Particle accelerators like the Large Hadron Collider (LHC) are wonders of modern engineering and vending machines for Nobel prizes, but they’re also large – as indicated by the LHC's name – and costly. A new theoretical study by the Lawrence Berkeley National Laboratory's Berkeley Lab Laser Accelerator (BELLA) Center suggests how lasers could dramatically shrink the size and cost of particle accelerator. If the models hold true, it could remove a significant bottleneck from physics research and open up such machines to industrial and medical applications.
There’s no doubt that the LHC is a modern marvel, if only because it helped discover the Higgs boson particle without destroying the planet, as some theoreticians feared might happen. It’s also the largest accelerator ever built with its main ring stretching 17 mi (27 km) at a cost of US$9 billion. However, its size isn’t just a matter of prestige. It’s also a way of working around the inherent limitations of conventional particle accelerators.
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A particle accelerator works by using electromagnetic fields to pump energy into charged particles in a vacuum pipe, to make them go faster. The faster they go, the bigger the bang when they hit the target, and the better the chances of something interesting getting thrown out, like the Higgs boson. The problem is that only so much energy can be pumped into an accelerator of a given length. If you go beyond that limit, then the energy starts to leak out of the accelerator regardless of what it’s made of, like putting too much pressure on a leaky garden hose.
Engineers get around this by making the accelerators longer and longer, so the particles keep accelerating without having to pump any more energy into a particular length of pipe. It works, but the disadvantages pile up very quickly. The really big accelerators soon get so big and so expensive that you can count the number of them on the fingers of one hand – assuming you’ve lost four fingers.
Worse, the construction becomes a herculean task in itself, the scarcity of the machines means their use is restricted solely to the most rarefied of high-energy physics studies rather than for industrial purposes, and physicists face the gloomy prospect that some questions may never be answered because they’d need an accelerator 1,000 light years in diameter to do the job. And nobody wants to write the grant application for that one.
First conceived at UCLA in 1979, laser-plasma accelerators seemed to be a way for doing for high-energy physics what the microchip did for the electronics field. According to the maths, if an accelerator could be built that uses lasers instead of electromagnetic fields, it would be possible to shrink the LHC into a linear tube shorter than a football field.
Instead of pumping energy directly into a particle, a laser-plasma accelerator, as the name implies, shoots a laser into a cloud of plasma behind an electron, causing the plasma to push it along with much higher energy levels.
"The effect is like the wake of boat speeding down a lake. If the wake was big enough, a surfer could ride it," says Wim Leemans, head of the BELLA Center. "Imagine that the plasma is the lake and the laser is the motorboat. When the laser plows through the plasma, the pressure created by its photons pushes the electrons out of the way. They wind up surfing the wake, or wakefield, created by the laser as it moves down the accelerator.”
The accelerator is designed so that when a light particle, such as an electron, separates from the plasma cloud, it gets a massive boost of energy that’s up to 1,000 times more than in a conventional accelerator, so energy levels in tens of billions of electron volts that took miles to achieve can now be reached in mere feet.
An example of this sort of scale that the team gives is of BELLA’s petawatt laser that occupies only 100 square meters (1,076 sq ft) of space, yet generates 400 times the power of all the world’s power plants for 40 femtoseconds – that’s 40 quadrillionths of a second.
The obvious next step would be to build gigantic lasers and put the LHC out to pasture, but such lasers can take as long as one second to recharge. That’s unfortunate because high-energy physics need lasers that can manage tens of thousands pulses per second if the plasma wave is to be sustained in the accelerator long enough to be practical.
Since this is far beyond current technology for the high-powered lasers, the BELLA effort looked at modeling an alternative of using a battery of smaller lasers instead of one big one. Again, it’s a logical step, but the new problem now arises of how to synchronize the lasers if they’re going to fire together in less than a femtosecond. According to the BELLA team, this turned out to be not as great a hurdle as first thought because the lasers can average out their effect in a series of pushes.
"Instead of one big push, we would give it many smaller pushes at roughly the same time,” says Leemans. “It's not quite perfect, but the swing doesn't really care. It averages over all these little pushes and up it goes."
If the BELLA model holds up in the real world, the team says that it could not only mean smaller and cheaper ways of studying high-energy physics, but could also move them out of the lab and into factories and hospitals.
The team’s results were published in the journal Physics of Plasmas.
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