Argonne Wakefield Accelerator supplies more Big Bang for buck
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For particle physicists, getting "more bang for the buck," means colliding particles at higher energies for lower cost. In a recent breakthrough, researchers at Argonne National Laboratory have demonstrated a technique — called wakefield acceleration — that can power a linear, high-energy particle accelerator by using a low-energy particle accelerator like a booster in a multistage rocket. This could make possible collisions powerful enough to generate particles not seen since the Big Bang.
The wakefield approach accelerates groups of electrons using the electromagnetic field generated by another high-current electron beam. Theoretical physicists first proposed this idea in the late 1970s. These wakefields — so-called because they rely on the wake created by the high-current electron beam — would accelerate the trailing electron bunches much like an ocean wave accelerates surfboards. Some of the earliest experiments in the field were performed at Argonne.
Using the Argonne Wakefield Accelerator (AWA), Argonne physicists have made the first ever measurement of wakefield acceleration in a new kind of accelerating structure. Researchers spent more than a decade pioneering the technological and design details necessary for making this a reality. Jim Simpson, now retired, started the program that is currently guided by Wei Gai in the High Energy Physics Division.
"We've developed several ideas for new technology and have shown that it is possible to get acceleration from a wakefield," says Argonne physicist Paul Schoessow, one of the principal investigators at the AWA project.
Eventually, this accelerator technology could mean particle colliders with higher efficiencies, lower operation costs and greater collision energies. Physicists could explore in greater detail the forces and particles inside atoms, reproducing in the laboratory the very fires of creation. It could also provide information needed to widen the model of fundamental particles and forces in nature and gain insight into the origin of the universe. In addition to its possible accelerator applications, the ideas and concepts developed in this research may be helpful to astronomers developing experiments for detecting signals made by elusive cosmic rays, like neutrinos.
High-energy physicists probe the fundamental nature of matter by colliding beams of particles like protons and anti-protons, or electrons and positrons. Sophisticated detectors and computers help them look at the aftermath of the collisions. Physicists want to deepen our understanding of matter by looking at the components of the neutrons and protons in the atom. These components — different kinds of quarks, leptons and force-carrying particles — are only seen when particles are smashed together at high energies. To delve ever deeper into the atom, researchers need more powerful machines at correspondingly higher costs.
Due to technical and financial limitations of the accelerators in use today, developers of next-generation particle accelerators are pursuing alternatives such as the wakefield accelerator.
Today's accelerators
In circular accelerators, such as CERN in Switzerland and the Tevatron collider at Argonne's Illinois neighbor, FermiLab, powerful superconducting magnets bend the particle beams in a circular path to collide head on. The maximum attainable collision energy is limited by the maximum attainable strength of the magnets. In addition, the accelerator's efficiency is limited by the energy spent generating "synchrotron radiation" — radiation emitted whenever a magnet bends a particle's path, especially particles with a small mass such as electrons. Some accelerators, such as Argonne's Advanced Photon Source, take advantage of the synchrotron radiation to produce X-rays for research in structural biology, chemistry and materials science. But in high-energy physics, it is just wasted energy.
Linear accelerators like the one at the Stanford Linear Accelerator Center (SLAC) avoid the energy loss and radiation issues of bending magnets, but to get more powerful, linear accelerators must get longer, and eventually this runs up against prohibitive power and real estate costs. SLAC is the highest energy linear accelerator, but it requires periodic power boosts to move particles down its 2-mile length. Using today's technology to get the energies needed for 21st century physics would require an accelerator 30 to 40 miles long, with the requisite power boosts along the way.
These accelerators are usually powered by klystrons – a kind of vacuum tube that generates microwaves to accelerate particles. Both economic and technical reasons prohibit scaling up klystron technology to the higher energies required for linear colliders.
