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A new wave in particle acceleration(New Scientist Via Acquire Media NewsEdge) WHEN the Large Hadron Collider was switched on last September - and ignominiously switched off a few days later - it was the subject of the kind of media frenzy usually reserved for rock stars and celebrity models. These, we were told, were the first moments of the most complex machine ever built. What we were not told was that something similar might never be built again. At issue is the sheer scale of the project. The LHC took some 20 years to design and build, and cost around $9 billion. The price didn't even include the 27-kilometre circular tunnel that houses the machine underneath the French-Swiss border near Geneva - that was built a couple of decades earlier for a previous accelerator. Some day, particle physicists will want to upgrade the LHC to an even bigger, better and more expensive model. In fact, they are already working on a successor known as the International Linear Collider. If the public baulks at paying the price, physicists can kiss goodbye to their dreams of teasing apart the laws of nature by smashing together particles at high energies. Given the current economic turmoil, the ILC might never be built. Perhaps it won't need to be. In laboratories around the world, the outline of an entirely new design of accelerator is being sketched that could revolutionise the economics of particle physics. Out are the many-kilometre-long tunnels; in is a compact construction a fraction of the size. More intriguingly still, this new breed of accelerator is being conjured up from little more than thin air. So why are particle accelerators so big? A simple answer is that to create the exotic and extremely massive particles that might tell us more about how the universe works, more familiar and less massive particles need to be smashed together at enormous energies. High energies mean high speeds, just a fraction below the speed of light - and that means giving the particles a long run-up. That's true, but misses out on an important subtlety. Machines such as the LHC use electric fields to accelerate protons, electrons and other charged particles. The stronger the field, the greater the acceleration. But making a faster particle blaster is not just a case of ramping up the electric field. Efficient acceleration can only take place in a vacuum: otherwise, the particle beam bangs into stray atoms and loses energy. If the electric field is too strong, it starts to rip out electrons from the material of the walls containing the vacuum. This causes sparks to fly, shorting out the accelerator and rendering it useless. Accelerator physicists must therefore take a softly, softly approach, using smaller electric fields to coax particles to higher energies over long distances. This, along with the limitations of the magnets used to bend the particle beams, is the real reason why protons need almost 17 million full revolutions of the LHC - a journey of about 450 million kilometres - to be boosted from an energy of 450 gigaelectronvolts to 7000 GeV. Solve the problem of material breakdown and you would be well on your way to doing everything an accelerator such as the LHC can do, but over a much smaller distance. In 1979, John Dawson of the University of California in Los Angeles and Toshiki Tajima at the University of Texas in Austin had a bold proposal to do just that (Physical Review Letters, vol 43, p 267). Instead of getting rid of the vexatious phenomenon of material breakdown, they proposed exploiting it. Such an audacious plan demanded an extraordinary material, and the two had one in mind: an exotic form of thin air known as a plasma. Unlike air, which is largely made up of neutral atoms and molecules, a plasma consists of positive ions sitting naked in a sea of electrons stripped from them. Dawson and Tajima envisaged sending a pulse of intense laser light through this plasma. The light's electric field would plough through the sea of lightweight electrons, pushing them from its path while leaving the heavier positive ions relatively unmoved. This would create an area of low electron density immediately where the pulse was passing. The displaced electrons would quickly snap back towards the positive ions in the pulse's wake, attracted by the opposing charge. They would briefly pile up together behind the pulse before overshooting and creating another area of lower density. Meanwhile, the pulse would be clearing electrons from the next region and repeating the process. The result is a wave-like pattern of varying electron densities along the light's path . "It's like a motorboat on a lake creating a wave in the water behind it," says Wim Leemans, a physicist who leads the development of plasma-based accelerators at the Lawrence Berkeley National Laboratory in California. And just like surfers on an ocean wave, electrons in just the right position at the crest of the density wave can surf down it - gaining a huge amount of energy as they go. The great advantage of this scheme is that the huge accelerating electric fields created by the displaced electrons are contained in a tiny bubble of plasma behind the light pulse. Unlike the walls of a conventional accelerator, the plasma material is already "broken", with all its electrons ripped from their mother atoms. "You can't do any more damage to it," says Robert Bingham, a physicist at the Rutherford Appleton Laboratory near Oxford, UK. So given a laser pulse with just the right intensity, duration and shape, there seemed virtually no limit to the acceleration that could be achieved over a short distance. It didn't take long for the idea to catch on. By the mid-1980s, physicists had confirmed Dawson and Tajima's scheme by zapping some plasma and accelerating a handful of electrons by a thousandth of a gigaelectronvolt. By the 1990s, it was a few billion electrons, and energies had reached 0.1 GeV over less than a millimetre - a distance many thousand times shorter than required in a conventional accelerator. Breakthough beamEven so, this was about 500 times less energy than could be delivered by the cutting-edge electron accelerator of the time - the Large Electron-Positron collider, which was then resident in the LHC's tunnel. And quite apart from that, only some electrons were accelerated the full whack. Depending on where they were in the density wave, some were accelerated less, and some not at all. This wide range of energies was useless for physicists wanting to bang together particles at precisely defined energies. A breakthrough came in 2004, when three groups - one under Leeman's leadership and others in France and the UK - independently announced that they had managed to create electron beams accelerated to a single energy, give or take a per cent (Nature , vol 431, p 535, p 538 and p 541). The researchers used various tricks to bring this about, but all three teams exploited the phenomenon of breaking waves. Just as a wave in shallow water that has grown too tall to support itself collapses, so the electron density wave in the plasma can grow too large and break. Just before this happens, the wave transfers its energy directly to the electrons, like a slingshot. By carefully controlling the laser power and the plasma density, it is