a container of lead. Image: CERN

This week the Large Hadron Collider began heavy ion physics, the process of colliding lead ions to learn about conditions in the primordial universe.

The accelerator is expected to perform five to 10 times better than it did in its first run of these collisions last November. Although the heavy ion program will last only from now until CERN’s annual winter shutdown just after the first week of December, operators started preparations months in advance. Here symmetry breaking examines what it really takes to put lead beams in the LHC.

The source

Making heavy ions is more complicated than preparing the protons used in regular LHC collisions, which come from hydrogen gas. Since hydrogen atoms have only one proton and one electron each, applying a voltage to them is sufficient to rip off their electrons, leaving a load of beam-ready, positively charged protons. But the source for heavy ions, enriched lead, starts with 82 electrons. Physicists do not have miracle flypaper to grab that many subatomic particles at once, so the process takes a few steps.

Meet Detlef Kuchler, a heavy-ion expert who tends the lead source, the first part of the heavy-ion acceleration process, by hand. He helped develop the method of extracting lead ions decades ago and can explain from memory its hundreds of associated, unlabeled diagrams. Although several people work on the source, a flowchart of what to do when things go wrong at this stage dead-ends everywhere with, “Call the expert.” It may as well say, “Call Detlef.” He spends a lot of nights and weekends at CERN during heavy ion season.

Kuchler prepares the oven. Image: Amy Dusto

The oven

The first thing Kuchler does when it’s time to make heavy ions is prepare the oven, a palm-sized cylinder on a long pole that evaporates metallic lead. The whole thing fits into another machine that begins a chain of hand-offs from the source to the LHC. The lead must heat slowly in the oven or fragments spill out. Kuchler refills the oven once every two weeks.

The lab’s 10-gram stock of enriched lead costs $12,000. But so little is used at a time — about 500 milligrams per oven fill — that the bill equates to roughly $2 per hour. Not bad, in the scheme of accelerator operating costs.

Kuchler carefully unscrews the oven, takes it apart, cleans its seals with ethanol, measures and fills it with lead, puts it back together, enters the correct settings and turns it on. When symmetry visited, he searched with gloved hands for a leak around water tubes used for cooling and requested new O-rings to fix a vacuum connection somewhere. He usually finishes all the physical tweaks in less than half an hour.

“I know every screw of this machine,” he said. “It’s fun.”

Operators test beams of lead ions in this linear accelerator. Image: CERN

The beam

After lead evaporates in the oven, it moves into a chamber of plasma heated by microwaves. The lead gas loses some electrons when it hits the plasma. In 30 milliseconds, the ions inside have each lost varying amounts of the negatively charged particles, but the majority have lost 29, making their charge 29+. As they leave the chamber, the 29+ ions are separated from the rest and collected into a beam.

A newborn beam of lead ions spends its first hours in testing. Here, the human-machine interactions become less TLC and more CPU. Kuchler adjusts computer settings to send a bit of beam a few meters into a small, straight section of accelerator, where it hits a diagnostic device called a Faraday cup. Kuchler sees how he did from what the cup tells him, makes adjustments accordingly, heats a little more lead in the oven and repeats. Eventually, when the beam looks good enough, he retracts the Faraday cup from the path so that the beam may continue its tour of CERN’s accelerator complex on the way to the LHC.

Electronics that control every aspect of source machine settings sit in shelves all around the oven and the accelerator. “During the day I keep an eye on [the machine],” Kuchler said. He can easily pop down from his office any time. During the weeks of heavy-ion physics, he elects not to take any personal holidays so that even when he’s not at work, he’ll always be nearby. He and dozens of other experts work day and night and remain on-call after hours in case anything goes wrong.

The heavy-ion beams make their presence known through synchrotron light, which shows up as subtly shimmering spots on the screen. Image: CERN

The chain

In August, eight weeks after Kuchler began working on the source, he began letting the beam through to the Low Energy Ion Ring, where other people took over its testing and fine-tuning.

On the way to the ion ring, the beam passes briefly through an area where it encounters a number of 300-nanometer-wide stripping foils. These take off more electrons, increasing the ions’ charges to 54+. Next, at the LEIR, a process called beam cooling begins to reshape and intensify the beam, which the machine accelerates.

The newly narrowed beam travels from there to another accelerator, the Proton Synchroton, behind a wall in the same building. This accelerator takes the beam up to a higher energy and sends it through another stripping foil. Ions leave here at the charge with which they will collide: 82+.

The last checkpoint before the LHC is the second biggest accelerator at CERN: the Super Proton Synchroton. This machine further accelerates the ions. The Proton Synchrotron accelerator started sending beam to the SPS in September. Over the course of eight weeks, a team of physicists spent hours refining the machine’s settings in order to optimize the beam and prepare it for its final destination: the LHC.

The LHC

Operators declared "stable beams" today, which means the LHC is ready for heavy ion physics. Image: CERN

During the few days of tightly-scheduled heavy-ion commissioning, many additional experts join the usual operators at the LHC controls. They help troubleshoot to make sure the beams reach the precise conditions needed for physics. Lead beams here accelerate to 1.38 TeV per single proton or neutron inside the ion.