The way a wakefield accelerates particles bears some similarities to the way a boat passing through an ocean leaves a disturbance behind it — its wake. In wakefield acceleration, fast-moving charged particles give off electromagnetic radiation — the wakefield — as they interact with their surroundings. With the right surroundings and equipment, Argonne researchers can use the electromagnetic field generated by a beam of low-energy, high-current electrons to accelerate another beam of electrons to high energy. Researchers want to increase the energy boost per unit length to minimize costs. High acceleration over a relatively short distance is the goal.
"We are looking at the acceleration gradient — how much energy can you impart to a particle beam in your accelerating cavity," says Schoessow.
Lasers create the low-energy, high-current electron beam, which the scientists call the "drive beam." Short pulses of ultraviolet laser light are shot at a target. The ultraviolet light dislodges electrons in bunches from the target's atoms. These electron bunches are immediately pushed with a microwave electric field to create a high-current drive beam that energizes the wakefield accelerator.
Argonne researchers developed the mechanism that transfers energy from the drive beam to the so-called "witness beam," where the acceleration takes place. It is a transformer, similar in principle to the way an electric transformer converts high-current, low-voltage electricity to low-current, high-voltage.
Radio-frequency energy is generated by the low-energy, high-current electron bunches as they pass through a tube of electrically insulating, or dielectric material, Schoessow explains. The energy is fed to another tube of dielectric material where the fields are stepped-up, or increased in magnitude. Inside the second tube, the energy accelerates a second beam to higher energies. "One way to look at the dielectric tube system is as a transformer," says Schoessow. "Another way is to think of it is as using the drive beam as the power supply to provide what is essentially a frequency multiplication of the microwave fields used to accelerate the witness beam."
Argonne researchers measured the energy gain of the second beam, showing that the wakefield theory works. In demonstrating the principle, the Argonne team first measured a modest acceleration of 10 MeV per meter. A new laser being installed is expected to raise that to a sustained 100 MeV per meter with boosts up to 500 MeV over small distances. The 100-MeV-per-meter beam will be an increase of about 20 percent over SLAC's proposed state-of-the-art accelerating structures.
Laying the groundwork
To demonstrate wakefield acceleration, Argonne researchers overcame several technological and design obstacles. The voltage step-up apparatus was crucial to the demonstration. A notable part of the "transformer" device was the use of ceramic-like dielectric materials to carry the electron beams. These advanced-ceramic tubes can support the high fields generated without emitting electrons that could physically break down the beam, as can happen in existing accelerating structures.
Argonne researchers developed new instrumentation for measuring the witness beam acceleration. The team also pioneered the adaptation of laser technology for use in high-current electron sources such as those in wakefield acceleration. For example, Argonne scientists designed a mirrored device that can split the electron beam and produce a precisely spaced train of electron bunches, which is an important aspect of wakefield accelerator technology.
The U.S. Department of Energy's High Energy Physics Office of Advanced Technologies is funding this research.
Technology development is the goal for Argonne's wakefield accelerator team. They are planning equipment upgrades, design improvements and further experiments to improve their understanding of wakefields. The team is in the process of upgrading the laser and drive beam equipment to increase the acceleration gradient. Also, they are making changes to create shorter lengths of electron bunches, which, in turn, will allow higher energies.
In addition to improving the existing equipment, the wakefield team is measuring the beam currents at every step of the acceleration to see where the power goes. "We want to understand the efficiency of each stage of the process," Schoessow says.
In addition to further study of the physics of wakefield acceleration during the next several years, scientists want to demonstrate increased performance of the wakefield accelerator to show that it is moving closer to viability as a high-energy physics machine alternative. So far, the experiments with the dielectric transformer device have been carried out with a single bunch of electrons. To move forward, the Argonne team would like to perform similar experiments with a train of several electron bunches.
Their near-term goal is to demonstrate a reliable acceleration of 100 MeV per meter —that is the energy at which wakefields would become interesting technology for experimental accelerators, says Schoessow. One day the group hopes to design and build a high-energy physics machine that accelerates particles to 1 GeV over 10 meters.
Article from ANL Logos, Summer 2001.
For more information, please contact Evelyn Brown (630/252-5501 or eabrown@anl.gov) at Argonne.