possible to maintain a single wave behind the pulse on the verge of breaking. The result is a beam of electrons of one tightly defined energy. The new technique was sufficient to propel electrons reliably to energies of more than 1 GeV - enough to excite the interest of researchers in other disciplines. This is because an electron beam with an energy of a few GeV, when it is deflected by a magnetic field, radiates away part of its energy in the form of highly energetic X-rays. The wavelength of this light is short enough to probe in detail how individual molecules, atoms and even electrons behave. The list of researchers wanting to get their samples under these super-submicroscopic probes is as long as your arm: everyone from biologists studying the structure of the molecules of life, to materials scientists and electrical engineers looking at ways to etch ever smaller designs onto computer chips. Unfortunately, the machines currently needed to generate these X-rays, although not as gigantic as the behemoths of particle physics, are still pretty substantial. The Diamond Light Source, opened in January 2007 on the Rutherford Appleton Laboratory's site, is one of the latest examples. It cost some ?400 million to build and, with a circumference of over half a kilometre, covers an area the size of several football pitches. Plasma accelerators will not make facilities such as Diamond obsolete overnight, as they cannot yet match the intensity and pulse frequency that the larger machines can produce. But any decent-sized lab should be able to procure its own source of high-energy X-rays within the next five years or so, says Victor Malka, leader of the group at the Ecole Polytechnique in Palaiseau, France, that was involved in the 2004 breakthrough. Revolutionary as such an advance could be, such low-energy folderol is unlikely to impress particle physicists hungry for power. Thomas Katsouleas's baby, on the other hand, just might. Katsouleas is an electrical engineer at Duke University in Durham, North Carolina, and part of a team involved in a series of potentially ground-breaking experiments at the SLAC National Accelerator Laboratory in Stanford, California. "The minimum energy for seeing new physics is probably a beam of 250 GeV hitting another at 250 GeV," he says. In 2007, Katsouleas and his colleagues took a giant leap towards that goal, creating a new record for the highest energy achieved using a plasma accelerator: 85 GeV. Admittedly, they did cheat a little. The electrons they started with were not the relatively low-energy, chilled-out electrons of the plasma itself, but the 42 GeV output of one of the world's most muscular conventional electron boosters: the 3-kilometre-long SLAC main accelerator. Even so, the doubling in energy that Katsouleas's team achieved in just one metre of plasma was little short of astounding (Nature , vol 445, p 741). In the SLAC team's experiment, the high-energy electrons played the part of the laser pulse in the previous experiments, creating the density gradients and accelerating electric fields in its wake. "The physics is pretty much the same," says Patric Muggli, a colleague of Katsouleas's. Unfortunately, the problems are, too: only some of the electrons in the original beam get caught up in the whirl, with most passing through unaffected, producing a frustratingly smeared spectrum of electron energies at the other end. Ideally, the researchers would send a second high-energy electron pulse into the plasma just behind the first, timing it perfectly so that it surfs on the wake of the first. But as the SLAC accelerator only delivers one pulse at a time, the team are looking at achieving the same result by shaping the single pulse so that it is effectively chopped in two. Results should be expected, they say, within the next few years. Beyond that, the SLAC team has ambitious plans, currently unfunded, to place a second plasma accelerator after the first so that the output of one is immediately boosted by the next. "Once you've demonstrated two stages, you've solved the problem," says Mark Hogan, part of the team: if you can link two accelerators, you can link three or more, continuing until there are the number needed to create physically interesting collisions. There are other challenges, of course. To do everything that today's accelerator can do, a plasma accelerator should also be able to accelerate positrons - positively charged electrons. In conventional accelerators, that is not too much of a problem: currents of opposite charges simply flow in opposite directions. In a plasma accelerator, though, things are not that easy. When a beam of high-energy positrons enters a plasma, it does not repel electrons, but attracts them, which is an entirely different kettle of fish. "We haven't yet solved the physics behind the problem, but we're working on it," says Katsouleas. Despite such open questions, the plasma technology is not far off the point where it could replace more conventional accelerator designs. Most immediately, experiments such as those at SLAC open the possibility of a new breed of hybrid accelerator that is compact and cost-effective for what it can do. The first stage would be a conventional kilometre-scale accelerator, the second a miniature bolt-on afterburner to take the electrons' energy to new heights. Super cheapHogan and Muggli have even more ambitious plans. They envisage a conventional linear accelerator a couple of kilometres long to create an electron beam of 25 GeV energy. Bolted onto the end would be 20 consecutive plasma stages to boost the beam to 500 GeV over just a few additional metres. Such a machine will require some serious work, but Hogan and Muggli reckon it could be up and running by 2025. The essential component of a plasma accelerator - a tube of gas - is, at least compared with the sums big physics is used to talking in, essentially free. For that reason, says Katsouleas, the technology is going to be hard to ignore if particle physicists ever want to have their most ambitious plans funded. For him, the question now is not whether plasma accelerators will come, but what will drive them: the laser pulses of the original experiments, or the electrons of the SLAC experiments? Katsouleas thinks the answer is clear-cut. Although the cost per watt of laser power is predicted to drop within the next decade from around $1000 to about $70, the equivalent price to provide an initial electron beam today is only around $10. With a 250 GeV beam requiring about 100 megawatts to power it, the difference in price will be a huge consideration. Leemans thinks this judgement is premature, and that the still-unresolved problem of developing a single-energy beam with an electron-powered plasma accelerator could be a significant stumbling block. He is seeking funding for a project to develop a 10 GeV accelerator using a more powerful laser light source as a first step to even more powerful machines. In March this year, the US Department of Energy is due to announce its assessment of both approaches, with a view to funding either one or the other. The decision could have far reaching consequences for the future of high-energy particle physics. What price the next generation of particle accelerator? Copyright ? 2009 Reed Business Information - UK. All Rights Reserved. |