Three minutes after midnight on Sunday, Nov. 6, LHC accelerator physicist John Jowett captured an image of one moment in the process that always thrills him. When the lead beams in the LHC reach about twice their injection energy, they begin emitting enough radiation, called synchrotron light, to be seen by a special telescope and camera system in the accelerator.

“We see a shimmering spot on a screen,” Jowett said. “It’s as close as we get to seeing the beams of lead nuclei with our own eyes.”

At the end of that week, on Saturday, Nov. 12, at 6:41 a.m., operators declared “stable beams,” the machine term that indicates the LHC is ready for physics.

Amy Dusto

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The multiplying effects of an accelerator economy

November 7, 2011 | 4:13 pm

ILC NewsLine first published this story on Nov. 3 2011. 

On the homepage of a US-based company website is a picture that, at first glance, looks like an advertisement for a major household appliance. A yellow starburst graphic assures you that delivery time is quick. A text-boxed baseline price is presented in glowing type. And at the centre is the touted item: an ILC/TESLA-type accelerator cavity.

Image of Niowave's website. Image: Niowave

Through the ad, Niowave, Inc., a superconducting linear accelerator company based in Lansing, US, showcases its capability for producing cavities with relatively short turnaround times. For the accelerator community, the ad could be a sign that the production of these superconducting structures is very slowly inching its way towards a place in the market as an in-stock product.

“It’s an indication of their confidence in this product as a catalog item,” said Global Design Effort Project Manager Marc Ross. “It means there’s somebody there willing to buy a number of them in that way.”

Niowave is one of several US companies developing superconducting technology for the ILC. To weld its cavity cells together, it relies on C.F. Roark, a partner company in Indianapolis, US.

It’s too early to predict how well cavities will do in the market as a prefabricated good, but Terry Grimm, who started the company in 2005 and worked extensively with particle accelerators as a laboratory and university physicist, knows there’s a niche for them outside high-energy physics.

“We see a real opportunity here – there’s a commercial market out there for these devices,” Grimm said. The devices aren’t necessarily for large, underground high-energy particle colliders. Plenty of biology and defense projects make use of tabletop accelerators that use similar technology as that for the ILC. “Going into the commercial market, we can talk about those smaller machines.”

Cavities for the ILC are a much taller order than those for commercial projects. The niobium is purer, the gradients are higher and the basic structures have complicated add-ons. Commercial projects don’t require as much of their cavities. But in the course of developing state-of-the-art niobium structures, you learn a few things that can only help when manufacturing simpler systems.

“That work could work out well for them, even for the sake of the technology, not just the company,” Ross said.

For Niowave, developing the technology isn’t just about maximising gradients. It also means perfecting a basic, so-called Entry Level model of superconducting cavity, much like the no-frills Chevrolet. This 20 megavolt-per-metre cavity can be produced at a significantly lower cost than the ILC’s 35 MV/m ‘Cadillac’ standard model, and since many of the company’s customers don’t need the highest possible gradients, the Entry Level could go over well with a fair share of their clients.

Niowave sees enough of a market that it stockpiles raw niobium so that it’s within arm’s reach, ready to be treated and shaped when an order comes in. It’s the only company in the world that can make that claim. Being able to pull niobium off the shelf is the convenient first step in an operation that develops turnkey accelerator systems, including cryogenics and beam testing. Conveniently, the company has a radiation generating license that allows them to conduct beam tests, something that’s otherwise reserved for laboratories.

Developing such systems is a relatively young enterprise for industry in the US.

“In the US, the niobium cavities were made at the laboratories themselves. That’s part of why it’s such a new business for industry,” Grimm said. “That’s where the ILC has been very helpful – getting industry and the labs working together. The ILC community keeps pushing for more work in that area.”

Niowave is housed in a former elementary school in Lansing, Michigan. The research and development of superconducting accelerators takes place in classrooms, offices and lounges – even in the boiler room. The name Niowave is a combination of ‘niobium’ and ‘microwave’. Image: Niowave

Other fields seem to be feeling the effect of the push. Niowave’s free-electron laser systems can help see through a cargo container that might be carrying illegal weapons materials. The company is also developing superconducting cavities for electron microscopes in which the high-frequency pulsed electron beam would make it possible to image a series of snapshots in a chemical reaction. There, researchers hope to be able see the chemical reactants transform into products in front of their eyes.

“Some of the scientists that are developing and using electron microscopes come to us. They have some of the things we need, but they have very little experience with microwaves and accelerating cavities,” Grimm said. “That’s where we come in.”

The same push to make better superconducting linacs has also been felt in the Lansing region’s economy, best known for automobile manufacturing. Also, Michigan State University, home of the National Superconducting Cyclotron Laboratory and future Facility for Rare Isotope Beams (FRIB), is a hub of atomic and particle physics expertise. A recent renewal of high-technology manufacturing has brought life to an area hit hard by the 2008 recession.

The multiplier effect, said Bob Trezise, CEO of Lansing Economic Development Corporation, is significant.

