By Lynn Yarris, [email protected]
Two generations of Americans have grown up watching Captains James T. Kirk and Jean Luc Picard and the crew members of their respective Enterprise starships "beam" from one destination to another. Millions have sat in movie theaters watching Luke Skywalker and other Star Wars characters blast their way to victory with beams of destruction. But beams are every bit as much a staple of real science as they are of science fiction.
A beam is a directed flow of energy. It can be made up of photons, or it can consist of either charged particles (electrons, protons, or ions) or neutral particles (neutrons, atoms, or molecules). It can even be a combination. Once upon a time, it was chiefly the tool of physicists who used beams of charged particles to explore the things that make atoms. In recent times, beams of photons have become a vitally important tool for chemists and material scientists to explore the things that atoms make.
As the uses of beams have become more diverse, the need for new and different types of beams has grown. Creating beams that can meet this growing need requires the type of research and development that is being carried out by the nearly 50 scientists and engineers at LBL's Center for Beam Physics (CBP).
The CBP, which until this past year had been known at LBL as the Exploratory Studies Group, is a multidisciplinary research and development unit in the Accelerator and Fusion Research Division (AFRD). Its mission to study the production, manipulation, storage and control of beams grew out of efforts in the design and construction of particle accelerators and detectors by AFRD and the Physics Division.
The CBP is led by physicist Swapan Chattopadhyay, who gained his expertise working for the Super Proton-Antiproton Synchrotron collider at CERN, in Geneva, Switzerland. At LBL, he worked on design of both the Advanced Light Source (ALS) and the Superconducting Super Collider, being built in Texas.
Chattopadhyay became the leader of the beam physics group in 1987 and was instrumental in assembling the center's current staff.
"Because our program serves broad areas of research, we have carefully and deliberately tailored the structure and composition of our staff over the past five years to make up an effective and complementary team," he says.
"We can now bring to bear on beam research and development a significant amount of expertise in the fields of accelerator physics, nonlinear dynamics, synchrotron radiation, advanced microwave techniques, plasma physics, optics, and free electron lasers."
The variety of scientific expertise at the CBP mirrors the variety of the projects in which they are currently involved, beginning with a big role in the "PEP-II" initiative (also known as the B-factory).
The PEP-II is a proposal to convert the Positron-Electron Project collider at the Stanford Linear Accelerator Center (SLAC) into an "asymmetric" collider that produces copious quantities of B mesons--subatomic particles that can be used to study CP violation, the mechanism which many scientists believe was crucial to the formation of the universe.
The idea for an asymmetric collider--one in which the two colliding beams of particles are not of equal energies--was first proposed in 1988 by physicist Pier Oddone, who was then the leader of LBL's Physics Division and is now the Laboratory's deputy director. It was greeted somewhat skeptically by the physics community until Chattopadhyay and his Exploratory Studies Group answered major beam physics questions, such as: Is it practical or even possible to focus and then separate two beams of unequal energy?
Oddone's idea, fortified by the studies of Chattopadhyay and his group, is now a major initiative involving LBL, SLAC, and the Lawrence Livermore National Laboratory (LLNL). The U.S. Department of Energy has included $36 million in its budget proposal for FY94 to begin construction, although the site (either at SLAC or at Cornell University in New York) has not been decided. For the LBL-SLAC proposal, CBP researchers, led by Mike Zisman, are tackling the technological problems posed by the B-factory's luminosity requirements.
"Luminosity" to a high-energy physicist refers to the collision rate between particles in colliding beams (a product of beam intensity). The B-factory design calls for a luminosity of 3 x 1033 (3 followed by 33 zeros) cm-2/sec-1, which is about 15 times higher than today's best.
Says Zisman, "The B-factory will be a first bold step over the luminosity frontier, a frontier that must be crossed as we look for rarer and rarer phenomena. This is a departure from the past, where the challenge in accelerator research and design was to reach for higher and higher energies."
Another accelerator-based initiative on which the CBP staff is working is the infrared free-electron laser (IRFEL).
