Innovation could hold the key to ultra-compact machines useful for materials science and medical imaging
Conventional particle accelerators are typically big machines that occupy a lot of space. Even at more modest energies, such as that used for cancer therapy and medical imaging, accelerators need large rooms to accommodate the required hardware, power supplies and radiation shielding.
This schematic illustrates the laser-driven electron accelerator experiment at the University of Maryland. The three images at the top directly depict three key phases of the process. At left, a laser pulse is directed into a dense jet of hydrogen gas, where it ionizes the gas to form a plasma and initiates an effect called relativistic self-focusing. (See left inset.) Electrons within the plasma are rapidly accelerated to nearly the speed of light, which produces a brief, intense flash of visible light. (See middle inset.) The accelerated ultra-short bunch of electrons continues to gain energy and then exits the plasma, where it produces intense radiation that can be used for ultra-fast, high-energy imaging applications. (See right inset.)
Credit: Howard Milchberg/George Hine
A new discovery by physicists at the University of Maryland could hold the key to the construction of inexpensive, broadly useful, and portable particle accelerators in the very near future. The team has accelerated electron beams to nearly the speed of light using record-low laser energies, thus relieving a major engineering bottleneck in the development of compact particle accelerators. The work appears in the November 6, 2015 issue of the journal Physical Review Letters.
"We have accelerated high-charge electron beams to more than 10 million electron volts using only millijoules of laser pulse energy. This is the energy consumed by a typical household lightbulb in one-thousandth of a second." said Howard Milchberg, professor of Physics and Electrical and Computer Engineering at UMD and senior author of the study. "Because the laser energy requirement is so low, our result opens the way for laser-driven particle accelerators that can be moved around on a cart."
"As an unexpected bonus, the accelerator generates an intense flash of optical light so short that we believe it represents only one-half of a wavelength cycle," Milchberg added. These ultrashort light flashes could lead to the development of optical strobe lights that can capture the motion of electrons as they swarm across their atomic orbits--a potentially important development for materials science and nanotechnology.
The UMD team began with a technique known as laser-driven plasma wakefield acceleration and pushed it to the extreme. Generally speaking, the approach works by shooting a laser pulse into plasma, which is a gas (in this case, hydrogen) that has been fully ionized to remove all the electrons from the gas atoms. An intense laser pulse can create a plasma wake that follows the laser, much like the water wake that trails a speedboat. A bunch of electrons following the initial laser pulse can "surf" the waves of this wake, accelerating to nearly the speed of light in millionths of a meter.
"Unless your laser pulse can induce the plasma wake in the first place--and it takes a very intense pulse to do that--you're out of luck," Milchberg explained. Prior efforts needed much bigger laser energies to accomplish this effect. So Milchberg and his team tried a different approach, instead forcing the plasma itself to transform a weak laser pulse into a very intense one.
When a laser pulse passes through plasma, the laser causes the electrons to wiggle back and forth in the laser field. The electrons in the center experience the most intense part of the beam, so they wiggle the fastest. As they do they become more massive, as dictated by Einstein's law of relativity, which says that faster objects must increase in mass.
The result is that the center of the beam--where the electrons become heaviest--slows down compared to the outer parts of the beam. This causes the beam to self-focus, gaining intensity as it collapses, finally generating a strong plasma wake. This effect is known as relativistic self-focusing, and becomes more pronounced as the plasma density increases.
The UMD team took advantage of this self-focusing effect, drastically increasing the density of the plasma to as much as 20 times that used in typical experiments. In the process, they dramatically reduced the laser pulse energy needed to initiate relativistic self-focusing and thereby generate a strong plasma wake.
"If you increase the plasma density enough, even a pipsqueak of a laser pulse can generate strong relativistic effects," Milchberg added.
"From a practical standpoint, the key difference here is the footprint of the accelerator. What once required a room full of equipment and a very powerful laser could eventually be done with a small machine on a movable cart, with a standard wall-socket plug," said Andrew Goers, a graduate student in Physics at UMD and the study's lead author. "We started with a very powerful laser and found that we were able to keep dialing the energy back. Eventually we got down to about 1 percent of the laser's peak energy, but we were still seeing an effect. We were blown away by this."
The UMD laser-driven accelerator produces a beam of electrons and radiation, including gamma rays, which can be used for safe medical imaging and other applications without the need for significant levels of radiation shielding outside the beam path. The secondary effect of bright, extremely brief flashes of light is the result of the initial accelerations of electrons within the plasma wake, as they are accelerated from rest to almost the speed of light in less than 1 millionth of a meter.
"Such a violent acceleration means they radiate like crazy," Goers said. "As much as 3 percent of the initial laser radiation is emitted in the flash in a millionth of a billionth of a second."
In terms of sheer acceleration, laser-driven particle accelerators have a long way to go before they are ready for applications in high-energy physics, where facilities such as Fermilab and CERN reign supreme. But for more immediate applications, such as ultra-fast medical and scientific imaging, the main barriers to laser-driven acceleration are cost, complexity, and portability.
"We may have found a solution to overcome all three of these barriers," Milchberg said.
This research was supported by the U.S. Defense Threat Reduction Agency, the U.S. Department of Energy, and the U.S. Air Force Office of Scientific Research. The content of this article does not necessarily reflect the views of these organizations.
The research paper, "Multi-MeV electron acceleration by sub-terawatt laser pulses," Andrew Goers, George Hine, Linus Feder, Bo Miao, Fatholah Salehi, Jared Wahlstrand and Howard Milchberg, was published online November 6, 2015 in the journal Physical Review Letters.
Media Relations Contact: Matthew Wright, 301-405-9267, firstname.lastname@example.org
About the College of Computer, Mathematical, and Natural Sciences
The College of Computer, Mathematical, and Natural Sciences at the University of Maryland educates more than 7,000 future scientific leaders in its undergraduate and graduate programs each year. The college's 10 departments and more than a dozen interdisciplinary research centers foster scientific discovery with annual sponsored research funding exceeding $150 million.
Matthew Wright | EurekAlert!
NASA detects solar flare pulses at Sun and Earth
17.11.2017 | NASA/Goddard Space Flight Center
Pluto's hydrocarbon haze keeps dwarf planet colder than expected
16.11.2017 | University of California - Santa Cruz
The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.
Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...
Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.
That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...
Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.
During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....
The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.
Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...
Pillared graphene would transfer heat better if the theoretical material had a few asymmetric junctions that caused wrinkles, according to Rice University...
15.11.2017 | Event News
15.11.2017 | Event News
30.10.2017 | Event News
17.11.2017 | Physics and Astronomy
17.11.2017 | Health and Medicine
17.11.2017 | Studies and Analyses