“This has certainly been true with Niowave, where almost all of its sales originate from customers outside of Michigan,” he said. “By combining the research at the new FRIB project and the commercialisation efforts of FRIB’s research at Niowave, mid-Michigan has established itself as a worldwide powerhouse for the superconducting accelerator industry.”

While reaching out to other areas of exploration, Niowave will continue developing cavities for the ILC.

“They have the opportunity to show that there’s a small business model for this technology,” Ross said.

Leah Hesla

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The Tevatron: a training ground beyond particle physics

October 25, 2011 | 8:41 am

Beyond smashing together billions of protons and antiprotons over the course of its 28 years of operations, Fermilab’s Tevatron also served as a launching pad for many careers, often in fields beyond particle physics.

Ron Moore now helps operate the 17 MeV cyclotron at Mass General Hospital. The accelerator produces radioisotopes used for medical diagnosis.

Take Ron Moore, the former head of Fermilab’s Tevatron Department, for example.

This month, Moore is starting a new endeavor as a medical physicist with a joint appointment at Harvard University and Massachusetts General Hospital in the Nuclear Medical & Molecular Imaging Division of the Department of Radiology. In his new role, he will help run a 17 million electronvolt (MeV) accelerator that makes radioisotopes used for medical diagnosis, such as PET scans, and conduct research about new imaging techniques.

Besides leaving the picturesque cornfields of Illinois for the collegiate charm of Boston, Moore realizes that he will have a few adjustments to make after working with the world’s largest proton-antiproton accelerator for the past 10 years.

“For starters, I can practically put my arms around the accelerator at Mass General,” Moore says with a grin.

Cutting your teeth on a machine like the Tevatron, however, prepares you for just about anything an accelerator might throw at you—no matter its shape, size or function.

“At Fermilab, you learn lots of accelerator skills because running a complex machine like the Tevatron is not trivial,” says Roger Dixon, head of Fermilab’s Accelerator Division. “Ron saw the technical problems, and he saw the solutions. That particular skill applies anywhere.”

The application
At 4 a.m. every morning, the accelerator at Mass General begins operating to produce Flourine-18, Carbon-11 and Nitrogen-13, the three main isotopes that the hospital uses. Once the accelerator makes the isotopes, they get transported through a pipeline in the hospital to the chemistry lab where chemists prepare them for the specified medical procedure. The isotope is then transported to imaging, where it will either get injected into the patient’s bloodstream or inhaled as part of a gas.

Mass General is currently one of 300 hospitals in the United States that has an in-house accelerator to produce medical isotopes for diagnostic procedures, including bone scans, gastrointestinal studies and cancer detection. Medical physicists like Moore keep the accelerators running on a daily basis in order to supply the hundreds of medical isotopes that are needed for patients every day.

“You’re running the accelerator every day for patients. Uptime is very important,” Moore says. “The focus is on running and not to set a record store, like it was at the Tevatron.”

When inside the human body, the isotope emits positrons—the opposite of electrons. When a positron hits an electron, it gives off energy and a detector tracks its movement and location to create an image that doctors use to diagnose disease and select effective treatments. Different isotopes have different purposes, and chemists can combine them with substances that aid in the diagnostic process.

When looking for a tumor, for example, chemists will combine Flourine-18 with a sugar to make the isotope go to areas of high metabolism in the patient. Because tumors have very high rates of metabolism, Flourine-18 laced with sugar will go right to that spot and illuminate it.

Prior to joining Mass General, Ron Moore was the head of the Tevatron Department at Fermilab.

The runner
Moore first became interested in medical physics when he needed to have a bone scan last December.

An avid runner, Moore started experiencing chronic pain in his shin. With the help of an isotope, a bone scan revealed the source of his pain—a small fracture in his tibia.

“I have always been interested in medical applications and how accelerators can be used for direct benefits for society,” Moore says.

A longtime member of the CDF experiment at Fermilab, Moore also brings decades of particle detector experience to Mass General and will work on medical imaging research.

For example, the medical physics community is actively working to develop real-time imaging using PET scans during proton cancer therapy treatments. Moore hopes to apply his background in high-energy physics detector development and electronics to the real-time imaging efforts.

And now that Moore’s tibia is all healed, when he isn’t running the accelerator, he’s hoping to find time to run along the banks of the Charles River, getting to know his new town.

Elizabeth Clements

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SLAC physicists using physics simulation tool to make cancer therapy safer

October 24, 2011 | 4:45 pm

This story first ran in SLAC Today on Oct. 20, 2011.

Joseph Perl described how he and his colleagues are turning the simulation toolkit Geant4 into a powerful application for medical physicists. Image by Helen Shen.

Tiny particles are making a big difference in the world of cancer therapy. And SLAC physicists—experts in particle transport—are using computer simulations to make those therapies safer.

At the Oct. 10 SLAC Colloquium, the lab’s own Joseph Perl described how he and his colleagues are turning the simulation toolkit Geant4 into a powerful application for medical physicists. Originally designed to track subatomic particles in high-energy physics experiments, Geant4 can also map proton paths through patients’ bodies during radiation treatment.