LBL's IRFEL is being designed to produce intense, coherent radiation that is tunable across infrared wavelengths between 3 and 50 micrometers. Scientists can use this light to identify and characterize molecules and to study the properties that result from the geometry and bond strength of molecular structures. In the past, such research has been hampered by a dearth of tunable infrared light sources.
An IRFEL is basically a linear accelerator or "linac" equipped with special optical components. A beam of electrons is accelerated in the linac to nearly the speed of light, then sent into an optical cavity that contains an undulator, an array of dipole magnets with alternating north and south poles. The magnetic field of the undulator forces the electrons to oscillate. This motion in turn causes them to radiate photons at specific wavelengths, depending upon the energy of the electrons and the strength of the magnetic field.
In an IRFEL, coherent light can be obtained in the same manner as with a conventional laser--by bouncing the emitted radiation back and forth between two mirrors, one of which is only partially reflecting. After about a thousand roundtrips between the mirrors, a laser beam emerges from the cavity.
The IRFEL proposed for LBL would accelerate its electrons through a pair of radio-frequency (rf) cavities--devices that energize electrons with their oscillating electromagnetic fields. On its first pass through these cavities, the beam would be energized to approximately 30 million electron volts (30 MeV). It could then either be sent into the optical cavity or directed around a loop and recirculated back through the rf cavities. A second pass through the rf cavities would boost the electron beam to nearly 60 MeV.
In the optical cavity, the extracted infrared light could be tuned to a specific wavelength within a narrow range set by the energy of the electron beam. When 30 MeV electrons pass through the undulator, the photon wavelengths could be tuned between 10 and 50 microns. When the electrons are at 60 MeV, the wavelengths could range from 3 to 10 microns.
As originally conceived, LBL's IRFEL was to accelerate its electrons through the use of rf cavities that operate at room temperature. However, CBP researchers, led by Kwang-Je Kim, have subsequently changed this design so that the IRFEL would now make use of rf cavities that are superconducting.
"The primary reason for this design change was the user requirement for a photon-beam wavelength stability of about 10-4 (one part in 10,000)," says Kim.
The use of superconducting rf cavities also produces a continuous train of electron bursts rather than a pulsating electron beam, which increases the power of the IRFEL's light from 20 watts to nearly 600 watts. Furthermore, once started, the acceleration field of a superconducting cavity will oscillate indefinitely with almost no loss of energy, making it much more efficient than a conventional cavity.
Even though its electron beam is continuous, LBL's IRFEL will emit its light in pulses as short as 10 picoseconds--a picosecond being one trillionth of a second. It is important that experimenters know the precise wavelength and duration of the infrared light being used. Wim Leemans and other CBP researchers have developed the first diagnostic system capable of providing these measurements for a single IRFEL beam pulse.
"The key to obtaining spectral and temporal information for each individual pulse of light is the use of an image dissector that was developed for visible light lasers but which we have adapted for IRFELs," says Leemans.
Being able to do precise beam diagnostics not only provides experimenters with crucial information, it also provides the feedback that can enable operators to improve the IRFEL's performance. Another performance-improving idea from the CBP is a concept called "hole-coupling," in which light passes out of the optical cavity through a hole with a variable aperture similar to that on a camera. This concept allows experimenters to select from the range of wavelengths offered by the IRFEL with a minimum amount of downtime or disruption of the beam for other users.
The beam diagnostic system has been successfully tested on an IRFEL at Stanford. The hole-coupling concept was tested at the CBP using a visible light helium laser and is scheduled for another round of tests at Stanford's IRFEL. Even though this IRFEL has substantially less power than the one proposed for LBL, the test results should prove whether or not the technology will work.
LBL's IRFEL would be housed inside a proposed Chemical Dynamics Research Laboratory that would be located adjacent to the Advanced Light Source. The ALS is LBL's new synchrotron radiation facility that produces laserlike beams of x-ray and ultraviolet light (roughly one to 100 nanometers in wavelength). Synchronizing ALS beams with IRFEL beams would yield research capabilities unmatched anywhere else in the world.
