Plasmas Show Promise As Next Step For Accelerators

Date: November 23, 1992 Even as the superconducting supercollider survives another funding battle and lurches toward completion, some physicists are already wondering what comes next. The power of SSC collisions will fall far short of re-creating the state of matter at the instant the universe was born--an eventual goal of particle physicists. In order to give high- energy colleagues a new generation of tools to continue probing the mysteries of matter, a few plasma physicists are working on a revolutionary type of particle accelerator that uses the new, powerful, ultra-fast pulse lasers to achieve particle acceleration. This new technology has potential applications in the life sciences, as well. The funding for developing alternative particle accelerators is not especially abundant, with the De- partment of Energy devoting about $5 million a year to the effort, but the possible first demonstration of laser particle acceleration made in a University of California, Los Angeles, lab could change that situation. Fearing a repeat of the cold fusion fiasco, plasma physicists familiar with this potential breakthrough are reluctant to say much in the press about the work, at least until it is peer- reviewed and accepted for publication. But if these preliminary results ultimately bear up to scrutiny, they say, increased excitement--translated into funds and opportunities--will be injected into this nascent field. Unlike existing circular particle accelerator designs, such as the SSC, which accelerate charged particles via magnetic fields, the new technology would fire powerful, short-pulse lasers into gas clouds, creating plasmas--the state of matter in which electrons and positively charged nuclei are pushed apart in a cloud that remains electrically neutral overall. In theory, these laser pulses could induce waves in the plasma, kicking out electrons with 1,000 times greater energies than the SCC's protons. When channeled into collisions, these electrons would be far more effective at probing the basic building blocks of matter and in reproducing conditions at the time of the Big Bang. C. Joshi, et al., "Ultralight gradient particle acceleration by intense laser-driven plasma density waves," Nature, 311:525-9, 1984. T. Katsouleas, "Plasma-based high-energy accelerators," IEEE Transactions on Plasma Science, 15:5-87, 1987. P. Sprangle, et al., "Laser wakefield acceleration and relativistic optical guiding," Applied Physics Letters, 53:22, 1988. Physicists involved in this effort are convinced that plasma accelerators will become cost-effective replacements for SSC-type devices and will spawn a new generation of microaccelerator products for use in biomedical research. "The trouble with the SSC-type devices is that they're pricing themselves out of the market," says UCLA physicist John Dawson, who first advanced the notion of plasma accelerators in a 1979 paper (T. Tajima and J.M. Dawson, "Laser Electron Accelerator," Physical Review Letters, 43:267-70). "To make [plasma accelerators] work on the scale of a trillion electron volts, like the supercollider, would require a lot of development," he acknowledges. "But theory and experiment show we should be able to get three orders of magnitude greater particle acceleration than present technology in perhaps one thousandth of the distance." Dawson is the acknowledged dean of plasma accelerator scientists, who number a few hundred among the 5,000 or so plasma physicists worldwide. Although this tiny field is not yet getting much recognition or funding, theoretical physicist Nat Fisch believes working with lasers and plasmas is a good career move because of its enormous potential. "I am responsible for graduate education here at Princeton [University], and I have given some thought to what fields are opening in the future," Fisch says. "Lasers are getting bigger and better, and pulses are getting shorter, which makes for very interesting physics. "The basic physics of laser-plasma interactions is going to be with us for a long, long time," he predicts. Indeed, it was these sorts of interactions that excited Dawson when he laid out the theory behind this new accelerating scheme. He knew that a laser fired into a cloud of hydrogen--chosen for its simple atomic structure--would superheat the gas, stripping the electrons from their nuclei and thus creating a plasma. What Dawson realized is that plasmas have many useful characteristics. Though electrically neutral overall, the electrons and positively charged nuclei inhabit separate regions of the plasma cloud. The tension between the positive and negative charges creates an electrical field, with the heavier nuclei remaining more or less stationary, pulling the electrons toward them. But instead of locking into orbit, the electrons keep overshooting the mark, creating a pendulum effect termed a plasma wave. Dawson postulated that if a second stream of electrons were injected into the plasma, the negatively charged wave would impart its energy to the injected electrons, kicking them out of the plasma. The power of that kick--called the acceleration gradient--would depend on the density of the electrons in the plasma wave. He calculated that plasma waves could create accelerating fields that would impart 100 million to 30 billion volts per cubic centimeter--an energy gradient orders of magnitude greater than the 2 million volts per cubic centimeter generated by huge conventional accelerators. To prove that laser accelerators can work, experimentalists have been trying to build devices to demonstrate the principles that Dawson described, according to Tudor Johnston, a plasma physicist at the University of Quebec, where an early prototype was attempted. "The plasma is smarter than we are," Johnston jokes. "What we are really figuring out is how to get the concept off the ground." Johnston says experimentalists have had trouble creating what Dawson described as a "beat wave" accelerator. The beat wave is created by targeting the plasma with two laser beams of different frequencies. The overlapping laser frequencies create peaks of high- and low-radiation energy. These