In radiation treatment, subatomic particles inflict DNA damage on dividing cells—both healthy and cancerous—causing them to commit suicide. The technique works because rapidly growing cancer cells are more likely to be dividing at any given time, and thus are more likely to be killed; but a smaller proportion of healthy cells are also susceptible to damage.

Minimizing collateral damage is a tough problem for medical physicists who design radiation treatments.

“To perfect this stuff, what we have to understand really is where are the particles going?” Perl said. “We have to understand particle transport when we’re designing the medical linacs,” the accelerators that deliver the particles to patients. “We have to understand particle transport when we’re talking about how the beams actually penetrate the body.”

Computer simulation tools such as EGS4, developed at SLAC in the 1970s, have helped medical physicists predict the behavior of X-rays and gamma rays. Now, Geant4 offers the ability to model proton beams, too.While Perl is SLAC’s only Geant4 team member working on medical physics applications, he works with four other partners on more general Geant4 capabilities. Together they constitute the second-largest team in the international Geant4 collaboration.

In contrast to the X-ray beams used in traditional therapies, which go all the way through the body, proton beams dump their energy at a specific depth. Medical physicists can target a tumor at one depth and avoid deeper and shallower tissues by tweaking the energy levels of one or more beams. Proton beam therapy may be particularly useful in children, for whom stray radiation can stunt growth and cause secondary cancers in adulthood.

Geant 4 relies on a technique called Monte-Carlo simulation, which models each proton moving through the body in a series of random steps. At each step, the program essentially casts a die to guess where the particle will move next. Over many steps, the program shows where protons are most likely to end up.

Unlike many other tools, Geant4 can also simulate the effect of tissues, such as the rib bones, that may move in and out of the proton beam as a patient breathes. Such obstructions can block some radiation from the intended target, while simultaneously allowing some tissue to soak up unnecessary radiation. Geant4 can potentially help medical physicists program beams to track a moving target and deliver a constant dosage to the tumor.

Geant4 is freely available to anyone who wants to use it, but in its current form may be challenging to some novices. “It’s a really fancy techno-Lego kit,” said Perl, but the box does not come with any ready-made toys.

To address this problem, SLAC’s Geant4 team has joined Massachusetts General Hospital and the University of California-San Francisco, in a four-year collaboration funded by the National Institutes of Health. The project, headed by Perl, will help medical physicists customize their simulations without disrupting the program’s innermost workings.

“If we can make it easier for people to use,” Perl said, “the more likely they are to use things right.”

- Helen Shen

Symmetry Intern

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Advancements in proton therapy cause for celebration

October 21, 2011 | 9:43 am

This article first appeared in Fermilab Today on Oct. 21, 2011.

A fully commissioned and operational gantry room. Image courtesy of ProCure Proton Therapy Centers

In 1946, founding Fermilab director Robert Wilson was one of the first to tout the benefits of proton therapy. The cancer treatment has since been lauded as a way to minimize damage to healthy tissue while focusing a finely calibrated beam directly on the tumor – an impossibility for radiation treatments based on X-rays or gamma rays. Yet protons were not used within a hospital facility until 1990 with Fermilab’s construction of a particle accelerator at Loma Linda University Medical Center. Since then, the industry has grown and the technology has evolved. Worldwide, 37 centers use proton therapy and more than 73,000 patients have been treated.

The ProCure facility in nearby Warrenville, Ill., one of only nine centers in the U.S., is completing its first year of proton therapy. The facility uses an accelerator called the Cyclotron to administer the proton therapy. One distinction that sets the Cyclotron apart from other accelerators is that the particles are sped up to just 230 MeV, an energy high enough to treat tumors as deep as 32 cm within a patient.

The proton therapy process begins with the transformation of hydrogen gas into plasma. The Cyclotron speeds up the particles and isolates the protons along a curve of magnets that lead to one of four rooms within the treatment center. With the aesthetics of a set piece from “2001: A Space Odyssey,” a large gantry device rotates the beam line on a track and the patient, lying on a platform, is precisely positioned by a large robotic arm similar to ones used in automotive manufacturing. Here the proton beam is slowed to an energy that will reach the tumor and be absorbed by the cancer cells but go no further, while a brass aperture on the nozzle narrows the beam into the shape of the tumor. For the beam to be a precise match to the tumor, the aperture is designed for each individual patient.

Mingcheng Gao worked with DZero as the detector went operational and is thrilled to use the skills he gained at Fermilab as a senior medical physicist at the ProCure facility.

“The technology is advancing quickly,” he said.

The price of building and running each proton therapy center has been the field’s greatest hindrance for decades. After 22 months of construction, the ProCure center ultimately cost $150 million to build. Yet with the increasing demand for proton therapy, Gao sees proton treatments playing a larger role in fighting cancer.

“The cost will definitely go down in the future,” he said.