For example, beams from the ALS could be used to mimic the "pumping" or energizing of atoms and molecules that occurs during a specific chemical reaction, such as combustion. Beams from the IRFEL could then be used to probe the results. In the case of combustion, this could give scientists a much better understanding of the dynamics behind what is now, and will continue to be for the foreseeable future, humanity's chief source of energy. Such understanding could do much to improve energy efficiency and reduce pollution from burning fossil fuels.
The ALS is the largest, most complex synchrotron and storage ring complex ever built at LBL, and CBP researchers played a major role in seeing that it was completed within budget and ahead of schedule (a rare event for the construction of any major accelerator-based facility). Led by physicist Alan Jackson, they designed the accelerator system and a storage ring that help make possible production of the world's brightest beams of soft (low energy) x-rays.
The ALS storage ring features a vacuum chamber, 200 meters in circumference, coursing through an armada of dipole, quadrupole, and sextupole magnets. These magnets focus electrons that have been accelerated to 1.5 billion electron volts into a hair-thin, ribbon-shaped beam and steer them around the ring for up to six hours--enough orbits to cover a distance of nearly 3 billion miles. In addition to steering and focusing magnets, the storage ring also contains undulator and wiggler magnets-- collectively called insertion devices. When the electron beam passes through the magnetic fields of one of these insertion devices, a tangential beam of x-ray or ultraviolet photons is generated that can be used for a wide variety of purposes in the chemical, materials, and life sciences.
One of the biggest challenges facing Jackson and his colleagues in the design and construction of the storage ring was fitting all of the magnets and other components into snug quarters.
"The tolerance requirements were extremely tight," says Jackson. "For instance, the typical tolerance for aligning the magnets installed around the storage ring was 150 microns, barely the thickness of two human hairs."
For all of its rigorous complexity, the ALS storage ring was officially commissioned a week ahead of schedule, on March 24, 1993, when it achieved 65 milliamps of electron-beam current. This was comfortably above the 50 milliamp current required for project completion. Less than a month later, the ring exceeded its design goal of 400 milliamps.
"It always helps to be in on the design of a facility right from the start of the project," says Jackson. "This really paid off in the case of the ALS."
The first three ALS storage ring insertion devices were installed this summer. These devices are undulators that produce a linearly polarized beam, meaning that the electric-field component of this light (remember that light is electromagnetic radiation, a combination of electric and magnetic fields) vibrates in one direction only in a plane perpendicular to the direction in which the beam is moving. Linear polarization is critical for triggering chemical reactions or for studying the orientation of atoms and molecules when they are absorbed on a surface.
Some molecules, particularly biological substances and magnetic materials, absorb light that is circularly polarized, meaning its electric-field component vibrates in a spiral path around the direction of the beam, either clockwise (right-handed polarization) or counterclockwise (left-handed polarization). Since the orientation of such molecules determines whether they absorb right-handed or left-handed light--a property called "dichroism"--circularly polarized light can be used to identify their structure.
ALS bending magnets can produce circularly polarized synchrotron radiation, but this light appears in a sweeping beam similar to the light produced by a beacon. Kim and his CBP colleagues have a proposal for generating circularly polarized light in a special insertion device called a "crossed undulator." This device consists of two arrays of dipole magnets, the second of which is aligned perpendicular to the first. The first array produces light that is linearly polarized in a horizontal direction, and the second produces light linearly polarized in a vertical direction. When an electron beam passes through these two arrays in sequence, the result is circular polarity.
"We also have a proposal for an elliptical wiggler," says Kim. "By combining a weak horizontal magnetic field with a strong vertical magnetic field, we can extract light that is elliptically polarized. This would be particularly useful for studying the magnetic characteristics of materials."