peaks and troughs of light energy separate electrons from nuclei, creating a plasma wave. The plasma electrons are pulled toward the nuclei, overshoot their mark, and are pulled back, creating a pendulum motion that creates a "wave" effect. Once it starts rolling, a fresh batch of electrons is shot into the plasma behind the laser beam, to "surf" the beat wave, accelerating to near-light speeds. But the interplay of laser and plasma is so complex that any misalignment can cause the positive ions to move (they are supposed to stay in the trough), causing the beat wave to crash before it can accelerate the injected electrons to particle- smashing speeds. Reports of progress in solving the beat wave problem has heartened Johnston and others in the field. Chan Joshi, a plasma physicist at UCLA, may have demonstrated the first solid evidence of electrons being accelerated in a beat wave. Until his work is peer-reviewed and published, Joshi says his only comment on this investigation is that he has submitted a paper to Physical Review Letters, and that it concerns the latest results from his seven- year effort to prove that plasma waves can create energy gradients at least 50 times greater than conventional accelerators. Explaining his reluctance to describe his work, Joshi says, "If there's one thing we've learned from the cold fusion fiasco, it is that it is not good to circumvent the peer review process." Joshi says he has received about $5 million in support to date from the Department of Energy, which is trying to stimulate a next generation of accelerators. But Quebec's Johnston, who knows of Joshi's work, is a bit less constrained when he says: "This is the sort of breakthrough that gets people interested in the field." Although Joshi's findings could offer encouragement to beat wave proponents, other investigators in the field of plasma accelerators are already trying to demonstrate a slightly different technique--one that would use a single laser to create a type of plasma wave called a wakefield. Don Umstadter, an experimental physicist at the University of Michigan, explains the idea behind the wakefield method. A single laser beam is fired into the gas cloud. The center of the laser beam has a stronger intensity than the fringes. Theoretically, the difference in intensity between the core and fringes of the beam would create a plasma. As Umstadter describes it, the core laser would plow through the plasma, creating an effect of peaks and troughs similar to the beat wave. Umstadter likens the core laser to a speedboat skimming over a lake. An electron beam injected into the wakefield would then "water-ski" over the peaks and valleys of electrons and positive ions in the plasma, accelerating to relativistic speeds. Umstadter says the wakefield scheme would avoid the necessity of fine-tuning two lasers, which has caused instability that has hampered beat wavers. But the wakefield concept has only recently become possible, thanks in large part to the development of powerful new short-pulse lasers by Gerard Mourou, also at Michigan, as well as others. Lasers are now available that are capable of delivering light pulses that last less than a picosecond (10-12 second)--time enough to create a plasma wave without setting the positive ions in motion and wrecking the effect. "But nobody has experimentally demonstrated the wakefield effect because the short-pulse lasers have only just become available," Umstadter says. To get the money to test the technique, the Michigan researchers have teamed up with Phillip Sprangle, chief scientist at the electron beam division of the Naval Research Lab. Several months ago they asked NRL, the National Science Foundation, and the Department of Energy to each provide $1 million to support a proof-of-principle experiment for the wakefield concept. They have not yet received an answer, which Sprangle calls disappointing rather than surprising, given the tight budgets everywhere in science. However, he thinks the amount requested is so small and the potential payoff so large that the money to test the wakefield will eventually be forthcoming. "It would seem wise to take a tiny fraction of the money needed for an SSC to develop a new technology," Sprangle says. Whether it's by water-skiing the wakefield or by surfing the beat wave, plasma scientists acknowledge they are years away from creating a useful particle accelerator. Johnston explains that even if plasma waves can accelerate electrons to near-light speeds, technologists will have to focus the accelerated electrons into the small, tight beams required to produce particle collisions with stationary or moving targets in high- energy physics experiments. "We're sure we can get the electrons moving fast enough," Johnston says, "but we're not so sure we can get the [accelerated] electron beams coherent enough." Proponents of these new-wave accelerators say that even as experimentalists spend the next decade trying to wrestle plasmas into sub- mission, their efforts will push the envelope of basic science, according to Princeton graduate adviser Fisch. "We're in a regime where lasers are only going to continue to get more intense, and when they interact with matter they'll produce a plasma, whether you want it to or not," says Fisch. Understanding both intentional and unintentional plasmas will continue to grow in importance, he says. Some of these possibilities have already been outlined by Dawson, who envisions directing plasma waves against a tungsten foil to create extraordinarily fast, powerful short-wavelength X-rays, capable of taking "pictures" of molecular reactions. And because the plasma accelerator could theoretically create these effects in areas as small as a few centimeters, Dawson suggests that scientists might one day be able to create X-ray machines that can be implanted in living subjects, to take views from the inside out. "All these things are dreams, so to speak, right now," Dawson says. "What we need are experiments." Tom Abate is a freelance science writer based in El Cerrito, Calif.

Plasmas Show Promise As Next Step For Accelerators

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