Brad Hooker

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Accelerator soup: Scientists to mix elements in LHC to study recipe for heavy-ion collisions

October 20, 2011 | 9:29 am

CERN physicist Detlef Kuchler holds a purified sample of lead used to create heavy ions for the LHC. Photo by M. Brice / CERN

Superman may have superpowers, but Batman has the ingenuity to be just as heroic with the help of gadgets. Scientists at CERN are channeling the Dark Knight to try to make their biggest gadget, the Large Hadron Collider, perform a feat that counters its very design principles but may give them a better understanding of the early universe. They’ll have about 40 hours total to make it work.

Instead of colliding two beams of protons or two beams of much heavier lead ions, as the LHC usually does, operators will try to collide one of each in the coming weeks. On October 31, they will test the process for 16 hours, and two weeks later they’ll get another 24. That’s all the time they decided they could take from the precious month of data-collecting they will give the experiments during the upcoming lead-lead run. If it works, a proton-lead ion research program could be in place for November 2012.

The scientists undertaking the task of colliding protons and lead want to collect benchmark information about single beams of lead ions to get a better picture of what’s going on in lead-lead collisions. For that, the tiny proton acts as a probe of the more massive lead ion.

Theorist Urs Wiedemann explained that it’s a bit like making soup. A meticulous chef wants to know exactly what happens at each step of a recipe. This includes both the initial state of the individual ingredients – onions sautéed or raw? – as well as the final outcome inside the pot. Otherwise the chef can’t make informed changes. Similarly a physicist needs to know the individual properties of both of the elements he or she wants to collide, as well as what their smashing produces, in order to get the full picture for analysis.

After years of experiments, the quarks and gluons in the proton are well characterized; those in the lead ion are not. So, in a proton-lead ion collision, all unexpected effects can be attributed to the lead ion. With this knowledge, scientists will be able to make predictions about lead-lead collisions. “I can only test my theory of ion collisions if I know what is collided,” Wiedemann said. Colliding a proton with a heavy ion serves to “switch off the confounding factors,” he said.

Snapshot of two lead nuclei just after impact. Image by Henning Weber / CERN

Colliding protons and lead ions also gives scientists the potential to discover new physics phenomena, Wiedemann said. For this reason, all the experiments around the ring are interested in taking data during the collisions.

Scientists still need to test whether the technique will work in the LHC, said accelerator physicist John Jowett. The machine was built with a 2-in-1 magnet design: Both beams are controlled with the same magnetic field. This helps keep everything inside the ring symmetric when the beams inside match. Collisions and kicks, when one beam’s electromagnetic field pushes the other beam slightly off course, happen in the same place on each turn, a fundamental principle of stability for circular accelerators.

Protons and lead ions have different masses and charges, so they are not equally affected by the same magnetic field. The biggest challenges in this type of run are to inject and then control the beams as they ramp up from low to high energy. At lower energies, a beam of protons makes one extra lap around the LHC every 15 seconds. Since a proton beam makes about 11,000 laps per second, this discrepancy is small. But it is still enough to shift where the proton and lead-ion beams kick each other with every turn. As energy in the accelerator increases to its peak of 3.5 TeV and the beams get closer to light speed, however, relativity makes the 15 seconds stretch out little by little. At some point the difference between the beams becomes small enough that it can be overcome with a minor adjustment in their orbits – essentially the proton ends up going just a bit farther than the lead does, so they both take the same time to make one revolution.

The CERN accelerator complex. Image: CERN

To further complicate things, the more intense the beams get, the worse the effects each beam has on the other. The need to reach a certain level of intensity, roughly a measure of the amount of particles in a beam, puts a limit on how much data the experiments can extract. So, even if researchers can manage to inject two different beams in the accelerator, they will also need to make them with enough intensity for the method to be considered feasible.

The accelerator does have built-in flexibility despite having only one magnetic field: It has independent RF systems for each beam. These adjust electromagnetic frequencies inside a special cavity at one point around the ring, which controls each beams’ speed and orbit length. The LHC also has a powerful automatic feedback system that reacts to effects in the pipe by serving counter-kicks. This helps to keep the beams stable.

The LHC physicists plan to take baby steps – but fast ones, given the strict time constraints. First, they will simply try and inject two different elements into the LHC. If that works, they’ll try accelerating the beams. Only after that would they consider attempting actual proton-lead ion collisions.

“I hope it all works and we’re not prevented by mundane effects” such as a power outage, Jowett said. “In a month from now we should know the answer.”

Amy Dusto

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Mirrors harness X-rays for science

October 17, 2011 | 9:09 am

SLAC first published this article on Oct. 11, 2011.

Up close, they look simple as can be: a pair of metal bars, each with one side polished to a brilliant shine. One bar faces up, the other to one side.

A K-B mirror system consists of two curved mirrors placed at right angles to each other. A parabolic mirror shape will focus a collimated beam to a point, while an elliptical mirror shape will focus a point to another point.

They’re known as K-B mirrors, sophisticated X-ray focusing tools that have a pedigree going back to the very birth of X-ray microscopy. The shiny sides can be within a nanometer of being perfectly smooth, are exquisitely shaped into a barely perceptible curve of an ellipse, and may have one or more atomically precise coatings of exotic elements like platinum or rhodium.