Getting optimal performance from any insertion device hinges on the quality of the electron beam going through it. While this quality in the ALS is sustained in the storage ring, it starts with the accelerator system, which consists of a linac that lifts electrons to 50 MeV and a synchrotron that boosts them to 1.5 billion electron volts (GeV). The booster synchrotron is a smaller version of the storage ring, sans insertion devices. The 50 MeV linac consists of a high-intensity electron gun that can generate single or multiple bunches of 120,000 eV electrons in pulses of 2.5 nanoseconds (billionths of a second), a pair of "subharmonic bunchers" that compress the beam down to 200 picosecond pulses, and two traveling wave accelerating structures that produce an extremely high-quality beam in pulses of 30 picoseconds.
Once the storage ring has been filled with electrons, the 50 MeV linac sits idle for several hours. CBP researchers want to make good use of this time. They are now constructing what they call the "Beam Test Facility," a branch line that will utilize the electron beam of the 50 MeV linac without disturbing the operations of the ALS.
Says Chattopadhyay, who oversees this project himself, "The magnetic lattice we have designed for our Beam Test Facility has already been optimized for two major experiments."
The first of these experiments, which is being designed by Leemans and his team, is testing the possibility of using a plasma lens to focus relativistic electrons. Today's accelerators use quadrupole magnets to bend the flight paths of relativistic electrons towards a converging point. As the energy of the beam escalates, the strength of the magnetic field must escalate also, producing an ever sharper bend in the flight paths. With too sharp a bend, however, synchrotron light is generated. This random emission of photons leaves the resulting electron beam with a fuzzy, rather than a sharply defined focus. The limit to how sharp a focus can be achieved with conventional magnets is called the "Oide limit" after the Japanese scientist, Kasimo Oide, who calculated it.
"You can overcome the Oide limit by using a large number of weak-focusing magnets instead of a few strong-focusing quadrupoles," says Chattopadhyay, "but this requires a lot of space and costs a lot of money."
A potentially better solution, he says, is to pinch the electrons in the electromagnetic field of a plasma, a gas whose constituent atoms have been ionized.
Says Chattopadhyay, "A number of physics issues will have to be resolved before plasmas can be used as lenses. For example, the ultimate achievable focusing strength will depend on whether we can control the density of the plasma and keep it uniform. It is important too, for use in high-luminosity linear electron- positron colliders where the focus is extremely tight, to know whether or not detectors can separate collision results from background radiation."
If plasma lenses prove feasible, they could play a future role in the second major experiment to be run on the Beam Test Facility: producing x-rays in femtosecond pulses.
A femtosecond is a millionth of a billionth (or one quadrillionth) of a second. To appreciate the brevity of such an increment, consider that a femtosecond is to one second what one second is to 30 million years. At room temperature, almost everything in nature vibrates to a femtosecond beat. If you want to study this motion, you need femtosecond pulses of light.
A visible-light femtosecond laser was developed by LBL Director Charles V. Shank when he was at AT&T Bell Laboratories. Femtosecond spectroscopy with visible light has been subsequently used by Shank's group at LBL to study phenomena on the basis of their optical properties, such as the chemical process that enables the eye to detect light. But scientists also need to study phenomena on the basis of atomic structures, and that requires the shorter wavelengths of x-rays.
For details, see the related article, LBL Develops Femtosecond Flashlight.
The pulse length of an x-ray beam extracted directly from an electron beam through the use of an undulator will be about equal to the pulse length of the electron beam. With current technology, it is difficult to generate a sufficiently intense electron beam much shorter than a few picoseconds.
To overcome this restriction, Chattopadhyay and Kim have proposed a technique involving orthogonal Thomson scattering--the scattering of photons after collisions with electrons at 90- degree angles. There have been previous proposals to employ Thomson scattering, but they invoked "backscattering," in which femtosecond laser-beam photons reverse their direction upon head- on collisions with a beam of electrons. This approach can be used to produce a beam of x-rays, but the pulse length will be roughly the average of the laser and the electron-beam pulse lengths-- still too long for the study of atomic motion.