The “B” in K-B mirrors stands for Albert Baez, father of folk singer Joan Baez, who helped invent this ingenious focusing system in the late 1940s while a graduate student at Stanford University. The “K” is for his Ph. D. advisor and co-inventor Paul Kirkpatrick, then head of the physics department.

Their invention signaled the beginning of X-ray optics – the ability to manipulate X-rays, an energetic, penetrating form of light. Today, thousands of scientists worldwide use X-ray beams to examine the material world in atom-by-atom detail, fueling advances in energy production, electronics, environmental cleanup, new materials, nanotechnology, medicine and many other fields.

“All these years later, K-B mirrors have become an essential tool for focusing X-rays to the tiny spots that scientists need for their experiments,” said Piero Pianetta, Stanford Synchrotron Radiation Lightsource deputy director. “Between SSRL and the Linac Coherent Light Source, we have about a dozen of these mirror systems here at SLAC.”  The world’s most accurate K-B mirror system currently in use, which was recently installed at LCLS, cost $1.2 million, when all of its alignment and stability aids are figured in.

For more than 50 years after Wilhelm Röntgen discovered X-rays in 1895, no one had figured out a way to focus this mysterious, penetrating form of light. Röntgen himself had proclaimed that it would be impossible to focus X-rays with lenses. In 1922, Arthur Compton reflected X-rays from polished surfaces at “grazing” or “glancing” angles of one degree or less. There matters sat for more than two decades, when Kirkpatrick began to explore the possibility of using Compton’s technique to focus X-rays.

Reflecting light with a single curved mirror produces a severe astigmatism, focusing rays from a point source to a line. “It was Kirkpatrick’s idea that a second curved mirror placed at a right angle to the first would ‘squeeze’ that line down to a point,” Baez said later in several retrospective articles and talks. He had been teaching calculus at Stanford and also working as a lab assistant in the Physics Department when Kirkpatrick asked if he’d like to work under him as a graduate student to develop this idea.

“I jumped at the chance,” Baez recalled.

In a historic 1948 paper, Kirkpatrick and Baez introduced their mirror pair as part of the world’s first X-ray microscope. But it would be another quarter century before three other developments enabled scientists to use the mirrors productively: In the mid-1970s, machining, polishing and metal-coating techniques improved enough to make ultra-smooth surfaces that could reflect X-rays without significant scattering. Second, James Underwood, an X-ray optics pioneer who later worked at Stanford, showed how to flex the mirrors precisely into the elliptical shapes that eliminated unwanted optical aberrations. Thirdly, new X-ray sources called synchrotrons produced intense beams of X-rays for probing and unlocking the secrets of molecules and materials.

With more than 70 synchrotron light sources now operating or under construction around the world, K-B mirrors are made by six manufacturers: two in France and one each in Germany, Japan, the United Kingdom and United States. There are several other ways to focus X-rays, but these mirrors are popular because unlike technologies based on diffraction or refraction, they reflect most  X-ray energies approximately uniformly.

LCLS mechanical engineer Jean-Charles Castagna (foreground, right) and Dan Harrington, SSRL engineering physicist, stand by the K-B mirror system at the Stanford Synchrotron Radiation Lightsource. Their hands are resting on the chambers that hold the actual mirrors. Photo by Mike Ross.

Despite their simple design, crafting K-B mirrors has become much more complicated over the years. The first K-B mirrors were made of glass or quartz coated with a reflective metal film. Today’s mirrors are typically made from silicon crystals polished to near-atomic smoothness and then coated with ultra-thin layers of hard silicon carbide or dense metals. The shorter the X-ray wavelength being focused, the more precise the mirror surface must be.

“The mantra for X-ray mirror designers is ‘figure and finish,’” said Underwood. “Figure is the shape of the mirror and finish is its surface roughness. Many innovations over the years have improved our ability to make more accurate focusing figures and smoother mirror finishes.”

The mirror surface must be formed – or flexed – into the correct curved shape to focus the beam. This can be done in several ways. Most often, the mirror surface is polished while flat and then positioned in the beamline, supported on both ends. Then, an actuator motor applies a bending force near each end of the mirror, flexing the mirror into an ever-so-slight concave shape. Controlling the placement and amount of the force, as well as the mirror block’s shape and its supports, determines the resulting curve.

“For point-to-point focus you need an elliptical mirror shape,” said SSRL engineering physicist Tom Rabedeau. “Focusing a collimated beam to a point or vice versa requires a parabolic mirror shape.”

The mirror has to keep that shape despite being pummeled with intense high-energy X-rays. Although the X-rays are supposed to reflect off the mirror at a glancing angle, some of their energy is inevitably absorbed by the mirror, and the resulting heat buildup can warp or otherwise distort it.

The high energies of the X-ray laser pulses at the LCLS can also cause ablation, ejecting atoms from the mirror surface and reducing focus quality. Coatings can help minimize such damage. And while the mirrors are usually held in a vacuum, stray molecules owing to imperfect vacuum may stick to the mirror surface,  degrading its focus.

Temperature fluctuations can deflect the mirrors’ focal point. And when using long mirrors, designers even have to account for sagging between the mirror’s end supports due to gravity, said Nicholas Kelez, LCLS mechanical engineer. Jitter in the actual location of the X-ray beam also affects its ultimate focus.

“K-B optics are nice, since you can adjust one axis at a time,” Rabedeau said. But even a tiny error in the adjustment can throw the beam’s focus way off. For a one-meter-long mirror, “a one-micron mirror deflection means a huge 60-micron move in the focus spot 30 meters distant,” he added. Increasingly, researchers are using automatic sensor-feedback “servo” controls to fine-tune the positions of the mirrors to keep the X-rays focused accurately onto their target.

“All in all, K-B mirrors give you much better control of the focused spot shape than other X-ray focusing methods,” said Dan Harrington, SSRL engineering physicist.

The world’s best K-B mirror system currently in use is in LCLS’ Coherent X-ray Imaging (CXI). Its $300,000 mirrors were made in Japan using a precision liquid-jet method to shape the desired elliptical figure directly into the silicon, avoiding the need for mirror-bending apparatus. Each of the 350 mm-long reflecting surfaces are smooth to better than 1 nanometer surface roughness over their entire length.

A German company then built the mirrors’ enclosure, alignment system and 6-ton granite base, bringing the total price tag of $1.2 million. It can focus to a one-micron spot, and its focal plane can be moved by about half a meter to accommodate the needs of various experiments, said Sebastien Boutet, CXI instrument scientist.

CXI’s current K-B system won’t be the star of the show very much longer, however. A near-twin of that system, designed to focus the LCLS pulses down to much smaller 0.1-micron-diameter spots, has been built in Japan and is being commissioned now in Germany. Installation should begin at CXI next month.

- Mike Ross

Guest author

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Tevatron operations: A way of life

September 30, 2011 | 10:38 am

Editor’s note: This afternoon, Sept. 30, Fermilab will shut down the Tevatron for the last time. A broadcast of the event will begin at 2 p.m. CDT and will be available online. Fermilab Director Pier Oddone will host the broadcast, which will feature the shutdown in the Main Control Room, and the CDF and DZero control rooms. Fermilab Today published this story to honor the operators – the men and women who have kept the accelerators running 24 hours a day, seven days a week, 365 days a year.

On July 23, current and former Fermilab operators celebrated 40 years of operations.

A steady, consistent beep is the soundtrack of the Main Control Room in Fermilab’s accelerator complex. The metronomic resonance, not unlike a submarine, is reassuring to those who know it best: The operators. The regular tone signals that all is well with the 10 accelerators maintained, and often improved, by the Accelerator Division at Fermilab. But when the monotonous pitch is interrupted by any one of several alerts, the operators leap into action – even in the middle of the night.

Operators – the men and women who keep the accelerators running – work 24 hours a day, seven days a week, 365 days a year. Four crews rotate through three shifts every weekday, with two 12-hour shifts on Saturday and Sunday. The weekday shifts are split into day, evening and owl. Every shift is different.

At the basic understanding of their job, operators run the accelerators. But it goes further than that. Not only do they send beam to whichever experiment needs it at the moment, they also provide constant care for the accelerators. The accelerators can be brought down by storms, heat, electrical surges and even frogs.

Operators work in unique conditions. They’re often the first to know that there’s a problem with an accelerator, and they’re often the first to propose a solution.

“We’re the first line of defense, the skin,” Duane Newhart, the Deputy Head of Accelerator Operations, said. “We have to logically respond to problems, while dealing with a variety of personalities, often in the middle of the night.”

In the 15 years that Newhart has worked in Operations at Fermilab, he said not a day has been the same. He recalled how one operator handled a particularly difficult mystery.

“Duane Plant once investigated this TeV ramp trip that kept happening, but it’d move forward 3 to 5 minutes every week,” Newhart said.

Plant found that a service van would drive by around that time. The sun reflected off of the van’s windows, bounced to the service building, hit the photocell on the dump switch, and then the switch would open, causing the ramp to trip off.

“Not only is it amazing to see how people solve these problems, but it’s also incredible to see all of these different people working together,” said Michael Backfish, Operator II. “I’ve seen people on all sides work together for science.”

The pressure can be intense, but the operators know how to alleviate their stress with a little good-natured fun.

“We played a few practical jokes back in the day,” Paul Czarapata, the Deputy Division Head and former Meson Crew Chief, said. “There was a tech who was crazy about keeping his tools secure and had a desk with an elaborate locking mechanism.”

One day, several operators jimmied the lock, removed the tools and put on innocent faces.

“The guy was flustered,” Czarapata said. “He thought it was funny, too, once we pointed out his tools sitting safely in the corner.”

They have fun playing pranks on each other, but the operators can become very serious, very quickly.

“We have fun, but it just takes one alarm to make everyone focus,” Newhart said. “We go from laughing to serious in a second.”

Despite the strange hours and unpredictable work day, or in some cases it’s because of these things, there’s an excitement in operations.

“You get to watch what happen,” said Bob Mau, the former Operations Department head. He’s now retired. “You get to go everywhere and see a little bit of everything.”

All of that on-the-job training gives operators exposure to the rest of the laboratory and the variety of jobs therein. Mau said that a former operator can be found in nearly every division and section.