Instead of backscattering, Chattopadhyay and Kim suggest using right-angle Thomson scattering, in which a femtosecond laser beam is fired at a 90-degree angle to the path of an electron beam. An electron-beam pulse approximately 10 picoseconds in length can be focused down to about 100 microns in width, which is the equivalent of 330 femtoseconds.
"By scattering the laser beam across the waist of such an electron beam, we can generate femtosecond pulses of x-rays in the same direction as the electrons," says Chattopadhyay. "A magnet can then be used to separate the electron beam from the x-ray beam."
At the Beam Test Facility, CBP researchers will initially demonstrate the feasibility of this technology using a femtosecond laser system that is now being built by Shank and Robert Schoenlein. The laser will be positioned on top of the Beam Test Facility's roof shielding block, and its light will be directed through a hole to cross paths with the linac beam. Once they have produced x-ray pulses in the 300-femtosecond range, as proof of principle, the researchers will try squeezing the electron beam (perhaps using a plasma lens) into a width of about 10 microns in order to generate beams with pulse lengths of only 30 femtoseconds. This will open the door to a multitude of important scientific experiments (see sidebar).
Producing femtosecond x-ray pulses will be a tricky proposition in itself. Detecting these pulses could be even trickier. There are special cameras that can detect picosecond x-rays, but nothing for the femtosecond regime.
Says Chattopadhyay, "We will have to design a test that could be done only with a femtosecond x-ray pulse. For example, we are seriously considering an idea of Shank's: to shine our beam on a surface of silicon to induce a specific phase transition that could be used for optical grating and would only take place if the photons were at x-ray wavelengths and in femtosecond pulses."
Down the road, CBP researchers plan to use the Beam Test Facility to study critical ideas for an ultraviolet free-electron laser (UVFEL), a device whose short-wavelength radiation (around 40 angstroms) would complement that of the ALS and be ideal for studying the so-called biological "water window"--the region of the spectrum where most of the activities of biological molecules take place.
Says Kim, "Today's storage-ring technology permits generation of coherent radiation down to a few hundred angstroms, but for shorter wavelengths, linacs appear more promising."
Instead of the optical cavities used for generating light in IRFELs or visible-light lasers, the UVFEL will probably make light using a technology called Self-Amplified Spontaneous Emission (SASE). This involves a single pass of an electron beam through a very long undulator (approximately 70 meters) in which many of the photons produced are trapped back into the beam, making it radiate even more photons.
"The key here is accelerator technology," says Kim. "We need an electron beam that is highly stable and operates at a high current in order to optimize its interaction with photons."
To enhance radiation-beam interactions, CBP researchers have suggested that an electron beam first be conditioned through use of special microwave cavities. This idea will be tested for its possible use in a UVFEL by CBP researchers, who are now designing such a cavity for the Beam Test Facility.
Another radiation-beam interaction study at the facility will involve electron-beam "chirping"--a term that refers to a systematic change in energy during a pulse. By tilting the energy of an electron beam so that the energy at the "head" of each pulse differs from the energy at its "tail," then sending this tilted beam through an undulator, CBP researchers believe they will be able to significantly shorten the pulse length of the light they eventually produce.
The Beam Test Facility will be used by another CBP group for a series of beam electrodynamics experiments. Led by John Corlett, this group specializes in the design of advanced rf equipment and systems, particularly high-current accelerator rf cavities and feedback systems such as those used in the ALS or those being developed for the B-factory.
Says Corlett, "The quality of any particle beam depends heavily upon its feedback system. We design feedback systems that not only detect deviations in the beam but also correct the problem."
Many problems arise when a beam, as it travels through an rf cavity, excites energy modes within the cavity other than those that promote particle acceleration. Not only do these "parasitic" modes rob the beam of energy, but they can also distort its shape and create other problems that must be dealt with.
"Once a problem starts, it quickly gets worse," says Corlett, "so our goal is to anticipate problems and prevent them from occurring."
The beam electrodynamics group will also use the 50-MeV linac to test a number of novel research techniques and devices. Among these is a "nondestructive" method for measuring beam energy that involves Compton backscattering of the electromagnetic field inside an rf cavity. Proposed by Walter Barry, this would mark the first time that a beam's energy could be measured without affecting the quality of the beam.