“Most operators are hired right out of school. They’re trying to figure out what they want to do,” Mau said. “In operations, they learn the complex. It’s a great spring board for people to find what they’re really interested in.”

Regardless of where they end up, operations becomes a part of who they are.

On July 23, hundreds of Fermilab operators gathered to celebrate the 40th anniversary of Operations. Current employees, retirees and their spouses and children shared stories over a potluck picnic and barbeque. “There are people here that I haven’t seen in 15 or 20 years,” Mau said. “It’s nice to see that these people thought enough of their early jobs to travel all this way.”

“In operations, you become a bit of a family,” Jim Morgan, an engineering physicist, said. “Everyone is always in each other’s face, and we take all of the good and all of the bad. We’re close.”

There are often differences that need to be navigated, but operators are tied together in pursuit of the common goal to make the accelerators work.

“Operations is our opportunity to contribute to something bigger,” Dave Capista, an engineering physicist with the Main Injector Department, said. “It’s our chance to contribute to science.”

Ashley WennersHerron

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Fermilab’s Antiproton Source: A rich history and an exciting future

September 29, 2011 | 1:04 pm

Fermilab Today first published this story on Sept. 29, 2011.

A mock-up of the proposed upgrades to the Antiproton Source and the new Mu2e building.

Fermilab’s Antiproton Source has long produced the antimatter that makes Fermilab’s particle collisions possible. While the Antiproton Source will shut down along with the Tevatron on Sept. 30, there are plans for its future.

The facility that houses the Antiproton Source will be reconfigured for two proposed experiments: Muon g-2 and Mu2e. Instead of creating antiprotons, both experiments will use the reconfigured facility to generate intense muon beams. While each would produce exciting and interesting data, they are both very different from the Antiproton Source’s original mission.

Fermilab’s Antiproton Source came out of an upgraded design of a similar machine that was housed at CERN.

Fermilab’s first antiprotons were produced in the Tevatron’s first collider run in 1988. At that time, it took more than an hour to make 10¹⁰ antiprotons. Now, the Antiproton Source can make 30×10¹⁰antiprotons an hour.

“The antiproton production rate in those early days now seems like an incredibly slow pace,” said Steve Werkema, deputy head of the Antiproton Source. “I think we would’ve been amazed by the production number now.”

The Antiproton Source was upgraded in the mid-1990s, before collider Run II. Improved accelerator optics and upgraded stochastic cooling systems aided in the jump of antiproton production. The rate of antiprotons produced jumped significantly during this time, due to the combined efforts of improved accelerator complex components.

“We really have to recognizethe efforts by the operators,” Werkema said. “There are long periods of routine running, and the operators are not happy to merely sit and watch the beam. They get to work tuning the machine. I think a good part of the improvements are due to the amazing things they’ve done.”

More than tuning is necessary to convert the Antiproton Source facility into an area appropriately equipped to house the Muon g-2 and Mu2e experiments. Fermilab’s muon program is a strong component of the laboratory’s new direction. The laboratory will use muon beams to study fundamental laws of physics. Both of these experiments are possible, in large part, because of the cost savings that comes from reusing the Antiproton Source infrastructure.

These muon experiments will be at the cutting-edge of high-energy physics for Fermilab’s push into the Intensity Frontier in the coming decade. Both experiments are currently in their design stage, with scientists and engineers finalizing conceptual designs. Muon g-2, the smaller of the two experiments, is aiming to start construction in 2013 and begin taking data in early 2016. Mu2e expects to finalize their conceptual design this year and is working toward a construction period that would allow for data production toward the end of the decade.

“There’s a lot of work to do moving forward,” Werkema said. “We’re sad to see the Antiproton Source go, but we’re going to be busy.”

Muon g-2 is the newest generation of a similar experiment performed at Brookhaven National Laboratory in 2001, which found a disagreement in the data value and the Standard Model prediction of the gyromagnetic ratio “g” of the muon.

“We’re doing the same type of experiment, but with higher precision,” said Chris Polly, project manager for Muon g-2. “The Antiproton Source infrastructure plays a key role in getting to that higher precision. By building a 1,000 meter long beam line at Fermilab, the muon beam will have a much higher quality than what could be achieved with the 80 meter beamline used at Brookhaven.”

The Antiproton Source was not designed to employ the high beam intensities needed for these experiments. However, the scientists see a lot of potential in the current facility.

“The Antiproton Source rings makes Fermilab the ideal place to do the Mu2e experiment,” said Bob Bernstein, the Mu2e co-spokesperson. “Their length happens to be just right for our physics needs.”

The Mu2e experiment will increase the intensity of muons stopping in their target by four orders of magnitude over any prior experiment. The discovery potential and the techniques developed for Mu2e will pave the way for future possibilities with Project X, a proposed high-intensity proton accelerator that would generate high-intensity beams for various experiments.

“Working with regular matter seems like it would be boring after making antiprotons, but, thankfully, it’s not,” Werkema said. “Both of these experiments come with unique and serious challenges. We won’t be bored.”

Ashley WennersHerron

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