Another technique that may be investigated is the use of "crab cavities" for enhancing beam collisions. A crab cavity provides a transverse kick that forces the beam passing through it to travel sideways. This enables the beam's particles to collide head-on with the constituents of another beam, even though the two beams cross paths at an angle.
Says Corlett, "It is possible we could deploy a series of crab cavities that would enable us to deflect a beam for collisions, then straighten that same beam out for other experimental use further down the accelerator line."
An excellent way to anticipate beam problems in the design stage is through a symbiosis of analysis and computer simulations. In the CBP's nonlinear dynamics group, Etienne Forest and Johan Bengtsson use computer simulations to track a single charged particle as it moves around a circular ring through an electromagnetic field. From this they learn a great deal about nonlinear beam dynamics.
Says Forest, "We are interested in keeping the particle inside the ring and examining all of the qualitative effects that are necessary to keep the beam stable and under control. For us, details such as the positions of the magnets are not as important as the qualitative effects on a single particle."
Computer simulations are also valuable for diagnosing problems that arise once operations begin. This was demonstrated early in the commissioning of the ALS storage ring.
Says Bengtsson, who worked with Malika Meddahi on modeling the ALS electron beam, "Modeling led to the diagnosis of a major lattice problem due to a possible short in a quadrupole. The culprit magnets were diagnosed within a few hours (by CBP's Rod Keller) and rectified, leading to the first circulating beam with 60 turns."
The computer simulations developed to model the nonlinear dynamics of a particle in a circular ring have a number of other applications as well, including the design and diagnoses of linacs, experimental beam lines, and electron microscopes.
Yet another CBP group is spearheading the development of a Two Beam Accelerator (TBA), an idea proposed several years ago by Andy Sessler, former LBL director, current CBP physicist, and an authority on FEL and high-energy accelerator technology. The TBA is a linear collider with a remarkably efficient power source that can impart enormous rates of energy gain on particles over short distances of acceleration.
"The energy of accelerating electrons or positrons in a TBA increases by several hundred million electron volts for every few feet the particles travel, compared to the 20 million electron volts now gained over the same distance in colliders today," says Sessler. "This greatly reduces the TBA's length and power requirements."
Sessler's TBA would feature two parallel beams of electrons. One would be an intense but low-energy beam (maybe 3000 amps at 10 MeV) that could be used either for an amplifier FEL or for a relativistic klystron, devices capable of extracting powerful microwaves from relativistic electrons. These microwave beams would then be used to accelerate the second electron beam to energies of trillions of electron volts.
"The steep accelerating gradient of the TBA would make it 10 times more energetic than accelerators of the same length are now," says Sessler.
Designs for a TBA are being developed and evaluated by Sessler's high-energy collider physics group at the CBP, in collaboration with researchers at LLNL and at the KEK high-energy physics laboratory in Japan. In a related effort, Sessler is collaborating with AFRD Director William Barletta and others on the development of an "FEL afterburner." This proposed device, which would combine the spent electron beam of an FEL amplifier with the electrical field generated in a "slow-wave" rf cavity while both are immersed in the magnetic field of a wiggler, is potentially a highly efficient source of microwaves and is considered a "promising candidate" to power a TBA.
For all of the many scientific accomplishments and all of the progress made in so many different areas, when CBP leader Chattopadhyay looks back over the past year, the event he found particularly gratifying was the recognition of his group as a Center.
"Historically, LBL and University of California scientists have often turned to us to launch, nurture, and provide detailed research and design directives for important institutional projects," he explains. "By recognizing the group as a divisional center, the laboratory gave us an advantage in recruiting, retaining, and enhancing our outstanding staff, and increased our visibility in the community."
Chattopadhyay is fond of the quote from Carl Sandburg, "Nothing happens unless first a dream." At LBL's Center for Beam Physics, scientific dreams often come true.